Earth's Water
As the saying goes ... "water, water, everywhere." Well, how much water is there; where is this water; how does it move around? This section of Water Science for Schools tells the story of where, how much, and in what forms water exists on Earth.
Water properties
Before we begin looking at the properties of water, maybe you'd like to take our True/False quiz about water properties. Some of the answers may surprise you.
What are the physical and chemical properties of water that make it so unique and necessary for living things? When you look at water, taste and smell it - well, what could be more boring? Pure water is virtually colorless and has no taste or smell. But the hidden qualities of water make it a most interesting subject.
Water's Chemical Properties
You probably know water's chemical description is H2O. As the diagram to the left shows, that is one atom of oxygen bound to two atoms of hydrogen. The hydrogen atoms are "attached" to one side of the oxygen atom, resulting in a water molecule having a positive charge on the side where the hydrogen atoms are and a negative charge on the other side, where the oxygen atom is. Since opposite electrical charges attract, water molecules tend to attract each other, making water kind of "sticky." As the right-side diagram shows, the side with the hydrogen atoms (positive charge) attracts the oxygen side (negative charge) of a different water molecule. (If the water molecule here looks familiar, remember that everyone's favorite mouse is mostly water, too).
All these water molecules attracting each other mean they tend to clump together. This is why water drops are, in fact, drops! If it wasn't for some of Earth's forces, such as gravity, a drop of water would be ball shaped -- a perfect sphere. Even if it doesn't form a perfect sphere on Earth, we should be happy water is sticky.
Water is called the "universal solvent" because it dissolves more substances than any other liquid. This means that wherever water goes, either through the ground or through our bodies, it takes along valuable chemicals, minerals, and nutrients.
Pure water has a neutral pH of 7, which is neither acidic nor basic.
Diagram about pH
Water's Physical Properties
Water is unique in that it is the only natural substance that is found in all three states -- liquid, solid (ice), and gas (steam) -- at the temperatures normally found on Earth. Earth's water is constantly interacting, changing, and in movement.
Water freezes at 32o Fahrenheit (F) and boils at 212o F (at sea level, but 186.4° at 14,000 feet). In fact, water's freezing and boiling points are the baseline with which temperature is measured: 0o on the Celsius scale is water's freezing point, and 100o is water's boiling point. Water is unusual in that the solid form, ice, is less dense than the liquid form, which is why ice floats.
Water has a high specific heat index. This means that water can absorb a lot of heat before it begins to get hot. This is why water is valuable to industries and in your car's radiator as a coolant. The high specific heat index of water also helps regulate the rate at which air changes temperature, which is why the temperature change between seasons is gradual rather than sudden, especially near the oceans.
Water has a very high surface tension. In other words, water is sticky and elastic, and tends to clump together in drops rather than spread out in a thin film. Surface tension is responsible for capillary action, which allows water (and its dissolved substances) to move through the roots of plants and through the tiny blood vessels in our bodies.
Here's a quick rundown of some of water's properties:
Weight: 62.416 pounds per cubic foot at 32°F
Weight: 61.998 pounds per cubic foot at 100°F
Weight: 8.33 pounds/gallon, 0.036 pounds/cubic inch
Density: 1 gram per cubic centimeter (cc) at 39.2°F, 0.95865 gram per cc at 212°F
By the way:
1 gallon = 4 quarts = 8 pints = 128 ounces = 231 cubic inches
1 liter = 0.2642 gallons = 1.0568 quart = 61.02 cubic inches
1 million gallons = 3.069 acre-feet = 133,685.64 cubic feet
Common water measurements
The U.S. Geological Survey has been measuring water for decades. Millions of measurements and analyses have been made. Some measurements are taken almost every time water is sampled and investigated, no matter where in the U.S. the water is being studied. Even these simple measurements can sometimes reveal something important about the water and the environment around it.
The results of a single measurement of a water's properties are actually less important than looking at how the properties vary over time. For example, if you take the pH of the creek behind your school and find that it is 5.5, you might say "Wow, this water is acidic!" But, a pH of 5.5 might be "normal" for that creek. It is similar to how my normal body temperature (when I'm not sick) is about 97.5 degrees, but my third-grader's normal temperature is "really normal" -- right on the 98.6 mark. As with our temperatures, if the pH of your creek begins to change, then you might suspect that something is going on somewhere that is affecting the water, and possibly, the water quality. So, often, the changes in water measurements are more important than the actual measured values.
pH is only one measurement of a water body's health; there are others, too. Choose from this list to find out what they are and how they can reveal something about water.
Temperature pH Specific conductance Turbidity
Dissolved oxygen Hardness Suspended sediment
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Water temperature
Water temperature is not only important to swimmers and fisherman, but also to industries and even fish and algae. A lot of water is used for cooling purposes in power plants that generate electricity. They need cool water to start with, and they generally release warmer water back to the environment. The temperature of the released water can affect downstream habitats. Temperature also can affect the ability of water to hold oxygen as well as the ability of organisms to resist certain pollutants.
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pH
pH is a measure of how acidic/basic water is. The range goes from 0 - 14, with 7 being neutral. pHs of less than 7 indicate acidity, whereas a pH of greater than 7 indicates a base. pH is really a measure of the relative amount of free hydrogen and hydroxyl ions in the water. Water that has more free hydrogen ions is acidic, whereas water that has more free hydroxyl ions is basic. Since pH can be affected by chemicals in the water, pH is an important indicator of water that is changing chemically. pH is reported in "logarithmic units," like the Richter scale, which measures earthquakes. Each number represents a 10-fold change in the acidity/basicness of the water. Water with a pH of 5 is ten times more acidic than water having a pH of six.
Pollution can change a water's pH, which in turn can harm animals and plants living in the water. For instance, water coming out of an abandoned coal mine can have a pH of 2, which is very acidic and would definitely affect any fish crazy enough to try to live in it! By using the logarithm scale, this mine-drainage water would be 100,000 times more acidic than neutral water -- so stay out of abandoned mines.
Diagram about pH Picture of a pH meter
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Specific conductance
Specific conductance is a measure of the ability of water to conduct an electrical current. It is highly dependent on the amount of dissolved solids (such as salt) in the water. Pure water, such as distilled water, will have a very low specific conductance, and sea water will have a high specific conductance. Rainwater often dissolves airborne gasses and airborne dust while it is in the air, and thus often has a higher specific conductance than distilled water. Specific conductance is an important water-quality measurement because it gives a good idea of the amount of dissolved material in the water.
Probably in school you've done the experiment where you hook up a battery to a light bulb and run two wires from the battery into a beaker of water. When the wires are put into a beaker of distilled water, the light will not light. But, the bulb does light up when the beaker contains salt water (saline). In the saline water, the salt has dissolved, releasing free electrons, and the water will conduct an electrical current.
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Turbidity
Turbidity is the amount of particulate matter that is suspended in water. Turbidity measures the scattering effect that suspended solids have on light: the higher the intensity of scattered light, the higher the turbidity. Material that causes water to be turbid include:
clay
silt
finely divided organic and inorganic matter
soluble colored organic compounds
plankton
microscopic organisms
Turbidity makes the water cloudy or opaque. The picture to the left shows highly turbid water from a tributary (where construction was probably taking place) flowing into the less turbid water of the Chattahoochee River in Georgia. Turbidity is measured by shining a light through the water and is reported in nephelometric turbidity units (NTU). During periods of low flow (base flow), many rivers are a clear green color, and turbidities are low, usually less than 10 NTU. During a rainstorm, particles from the surrounding land are washed into the river making the water a muddy brown color, indicating water that has higher turbidity values. Also, during high flows, water velocities are faster and water volumes are higher, which can more easily stir up and suspend material from the stream bed, causing higher turbidities.
Turbidity can be measured in the laboratory and also on-site in the river. A handheld turbidity meter (left-side picture) measures turbidity of a water sample. The meter is calibrated using standard samples from the meter manufacturer. The picture with the three glass vials shows turbidity standards of 5, 50, and 500 NTUs. Once the meter is calibrated to correctly read these standards, the turbidity of a water sample can be taken.
State-of-the-art turbidity meters (left-side picture) are beginning to be installed in rivers to provide an instantaneous turbidity reading. The right-side picture shows a closeup of the meter. The large tube is the turbidity sensor; it reads turbidity in the river by shining a light into the water and reading how much light is reflected back to the sensor. The smaller tube contains a conductivity sensor to measure electrical conductance of the water, which is strongly influenced by dissolved solids (the two holes) and a temperature gauge (the metal rod).
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Dissolved oxygen
Although water molecules contain an oxygen atom, this oxygen is not what is needed by aquatic organisms living in our natural waters. A small amount of oxygen, up to about ten molecules of oxygen per million of water, is actually dissolved in water. This dissolved oxygen is breathed by fish and zooplankton and is needed by them to survive.
Rapidly moving water, such as in a mountain stream or large river, tends to contain a lot of dissolved oxygen, while stagnant water contains little. Bacteria in water can consume oxygen as organic matter decays. Thus, excess organic material in our lakes and rivers can cause an oxygen-deficient situation to occur. Aquatic life can have a hard time in stagnant water that has a lot of rotting, organic material in it, especially in summer, when dissolved-oxygen levels are at a seasonal low.
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Hardness
The amount of dissolved calcium and magnesium in water determines its "hardness." Water hardness varies throughout the United States. If you live in an area where the water is "soft," then you may never have even heard of water hardness. But, if you live in Florida, New Mexico, Arizona, Utah, Wyoming, Nebraska, South Dakota, Iowa, Wisconsin, or Indiana, where the water is relatively hard, you may notice that it is difficult to get a lather up when washing your hands or clothes. And, industries in your area might have to spend money to soften their water, as hard water can damage equipment. Hard water can even shorten the life of fabrics and clothes! Does this mean that students who live in areas with hard water keep up with the latest fashions since their clothes wear out faster?
More information: Hard water and water softening - Stephen Lower
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Suspended sediment
Suspended sediment is the amount of soil moving along in a stream. It is highly dependent on the speed of the water flow, as fast-flowing water can pick up and suspend more soil than calm water. During storms, soil is washed from the stream banks into the stream. The amount that washes into a stream depends on the type of land in the river's watershed and the vegetation surrounding the river.
If land is disturbed along a stream and protection measures are not taken, then excess sediment can harm the water quality of a stream. You've probably seen those short, plastic fences that builders put up on the edges of the property they are developing. These silt fences are supposed to trap sediment during a rainstorm and keep it from washing into a stream, as excess sediment can harm the creeks, rivers, lakes, and reservoirs.
Sediment coming into a reservoir is always a concern; once it enters it cannot get out - most of it will settle to the bottom. Reservoirs can "silt in" if too much sediment enters them. The volume of the reservoir is reduced, resulting in less area for boating, fishing, and recreation, as well as reducing the power-generation capability of the power plant in the dam.
Capillary action
Even if you've never heard of capillary action, it is still important in your life. Capillary action is important for moving water (and all of the things that are dissolved in it) around. It is defined as the movement of water within the spaces of a porous material due to the forces of adhesion, cohesion, and surface tension.
Surface tension is a measure of the strength of the water's surface film. The attraction between the water molecules creates a strong film, which among other common liquids is only surpassed by that of mercury. This surface tension permits water to hold up substances heavier and denser than itself. A steel needle carefully placed on the surface of a glass of water will float. Some aquatic insects such as the water strider rely on surface tension to walk on water.
Capillary action occurs because water is sticky, thanks to the forces of cohesion (water molecules like to stay closely together) and adhesion (water molecules are attracted and stick to other substances). So, water tends to stick togther, as in a drop, and it sticks to glass, cloth, organic tissues, and soil. Dip a paper towel into a glass of water and the water will "climb" onto the paper towel. In fact, it will keep going up the towel until the pull of gravity is too much for it to overcome.
This is more important than you think. Consider:
When you spill your glass of BubblyBerryPowerGo (which is, of course, mostly water) on the kitchen table you rush to get a paper towel to wipe it up. First, you can thank surface tension, which keeps the liquid in a nice puddle on the table, instead of a thin film of sugary goo that spreads out onto the floor. When you put the paper towel onto your mess the liquid adheres itself to the paper fibers (actually, the liquid moves to the spaces between and inside of the fibers).
Plants and trees couldn't thrive without capillary action. Plants put down roots into the soil which are capable of carrying water from the soil up into the plant. Water, which contains dissolved nutrients, gets inside the roots and starts climbing up the plant tissue. As water molecule #1 starts climbing, it pulls along water molecule #2, which, of course, is dragging water molecule #3, and so on.
One common experiment to demonstrate capillary action is to place a stalk of celery in a glass of water that has been colored with food coloring (you might want to use a piece of celery that has begun to whither, as it is in need of a quick drink). This effect happens because, in plants, water molecules move through narrow tubes that are called capillaries.
The water in you
Think of what you need to survive, really just survive. Food? Water? Air? MTV? Naturally, I'm going to concentrate on water here. Water is of major importance to all living things; in some organisms, up to 90 percent of their body weight comes from water. Up to 60 percent of the human body is water, the brain is composed of 70 percent water, and the lungs are nearly 90 percent water. About 83 percent of our blood is water, which helps digest our food, transport waste, and control body temperature. Each day humans must replace 2.4 litres of water, some through drinking and the rest taken by the body from the foods eaten.
There just wouldn't be any you, me, or Fido the dog without the existence of an ample liquid water supply on Earth. The unique qualities and properties of water are what make it so important and basic to life. The cells in our bodies are full of water. The excellent ability of water to dissolve so many substances allows our cells to use valuable nutrients, minerals, and chemicals in biological processes.
Water's "stickiness" (from surface tension) plays a part in our body's ability to transport these materials all through ourselves. The carbohydrates and proteins that our bodies use as food are metabolized and transported by water in the bloodstream. No less important is the ability of water to transport waste material out of our bodies.
Domestic water use
Of course, some of the most important uses for water are at our homes. Domestic water use is water used for indoor and outdoor household purposes— all the things you do at home: drinking, preparing food, bathing, washing clothes and dishes, brushing your teeth, watering the yard and garden, and even washing the dog. In this Web site, domestic water use just covers self-supplied domestic water withdrawals - those people and organizations that use their own wells to supply their water, as opposed to public-supplied water.
Water generally gets to our homes in one of two ways. Either it is delivered by a city/county water department (or maybe from a private company), or people supply their own water, normally from a well. Water delivered to homes is called "public supplied" and water that people supply themselves is called "self supplied." People who supply their own water almost always use ground. The vast majority of America's population gets their water delivered from a public-supply system. This makes sense, as America's population now largely live in urban centers. You might want to check out a bar chart below that shows how the trend over the last 40 years of people moving to urban centers is reflected in the water use for domestic purposes.
Self-supplied domestic withdrawals for the Nation, 2000
For 2000, withdrawals were an estimated 3,720 Mgal/d or 4,170 thousand acre-feet per year. Self-supplied domestic withdrawals were about 1 percent of total freshwater withdrawals and about 2 percent of total freshwater withdrawals for all categories excluding thermoelectric power. About 45 million people were self-supplied. Ground water was the primary source of the water (98 percent). Between 1995 and 2000, self-supplied domestic withdrawals increased about 10 percent and the self-supplied domestic population increased almost 7 percent. However, the self-supplied domestic population remained at 16 percent of the total U.S. population
Domestic water use by State, 2000
[d] - Data for this chart are available.
Self-supplied population, by State, 2000
Trends in domestic water use, 1955-2000
Since the end of World War II there has been a trend of people moving out of the rural countryside and into the ever-expanding cities. This has important implications for our water resources. Communities have had to start building large water-supply systems to deliver water to new populations and industries.
In times past, when most people lived in rural areas, they had to find ways to supply their own water - often by drilling a well and pumping water to their homes. Not many city dwellers have a well in their backyards today. A public-water supply system, such as your local water department, nowadays delivers water to most homes. The bar chart below shows the trend toward urbanization over the last 45 years. Notice how the blue bars (representing the millions of poeple served by a public water-supply system) keeps going up while the green bars (representing the number of people who supply their own water) trended downward until 10 years ago. For the last 10 years, the percentage of the population relying on public-supply systems has remained at about 84 percent.
Here is a bar chart showing the public-supplied and self-supplied populations in the U.S. from 1955-2000.
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Water, Hydration and Health
Liquid water is an absolute requirement for all active life. It is the most important nutrient throughout the living world. In particular, we cannot live without it for more than about 100 hours, whereas other nutrients may be neglected for weeks or months. Although commonly it is treated rather trivially, no other nutrient is more essential or needed in as great amounts.
The water content of our bodies (methodology reviewed [961]) varies and is variable between individuals, generally dropping, throughout our lives from above about 90% of total weight as a foetus to 74% as an infant, 60% as a child, 59% as a teenager (male; female 56%) 59% as an adult (male; female 50%) to 56% (male; female 47%) in the over-50’s. The gender differences, from the teenager years onwards, are due to their differing fat levels, as is the drop in the elderly who replace muscle mass with fat. There is little difference with gender or age from childhood onwards, if allowance is made for this fat content. Body water is distributed between the cells (intracellular fluid, ICF, ~65%; ~30 L in a 75 kg man, ~20 L in a 60 kg woman) and the extracellular fluid (ECF, ~35%; ~15 L in a 75 kg man including the ~3 L of plasma, ~10 L in a 60 kg woman). Water is free to move between the ICF and the ECF with any net movement controlled by the effective osmotic and hydrostatic pressures. The majority of the ions in the ICF are K+ and protein anions whereas in the ECF they are Na+, Cl- and bicarbonate.
Water intake and output are highly variable but closely matched to less than 0.1% over an extended period. Water balance in humans has been modeled [583]. Electrolyte intake and output are also closely linked, both to each other and the hydration status. Typical values for an adult in a temperate climate are given below:
Water balance Water input, ml/day Water output, ml/day
Drinksa 1500 Urined 1500
Foodb 700 From skin, sweate 500
Metabolic waterc 300 Respirationf
400
Fecesg 100
a Water, fizzy drinks, tea, coffee, alcoholic beverages, etc. All water intake counts equally including coffee and alcoholic drinks as any diuretic effect is minimal or non-existent; once accustomed to caffeinated drinks, these count and act as any other water intake [615].
b Water contained in foodstuffs, varying from ~6% in peanuts, ~35% in bread to ~85% in fruits and vegetables.
c Water produced on metabolizing the foodstuffs and drinks (e.g. 1 g fat gives ~1.1 g H2O).
d A significant fraction of this is required for the removal of urea and other solutes. The rest is variable to equalize water input and output
e Used for temperature control, varying with energy intake and expenditure, ambient temperature and humidity
f Varies with energy intake and expenditure, ambient temperature and humidity. Together with losses from skin this typically amounts to about 50 ml/100 kcal food energy intake.
g Varies with diet, particularly increasing with increased dietary fiber.
All values will vary with diet, activity and climate. The water ingested is determined by social, practical and psychological factors with need indicated by thirst, when the body is becoming dehydrated. Water output is regulated by hormonal action and the production of urine by the kidneys, which usually can adapt to the body‘s hydration status.
The actual amount of liquid water (from drinks) that an individual needs depends on their age, gender, physical activity, physiological condition or illness and the temperature and humidity of their physical environment. A healthy individual may have slightly lower or somewhat higher water intakes without harm by varying their urine output. The recommended amounts are somewhat higher (1.0-1.5 ml/kcal) than the average intakes, being about 3.0 L for men and 2.2 L for women (rising to 2.3 L if pregnant or 3.1 L if lactating) [962]l. These higher levels of water intake seem to reduce the occurrences of kidney stones, gall stones and some cancers and may be otherwise beneficial [963]. However, there seems to be no scientific source for the argument in favor of much increased water intake (e.g. for the statement "Drink at least eight glasses of water a day" or similar) [474] with both benefits and potential hazards of extra water intake being documented [474]. Generally, low levels of water intake do not seem to show any health benefits.
Men require more water than women due to their higher (on average) fat-free mass and energy expenditure. Infants and young children have need for more water in proportion to their body weight as they cannot concentrate their urine as efficiently as adults and their surface area relative to their weight is more extensive, giving rise to greater water loss from the skin. The elderly should take care to ensure adequate hydration, as ageing diminishes the sensation of thirst as well as the ability to concentrate the urine.
Water plays many roles within the body; as a media for, and contributor to, molecular interactions; as a solvent and separating medium, to carry and distribute nutrients, metabolites, hormones and other materials around the body and within cells; to remove waste products, mainly via the urine and feces; as a reactant in many metabolic reactions; as a thermoregulator due to its high specific heat and heat of evaporation; as a lubricant between bodily structures and in forming mucous as well as facilitaing necessary structural shifts in macromolecules such as proteins and nucleic acids; as a structure-former , maintaining cellular shape; and as a protective shock absorber, e.g. for the brain.
Hydration status is difficult to define or determine precisely or accurately. An indicator of hydration status is the osmolality of the blood. However, it is normally closely controlled around about 284 mOsmol/kg (increasing slightly (1-2 %) in the elderly and decreasing ~3% during pregnancy) and is, therefore, a relatively poor indicator of hydration status. Short term hydration status may be determined simply and accurately by weight as only water content affects weight over short periods when food intake, fecal output and other possibly confounding factors (such as sweaty or changed clothing) are controlled.
Dehydration (starting at about 2-3% loss of body weight) causes a range of symptoms from tiredness, headaches and decreased alertness to collapse and death (at more than 10% loss of body weight). Mild symptoms may be seen in the lack of concentration of schoolchildren towards the end of their school day. Severe symptoms of dehydration are sometimes evident in the elderly, due to restricted water intake for medical, psychological or social reasons. Increased water intake is normally easily controlled due to the effective functioning of the kidneys to produce more urine. If this does not occur, due to greatly excessive water intake (e.g. > 1.0 L/hr) or kidney disorder then the extra water (hyperhydration) may produce low blood sodium levels and cause the brain to swell, resulting in death.
Water should be drunk little but often throughout the day such that we are never thirsty. It is particularly important to hydrate last thing at night to prepare for the significant loss of water during sleeping and rehydrate first thing in the morning as this is a time when the blood is most viscous and strokes particularly prevalent. We should also drink before, during and after exercise to maintain our level of hydration. The thirst-quenching ability of soft drinks has been assessed [964]. Acidity was found to be the taste attribute most closely related with thirst-quenching with sweetness and ‘thickness’ (viscosity) being the most contra-indicated.
There is no such thing as naturally pure water; all waters we drink contain dissolved solutes and many contain some microorganisms [965]. There are several forms that the water we drink may take, which vary subtly from each other; drinking water, spring water, tap water, natural mineral water and water preparations promoted with various health claims. Bottled waters are subject to international regulations but are not necessarily safer than tap water. Clearly, all such water must be drinkable, contain solutes (including those classed as contaminants) below the legally-allowed limits, to be bacteriologically safe and be subject to continued monitoring.
Tap water Water, from any source, treated to meet legal and quality standards. It may contain low or moderate amounts of minerals depending on the source of the water (e.g. hard or soft water areas). This is the major water product with over a billion glasses a day being consumed in the US alone, although most domestic tap water is used for washing, flushing the toilet and through wastage. Often it is chlorinated, which ensures microbiological safety for long periods of storage and eliminates all risks from otherwise devastating diseases such as cholera and dysentery. Although chlorination has been shown to possibly produce potentially hazardous byproducts, the association between exposure and demonstrable adverse health effects is still unproven and the protection chlorination offers far outweighs this risk. Fluoridation of water (e.g. by adding SiF62-) for the purpose of reducing dental caries, is generally regarded as safe [966a]. However, groundwaters containing excessive amounts of fluoride (> 1 mg/liter) are widespead [966b]. The health claims for fluoridation remain contraversial [1048].
Drinking water Water intended for human consumption and may contain disinfectants and/or other solutes within legal quality standards. Such bottled water is not necessarily better for health than tap water, as shown in 2004 when Coca Cola was awarded an Ig® Nobel prize for producing Dasani in the UK. Dasani was a bottled 'pure' water prepared from London tap water. It was found that it contained high levels of the carcinogen bromate, which is (and was) not present in the tap water. The bromate was introduced by reaction between the added ozone and calcium chloride containing calcium bromide during production (for background science see [1000]).
Natural mineral water Water from a spring, artesian well or well that naturally contains dissolved salts [967]. It may be carbonated. It is characterized by its mineral content, which may vary between far lower to much higher than tap water, according to source. Mineral waters must be naturally safe with no parasitic or pathogenic organisms as they are not subject to disinfection. The presence of safe microorganisms is used as proof that no disinfection has taken place. Higher silica content distinguishes mineral water from surface (e.g. reservoir) water. The price of mineral water is over a thousand times that of quality tap water.
Spring water Water from an underground aquifer, collected as it flows and bottled at source.
Processed water with health claims There is an increasing market in bottled water and domestic water processing equipment claiming that the water has considerable health benefits varying from more rapid hydration to cures for AIDS and cancer. Generally there are no proper scientific trials to prove these claims, only isolated testimonial evidence. Oxygenated drinks have been proposed to improve the immune status. However, a randomized blinded clinical study [968], although showing a transient moderate increase in oxygen radicals (using 6 mM O2) and signs of activation of the immune response, was not conclusive.
One factor often used to promote these ‘health’ waters is supposed greater cellular hydration or ease of hydration. It is unclear whether increased cell hydration is actually health-promoting. A recent paper has argued that this may be a determining factor in the initiation of cancer [969]. It has been found that cancer cells do have greater water with increased fluidity but the cause and effect relationship (i.e. whether increased cellular hydration initiates cancer or cancer initiates high cellular hydration) has not yet been established.
‘Sports’ drinks Sports drinks [973] are intended to reduce fluid, mineral (e.g. particularly Na+) and energy imbalance due to exercise. The carbohydrate content and osmolality must both be low to encourage efficient hydration (i.e. the drink must be hypotonic (<280 mOsmol/L) or isotonic (~280 mOsmol/L)). Na+ ions (usually as NaCl) are a necessary ingredient as they stimulate both sugar and water uptake in the small intestine as well as replacing material lost by sweat. Hypotonic drinks give more rapid hydration but clearly contain less sugar and minerals. Chilling improves palatability so encouraging consumption. Some sports drinks contain ‘power’ ingredients such as caffeine or taurine, where there is patchy evidence of some sports benefit. These products are usually promoted with testimonials from athletes or sports teams, but without double-blinded trial evidence.
In the light of the increased promotion of 'special' water preparations, it is important to take notice that there are definite and proven health benefits from simply drinking more water and from changing fluid intakes from coffee, tea, alcohol, and hypertonic soft drinks to mineral or tap water [413]. That cup of coffee first thing in the morning is best, perhaps, replaced by a glass of water in order to reduce the higher risk of heart attacks at this time of day.
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Water Activity
When water interacts with solutes and surfaces, it is unavailable for other hydration interactions. The term 'water activity' (aw) describes the (equilibrium) amount of water available for hydration of materials; a value of unity indicates pure water whereas zero indicates the total absence of 'free' water molecules; addition of solutes always lowering the water activity. Water activity has been recently reviewed [788] and has particular relevance in food chemistry and preservation. Water activity is the effective mole fraction of water, defined as aw = λwxw = p/p0 a where λw is the activity coefficient of water, xw is the mole fractiong of water in the aqueous fraction, p is the partial pressure of water above the material and p0 is the partial pressure of pure water at the same temperature (i.e. the water activity is equal to the equilibrium relative humidity (ERH), expressed as a fraction). It may be experimentally determined from the dew-point temperature of the atmosphere in equilibrium with the material [473, 788]; e.g. by use of a chilled mirror.f, h A high aw (i.e. > 0.8) indicates a 'moist' or 'wet' system and a low aw (i.e. < 0.7) generally indicates a 'dry' system. Water activity reflects a combination of water-solute and water-surface interactions plus capillary forces. The nature of a hydrocolloid or protein polymer network can thus effect the water activity, crosslinking reducing the activity [759]. Note that the water activity of any aqueous solution in equilibrium with ice (awi) is equal to the water vapor pressure over ice to the water pressure over pure liquid water and does not depend on the solute's nature or concentration [457]. Solutions with the same ice melting point therefore have the same water activity.
Shown right is an indicative water activity isotherm displaying the hysteresis often encountered depending on whether the water is being added to the dry material or removed (drying) from the wet material. This hysteresis is due to non-reversible structural changes and non-equilibrium effects. There are many empirical equations (and tables) that attempt to describe this behavior but, although indicative, none predict with sufficient accuracy and the water activity isotherm should be experimentally determined for each material. In the food industry, such empirical equations combine contributions from the ingredients to give an estimate of aw, which is then used to estimate the mold-free shelf life (MFSL; Log10(MFSL,days)=7.91-(8.1xαw) , 21°C, [443]).
The water activity (aw) usually increases with temperature and pressure increases.e For small temperature increases (T1 T2) at low aw, an often-applicable relationship is: where ΔH is an enthalpy change (e.g. absorption or mixing), R is the gas constant and T is in Kelvin. Such changes in water activity may cause water migration between food components. Increasing the temperature reduces the mold-free shelf life.
The multi-ingredient nature of food and its processing (e.g. cooking) commonly result in a range of water activities being present. Foods containing macroscopic or microstructural aqueous pools of differing water activity will be prone to time and temperature dependent water migration from areas with high aw to those with low aw; a useful property used in the salting of fish and cheese but in other cases may have disastrous organoleptic consequences. Such changes in water activity may cause water migration between food components. As the humidity of the air is typically 50-80% (aw = 0.5-0.8), foods with lower aw will tend to gain water whilst those with higher aw tend to lose water.
Control of water activity (rather than water content) is very important in the food industry as low water activity prevents microbial growth (increasing shelf life), causes large changes in textural characteristics such as crispness and crunchiness (e.g. the sound produced by 'crunching' breakfast cereal disappearing above about aw = 0.65) and changes the rate of chemical reactions (increasing hydrophobe lipophilic reactions but reducing hydrophile aqueous-diffusion-limited reactions). Highly perishable foodstuffs have aw > 0.95 (equivalent to about 43 % w/w sucrose), Growth of most bacteria is inhibited below about aw = 0.91 (equivalent to about 57 % w/w sucrose); similarly most yeasts most yeasts cease growing below aw = 0.87 (equivalent to about 65 % w/w sucrose) and most molds cease growing below aw > 0.80 (equivalent to about 73 % w/w sucrose). The absolute limit of microbial growth is about aw = 0.6.b As the solute concentration required to produce aw < 0.96 is high (typically > 1 molal), the solutes (and surface interactions at low water content) will control the structuring of the water within the range where aw knowledge is usefully applied. Changes in the natural clustering of water due to low concentrations of solutes will only occur at aw > 0.98. Although low density water (ES) will possess less aw than collapsed water clustering (CS) and the consequences are very important in biological systems, such changes in the absolute value of aw are small.
Indicative values of water activities Substance λw xw aw
Saturated LiCl 0.19 0.57 0.11
Saturated MgCl2 0.83 0.40 0.33
Saturated SrCl2 1.03c 0.69 0.71
Saturated BaCl2 1.18c 0.76 0.90
Bread - 35d 0.96
Cheese - 37d 0.97
Dried fruit (e.g. sultanas) - 18d 0.76
Raw meat - 60d 0.98
Dry pasta - 12d 0.50
Cooked pasta - 72d 0.97
Preserves (e.g. jam) - 28d 0.88
a Water activity is defined as equal to the ratio of the fugacity (the real gas equivalent of an ideal gas's partial pressure) of the water to its fugacity under reference conditions, but it approximates well to the more easily determined ratio of partial pressures under normal working conditions. The activity coefficient (λw) has dependence on the partial molar volume and hydrogen bond strength (which includes dependence on the temperature and dielectric constant) of the water and only in dilute solutions (i.e. aw > 0.95) can it be approximated by unity. The water activity (aw) is related to the chemical potential (μw; at equilibrium, μw of liquid water and its vapor phase are identical) by μw = μw° + RTLn(aw) where μw° is the standard chemical potential of water. Prediction equations for the water activity of multicomponent systems have been developed [552], based on the Gibbs-Duhem equation , which at constant temperature and pressure simplifies to and therefore , where the terms ni are the relative proportions of components n of chemical potential μ and activity a. The resultant equations, although starting on this firm theoretical base, require empirical simplifications due to the problems involving the interactions between the components and the paucity in our knowledge of the molecular interactions of the components with water. Water activity prediction may also be achieved by combining the effects of the chemical groups (rather than molecules) present, where suitable parameters are available [557]. In conclusion, prediction of the water activity of mixed components presents difficulty and, except in cases of simple interpolation, is best determined experimentally. [Back]
b Note that the required aw necessary to prevent growth will depend somewhat on the solutes present; e.g. glycerol lowers aw efficiently but still may allow microbial growth. [Back]
c An activity coefficient (λw) greater than unity may be due to non-ideal behavior caused by the volume taken up by large ions (and other solutes, e.g. sucrose) at high concentrations [442]. Alternatively it may be simply seen as due to the removal of some of the the 'free' water by binding to the ions (see e.g. [997]). [Back]
d % w/w. [Back]
e In some materials (e.g. salts and some sugars) water activity may reduce with temperature increase. At high pressures, water behaves similar to solutions with increasing salt content in that the water activity apparently reduces with increased pressure [457]. [Back]
f There are a number of methods for measuring water content [470] including the poorly understood Karl Fischer titration [471]. [Back]
g The mole fraction equals the number of moles of water divided by the total number of moles of all materials, including water, in the same volume. [Back]
h The activity coefficients for solutes may be determined in several ways, including boiling point elevation, freezing point depression, equlibrium vapor pressure, equilibrium relative humidity, osmotic pressure, heat of dilution and excess heat capacity [929]. Due to deviations from the theoretical relationships applied, different methods may give different results, particularly at high solute concentrations.
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