Temperature distribution in a lake in summer and winter

These properties of water have important consequences in its role in aquatic ecosystems. In summer water at a temperature of 4°C will always accumulate at the bottom of freshwater lakes, irrespective of the temperature in the atmosphere.

Since water and ice are poor conductors of heat (good insulators) it is unlikely that sufficiently deep lakes will freeze completely, unless stirred by strong currents that mix cooler and warmer water and accelerate the cooling.

In the early spring at warming weather, chunks of ice float, rather than sink to the bottom where they might melt extremely slowly. These properties therefore allow aquatic life in the lake to survive during the winter.

· Water is also central to acid-base neutrality and enzyme function. An acid, a hydrogen ion (H⁺, that is, a proton) donor, can be neutralized by a base, a proton acceptor such as hydroxide ion (OH⁻) to form water. Water is considered to be neutral, with a pH (the negative log of the hydrogen ion concentration) of 7. Acids have pH values less than 7 while bases have values greater than 7.

· Solutes such as salts and sugars found in water affect the physical properties of water. The boiling and freezing points of water are affected by solutes, as well as air pressure, which is in turn affected by altitude. Water boils at lower temperatures with the lower air pressure which occurs at higher elevations.

· One mole of sucrose (sugar) per kilogram of water raises the boiling point of water by 0.51 °C, and one mole of salt per kg raises the boiling point by 1.02 °C; similarly, increasing the number of dissolved particles lowers water's freezing point.

· Solutes in water also affect water activity which affects many chemical reactions and the growth of microbes in food. Water activity can be described as a ratio of the vapor pressure of water in a solution to the vapor pressure of pure water.^[60] Solutes in water lower water activity. This is important to know because most bacterial growth ceases at low levels of water activity. Not only does microbial growth affect the safety of food but also the preservation and shelf life of food.

· The content of removable calcium carbonate salt (CaCO₃) in fresh water hardness is defined as hardness. In UK and YSA water hardness is measured in grains;

1 grain = 64 mg calcium carbonate

Soft water contains from 1 to 4 grains,

Medium water contains from 5 to 10 grains;

Hard water contains from 11 to 20 grains.

Heavy water

Heavy water, formally called deuterium oxide or ²H₂O or D₂O, is a form of water that contains the hydrogen isotope deuterium (D), instead the common protium (H) isotope.

Heavy water is not radioactive. It has a density about 11% greater than water, but otherwise, is physically and chemically similar.

This difference in density (especially affecting the biological properties) is larger than in all other isotope-substituted compounds because deuterium is unique among heavy stable isotopes in being twice as heavy as the lightest isotope.

This difference increases the strength of water's hydrogen-oxygen bonds, and this in turn is enough to cause differences that are important to some biochemical reactions. The human body naturally contains deuterium equivalent to five grams of heavy water, which is harmless. When a large fraction of water (> 50%) in higher organisms is replaced by heavy water, the result is cell dysfunction and death.

The scientists from the Institute for the Biology of Ageing (Moscow) conducted an experiment to determine the effect of heavy water on longevity using fruit flies. They found that while large amounts were deadly, smaller quantities increased their life spans by up to 30%.

Semiheavy water, HDO, exists whenever there is water with light hydrogen (protium, ¹H) and deuterium (D or ²H) in the mix. This is because hydrogen atoms (hydrogen-1 and deuterium) are rapidly exchanged between water molecules.

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