The ability of water to stabilize temperature depends on its relatively high specific heat. The specific heat of a substance is defined as the amount of heat that must be absorbed or lost for 1 g of that substance to change its temperature by 1º C. The specific heat of water is 1.00 cal/g ºC. Compared to other substances, water has an unusually high specific heat. For example, ethyl alcohol, the type in alcoholic beverages, has a specific heat of 0.6 cal/g ºC. Because of the high specific heat of water relative to other materials, water will change its temperature less when it absorbs or loses a given amount of heat. The reason you can burn your finger by touching the metal handle of a pot on the stove when the water in the pot is still lukewarm is that the specific heat of water is ten times greater than that of iron. In other words, it will take only 0.1 cal to raise the temperature of 1 g of iron 1ºC. Specific heat can be thought of as a measure of how well a substance resists changing its temperature when it absorbs or releases heat. Water resists changing its temperature; when it does change its temperature, it absorbs or loses a relatively large quantity of heat for each degree of change. Given that a big amount of energy its needed or lost to raise water temperature, animals that live in oceans of big lakes enjoy of a relative constant temperature. In the same way, given that there is a high water content in living organisms they maintain a constant inner body temperature. This water property is crucial because all of the relevant biochemical reactions take place only in a narrow interval of temperatures.
It is the physical change from a liquid to a gas. For a water molecule to separate of its neighbor molecules, hydrogen bonds must break. This requires an input of energy (540 cal/g). As a consequence, when water turns into vapor on a leaf or on the skin surface, the escaping molecules carry large amounts of heat with them. In this way, evaporation has a cooling effect. This is a way for organisms to unload heat excess and stabilize their temperature.
It is the physical change from liquid to solid. In most liquids density raises when temperature drops. This raise in density is a product of individual molecules moving slowly so space between them become smaller. Water density, behaves in the exact same way until it reaches 4C., at this temperature water molecules get so close to each other that they can form 4 hydrogen bonds, making the strutcture very stable in the form of crystals. The space needed to form these 4 hydrogens bonds is bigger than when there are only 3 (liquid phase). The result of this is a drop of ice density which allows it to float. This is of relevance because, without this property lakes would freeze from the bottom to the top and no life could take place. Also, the solubility properties of water are affected by this property. The presence of solutes in water drops water's freezing point. Some polar fish that are cold resistant, have in their blood a protein called antifreeze protein which prevents the formation of ice crystals in the blood.
Water is a Solvent
Other molecules and ions will dissolve in water. Any liquid capable of dissolving or dispersing other substances is a solvent. Water is a very good solvent because of the two opposite charges it carries on the same molecule. Almost every substance will dissolve in water in some extent. Because of it polarized nature, however, water is a better solvent for ionized or polarized substances than it is for nonionized or polarized substances.
Sodium chloride dissolves readily in water because the positively charged sodium ion attracts the partially negative oxygen atoms of water molecules. Water molecules surround the ion, forming a shell with the negative oxygen atoms facing inward and the positive hydrogens facing outward. A similar shell forms around chloride ion, but in this case the positively charged hydrogen atoms are attracted to the negatively charged chloride ion and form a shell with the hydrogen atoms facing inward. Strong interactions between water molecules and other polar substances cause the dissolving substance, the solute, to break up and disperse through the water. The result is called a solution. Any substance capable of dissolving in water is said to be hydrophilic, meaning water loving.
Some substances, however, do not form solutions when placed in water (oil, gasoline). These molecules do not ionize (like sodium chloride) or polarize (like water), so there are no charges to attract the water molecules. Such substances are called hydrophobic, meaning water fearing.
Almost all vital chemical reactions necessary to keep cells alive take place in water. Water is the universal solvent for all life's molecules, big and small. From the moment they first formed on the primitive earth, the simple precursor molecules of life dissolved in water. From this solution the higher orders of complexity arose and the first cells were created. Without the special solvents properties of water, life would not be possible.
Water can Ionize
As we have seen before, electrons in water molecules are not equally shared between the oxygen and hydrogen atoms, giving the molecule a slightly ionic character. Occasionally, however water molecules break into two unequal pieces to give an hydronium ion (H3O+) and a hydroxyl ion (OH-). A process such as this is called ionization.
Water can ionize quite readily. In the billions and billions of water molecules in a cup of pure drinking water, there will be about 1, 000,000,000,000,000 hydronium ions and exactly the same number of hydroxyl ions. Although this seems a lot, it is only a small fraction of the total water molecules in the cup. It is important to note that for each hydronium ion there is a corresponding hydroxyl ion.
This is not always the case however. Certain substances when added to water, can change the number of hydronium ions in the water, turning the solution acidic. For example HCL ----> H+ + Cl- Solutions with higher hydronium ion concentrations than that of pure water are called acids.
Other substances dissolve in water to give hydroxyl ions. For example sodium hydroxide (NaOH), when in solution with water, gives sodium ions (Na+) and hydroxyl ions (OH-). NaOH ----> Na+ + OH-
Hydroxyl ions react with the naturally ocurring hydronium ions to give water molecules again, depleting the concentration of hydronium ions in the solution. OH- + H3O+ ------> H2O
A solution with a hydronium concentration lower than that of pure water (and consequently with a higher hydroxyl ion concentration) is said to be alkaline or basic.
Hydronium ion concentration is conveniently represented by a scale of values that goes from 1 to 14. This is the pH scale. Pure water is right in the middle of the scale with a pH value of 7 (1x10-7). As the concentration of hydronium increases, the numbers decrease, so a strong acid with a very high hydronium ion concentration has a value on this scale of 1 to 2, whereas a strong alkaline solution with a very low hydrogen ion concentration has a value of 11 to 14.
Water is more than just a solvent into which life's molecules dissolve and react. Water molecules also participate in many critical synthetic and breakdown reactions that build and destroy cells and cellular structures.
Take a look at this video on water properties.