Properties of water
WATER HAS many properties including polarity, adhesion, cohesion, density, heat capacity, and solvent properties. These properties cause water to exhibit some unique behaviours. Solid water (ice) is less dense than the liquid, so it floats on top of the water. In areas such as ponds and lakes which freeze in the winter, organisms can still survive at the bottom as the water freezes from the top to the bottom.
The high specific heat capacity of water means that large amounts of energy are needed to raise the temperature by 1°C. Organisms that depend on water will, therefore, exist in a stable environment as the water’s temperature remains relatively constant.
A water molecule is a polar molecule since opposite ends of the molecule have opposite charges. The hydrogen atoms have a slightly positive charge while the oxygen atom in a water molecule has a slightly negative charge. Polarity of water means that it has unusually stronger bonds than other simple covalent molecules. This results in higher-than-expected boiling and melting points.
The polarity of water molecules influences the
properties of cohesion and adhesion. Cohesion refers to the attraction of water molecules to each other. Attraction at the surface allows tiny objects and some animals (such as insects) to walk on the water’s surface. Adhesion refers to the attraction of water molecules to other molecules. An example of this is the formation of a meniscus at the surface of water in a container (for example, a measuring cylinder).
The ability of water to be pulled up against the force of gravity is called capillary action and is the result of cohesion and adhesion working together. Capillary action is responsible for the transportation of water from the roots to the stem and leaves of plants.
By far, the most important use of water is as a solvent. Since water is polar, it can dissolve both ionic and other polar covalent compounds. It is described as a universal solvent. WHAT MAKES WATER HARD?
When magnesium and calcium ions dissolve in water, it causes the water to become ‘hard’. These ions enter the water when slightly acidic rain water or river water containing carbonic acid (H 2CO3) flows over rocks containing calcium or magnesium salts. Examples of these rocks are limestone (calcium carbonate), gypsum (calcium sulphate) and dolomite (calcium/magnesium carbonate).
MgCO (s) + H2CO3 (aq) ___ Mg2+ (aq) + 2HCO3- (aq)
CaCO3 (s) + H2CO3 (aq) ___ Ca2+ + 2HCO3- (aq)
Water that contains Ca2+, Mg2+ and HCO ions is said to be ‘temporary hard’ water. When boiled, the calcium and hydrogen carbonate ions can react to form insoluble calcium carbonate, which can then be removed from the water. This is usually seen as limescale in kettles and hot water pipes. Ca2+ + 2HCO (aq) (heat) ____ CaCO (s) + H O (l) + CO2 (g)
Water that contains other calcium compounds (for example, calcium sulphate from gypsum) is called ‘permanent hard’ water. Sulphate ions (SO cannot be removed by boiling.
REMOVING HARDNESS FROM WATER
Permanent hardness can be removed by using washing soda (Na2CO3), which reacts with the calcium ions to form insoluble calcium carbonate. Ca2+ (aq) + CO 2- (aq) ___ CaCO (s) insoluble 3 3 calcium carbonate This is an example of a precipitation reaction. Another method is the use of ion-exchange columns containing resins with sodium (Na+) or hydrogen (H+) ions. The calcium ions are replaced in the hard water by sodium or hydrogen ions. Calcium ions remain stuck in the resin.
In distillation, the water evaporates, leaving the dissolved ions behind. The vapour is condensed and collected as pure water. This method is used in desalination plants to produce fresh water from sea (salt) water. Francine Taylor-Campbell is an independent contributor. Send comments to kerry-ann. hepburn@gleanerjm.com.