Water and its properties
Water and its properties make it one of the most important compound on earth. More than 75% of the earth’s surface is covered with sea, rivers and lakes and this makes water the most common compound on earth. Water is also found in abundance in bodies of all living organisms. For example, a human being’s bones contain about 72% water, kidneys contain about 82% water and blood contain about 90%.
Uses of water
Besides sustaining life, water has a lot of other important uses such as, at home we use water for:
Industrially we use water:
- as a solvent
- as a coolant
- for cleaning
- as a chemical reactant
On farms we use water:
- to provide for animals to drink
- to water crops.
In power stations we use water:
- to make steam which then drives the turbines that generate electricity if its a thermal power station.
- to directly drive turbines if its hydroelectric power station.
- as a coolant for turbines and generators.
Natural occurrence of water
Natural water never occurs in pure form due to its excellent solvent properties. It always contain dissolved and suspended impurities. This leads to many problems because clean water is essential to the health and well-being of the world’s population. Biological and chemical pollutants are affecting the quality of our water. Some of the pollutants are artificial and some are natural.
Natural supplies of water such as rain, springs, rivers and seas all contains up to about 3.5% by mass of dissolved solids such as sodium, calcium, magnesium and potassium salts.
Though natural water can sometimes look clear, it is not clean. It usually contains microbes: bacteria and other microscopic organisms that can make a human being ill. Right now over 1 billion people around the world have no access to clean water, they rely on dirty river water as their drinking water. As a result, over 2 million people, mainly children, die each year from diarrhoea and diseases such as cholera and typhoid, caused by drinking infected water.
Artificial pollutants in rivers, lakes and seas also include sewage, industrial waste and agricultural waste. Many towns and cities discharge and toxic industrial waste directly into rivers that pour into lakes and seas. Agricultural waste include nitrates washed off by rain from fertilisers, insecticides, oil and lead compounds.
- Nitrates promote the growth of algae which use up dissolved oxygen, thus killing fish and other water organisms that need oxygen.
- Insecticides do not decompose readily. They accumulate in water and kill many organisms that live in water.
The problem with water is that it is very good at dissolving substances. This is why it is very rare to find really pure water on the earth’s surface. Even rain water as it falls through the atmosphere it dissolves a tremendous variety of substances such as oxides of nitrogen, sulphur and carbon leading to the formation of acid rain.
The water cycle
The water cycle is a description of how water circulates around the Earth. Solar energy is the driving force for the water cycle.
- Heat from the sun evaporates water from open water bodies such as the oceans, seas, rivers and lakes and also from plants through transpiration.
- As the water vapour rises it cools and condenses forming tiny droplets of water. These droplets form clouds.
- The clouds are moved along by air currents and they continue cooling until the tiny water droplets join to form larger droplets. The droplets then drop as rain when they reach a certain size.
- Rain water then accumulates in streams and rivers which flow into lakes, seas and oceans.
- Some of this water is diverted from rivers into reservoirs and purified and used for domestic and industrial uses. The waste water from households and industries is treated at the sewage works before being returned to the rivers, lakes and sea where it eventually evaporates back into the clouds. That is how the cycle continues.
The method used for water treatment to make it fit to drink depends on its source, it quantity and its quality. For some sources, such as mountain streams, boiling it is usually enough to kill any microbes present.
For other sources such as rivers and lakes, the contamination may include human waste, industrial waste and agricultural waste. In this case it will need a combination of filtration, settling and chlorination. The outline of the process is as follows:
- The water is first filtered through a coarse filter to filter out larger, insoluble particles and floating debris.
- It is then passed into a sedimentation tank that contains flocculants, such as aluminium sulphate, which make the smaller clay particles coagulate, stick together and sink to the bottom of the tank.
- These coagulated particles are removed by further filtration through a fine sand filter. At this stage, a carbon slurry is usually added to remove unpleasant tastes and odours, and then a
lime slurry is added to adjust the pH of the water.
- Chlorine gas is then added. Chlorine kills any microbes and bacteria in the water. This also makes the water acidic through the formation of weak solution of hydrochloric and hypochlorate acid. The water is neutralise by adding appropriate amounts of sodium hydroxide solution. Fluoride is sometimes added to water if there
is insufficient amounts of it occurring naturally to help in preventing tooth decay.
- The water is then pumped to the storage reservoir where it is stored ready for pumping to homes.
The unique properties of water
- Pure water is a colourless, odourless liquid with a flat taste. Dissolved gasses and solids can give natural water a pleasant taste.
- At sea level it freezes at 0° C or 273 K and boils at 100° C or 373 K. If its pure, it boils away completely without leaving an residue.
- It has its highest density at 4° where 1cm3 has a mass of 1g. As it freezes it expands, thereby decreasing its density, instead of contracting and increasing its density. A decrease in density when it freezes makes ice float on top of liquid water instead of sinking.
- It has a pH of 7 which means that it is neutral to litmus and universal indicator.
- Ordinary distilled water is not chemically pure because it contains small traces of dissolved glass from the distillation equipment.
- Many gasses including oxygen dissolves in water and that is why many organisms can live in water.
- Water is an excellent solvent for many ionic substances.
- It has an unusually high boiling point for a molecule of relatively low molecular mass and a greater specific heat capacity than almost any other liquid.
The tests for water
To test whether a liquid is water or not we test the following properties:
- boiling point and melting point – measure the boiling and melting point of the liquid at 1 atmospheric pressure. If it is water it will boil at 100 °C and freezes at 0 °C.
- density – measure the density of the liquid at 4° C. The density of water at 4° C is 1g/cm3.
- effect on anhydrous copper sulphate – to find out whether a colourless liquid contains water, add the liquid to anhydrous copper(ii) sulphate. If this changes from white to blue, then the liquid contains water.
- effect on anhydrous cobalt chloride – dip blue cobalt chloride paper into the liquid. If the paper turns pink, then the liquid