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Water - A Universal Solvent
A solvent is a substance which dissolves other substances (solute). For example, in a salt solution, water is the solvent and salt is the solute. Water has a unique property to dissolve more substances than any other solvents. It can dissolve solids such as salt and sugar, liquids such as honey and milk and gases such as oxygen and carbon dioxide in it. Therefore, it is called as universal solvent.
Place a sample of tap water on a clean watch glass and place it over a beaker containing water, as shown in the figure. Boil the water in the beaker. When all the water has evaporated from the watch glass, remove it from the burner and let it cool. What do you see on the watch glass?
(i) We can observe a number of concentric rings of solid matter deposited on the watch glass.
(ii) These are the dissolved solids left behind after the evaporation of water. Salts, minerals and impurities are the solids dissolved in water.
You can see a number of concentric rings of solid matter deposited on the watch glass. These are the dissolved solids left behind after the evaporation of water. Salts, minerals and impurities are the solids dissolved in water. Dissolved salts are important for the following reasons.
* They are essential for the growth and development of plants.
* They add taste to water.
* They supply the essential minerals needed for our bodies.
* Most of the chemical reactions important for our living take place in the cells of our body with the help of water.
Tap water, river water and well water contain dissolved solid but rainwater and distilled water do not contain dissolved solids. Hence concentric rings are not formed in the rain water and distilled water after evaporation.
Apart from solids and minerals, air is also dissolved in water. Air is present in dissolved state in all natural sources of water. The solubility of oxygen in water is higher than the solubility of nitrogen. Air dissolved in water contains approximately 35. 6% oxygen along with nitrogen and carbon dioxide. Air being dissolved in water is important for the following reasons.
* Air dissolved in water is important for the living organisms to survive.
* Fish extracts the oxygen from the water and expels water through the gills. Fish can survive in water only through the dissolved oxygen present in water.
* Aquatic plants make use of dissolved carbon dioxide for photosynthesis
* Carbon dioxide dissolved in water reacts with limestone to form calcium bicarbonate. Marine organisms such as snails, oysters, etc. , extract calcium carbonate from calcium bicarbonate to build their shells.
Take a beaker and fill it half with fresh tap water and heat it. You will see small bubbles appearing on the side of the beaker long before the water reaches its boiling point. These bubbles are oxygen gas dissolved in water.
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