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Water - It's Composition and Reaction

From several experiments, water has been found to be composed of two elements - hydrogen and oxygen. If hydrogen is burn in oxygen, the only product obtained is water.

Two volumes of hydrogen and 1 volume of oxygen form two volumes of steam,

 i.e. 2H₂(g) + O₂(g) → H₂O(g)

Or 1 volume of hydrogen and half volume of oxygen form 1 volume of water,

i.e. H₂(g) + ½ O₂(g) → H₂O(g)

Hence, the formula of water is H₂O

Water can be identified in the following ways:

(i). By it’s turning of white anhydrous copper(II) sulphate (CuSO₄) to blue crystals,

i.e., CuSO₄(s) + 5H₂O(l) → CuSO₄ . 5H₂O(s) White blue

(ii). By its turning of blue anhydrous cobalt(II) chloride, CoCl₂ to pink crystals, i.e. CoCl₂(s) + 6H₂O(s) → CoCl₂ . 6H₂O Blue pink

Water can dissolve almost every substance on earth to some degree. For this reason, water is said to be a universal solvent. In the laboratory, water is used to dissolve particles of reacting masses, so that they are able to move freely and hence undergo reactions easily - water however will not react with them, it is only a medium.

Example, AgNO₃(aq) + NaCl (aq) → AgCl(s) + NaNO₃(aq)

lf the two reactants are solids, the reaction will not occur, therefore, they have to be in solution.

Water, being able to dissolve most substances dissolves also certain atmospheric gases, such as oxygen, carbon(IV) oxide and nitrogen. Oxygen is slightly soluble in water. However, its presence in seas, rivers and other sources of water is very important to fish and other marine organisms which require oxygen for respiration. Carbon(IV) oxide is needed by plants for photosynthesis. Most aquatic organisms are unable to use atmospheric gases directly except when dissolved in water.

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