Question

Paheli had heard that rainwater is as good as distilled water. So she collected some rain water in a clean glass tumbler and tested it using a tester. To her surprise she found that the compass needle showed deflection. What could be the reasons?

Answer 1

Hint: Rain water contains dissolved salts in it. Salt when dissolved in any liquid is ionized on applying potential through it and makes the liquid conduct. Distilled water does not conduct electricity because it does not contain any charge carriers.

Complete step-by-step answer:
Distilled water is a very poor conductor of electricity. This property of pure water can be used to test if water is pure or not. Rain water is considered to be the purest form of water since impurities present in water are left behind during vaporization by the sun. However, when it rains, the rain water we receive on earth is not essentially pure because it brings down impurities and particles present in the atmosphere along with it. Since rain water contains dissolved salts which it gets from the atmosphere, it becomes conductive to electricity. So when electricity is allowed to pass through the water, the compass needle shows deflection.
Since, rain water allows electricity to pass through it. This may be the reason Paheli found the compass needle show deflection.

Additional Information: In the atmosphere, many gases such as sulphur dioxide and nitrogen oxides are present which are the side products of burning fossil fuels. These gases react with rain water and form acids. This acidified rain is termed as acid rain.

Note: Pure water does not contain any charge carriers due to which it does not conduct electricity. When salt is dissolved in water, it is broken down into ions which make water conducting.
Rain water may not be safe for drinking especially if it is collected from an area near chemical plants.