Important Questions – 2012
Class – X
Subject – Science (Chemical Reaction)

2. A student has been colleting silver and copper coins. One day he observed that a black coating on silver coins and a green coating on copper coins. Give the chemical names of the black and green coatings. How are they formed?

3. Tap water conducts electricity whereas distilled water does not. Explain, Why?

4. Five metals A, B, C, D and F are added to the following solutions one by one. The observations made are below :

Answer the following questions based on above observations-
i. Which is the most active metal and which is the least reactive? Why?
ii. What would be observed when (a). B is added to a solution of copper (II) sulphate and (b). E is added to the solution of iron sulphate? Explain, why?
iii. Arrange the metals A, B, C, D and E in order of increasing reactivity.
iv. Container of which metal can be used to store both zinc sulphate solution and silver nitrate solution.
v. Which of the above solutions can be easily stored in a container made up of these metals?

5. Identify the type of chemical reaction taking place in each of the following–
i. Barium chloride solution is mixed with copper sulphate solution and a white precipitate is formed.
ii. On heating copper powder in air in a china dish, the surface of the copper powder turns black.
iii. On heating green coloured sulphate crystals, reddish brown solid is left and smell of gas having odour of burning sulphur is experienced.
iv. White silver chloride turns grey in sunlight.
v. Barium hydroxide reacts with ammonium chloride solution to give ammonia, barium chloride and water.
vi. Iron nails when left dipped in blue copper sulphate solution become brownish in colour and the blue colour of copper sulphate fades away.
vii. Quick lime reacts vigorously producing a hissing sound with water releasing a large amount of heat.

6. What is the colour of ferrous sulphate crystals? How does this colour change after heating? Name the products formed on strongly heating ferrous sulphate crystals. What type of chemical reaction occurs in this change?

7. What is an oxidation reaction? Give an example. Is oxidation an exothermic or endothermic reaction?

8. Define combustion reaction. Give an example of combustion reaction which is also exothermic.

9. Why should magnesium wire be cleaned before burning in air?

10. Write a balanced chemical equation for the reaction which is used in black and white photography.