VARDAAN LEARNING INSTITUTE

Solve the following chemical riddles:

1. A shiny brown colored element 'X' on heating in air becomes black in colour. Name the element 'X' and the black coloured compound formed. Write the balanced chemical equation.
2. A substance 'X' which is an oxide of a group 2 element is used intensively in the cement industry. This element is present in bones also. On treatment with water it forms a solution which turns red litmus blue. Identify 'X' and write the chemical reactions involved.
3. A metal 'X' reacts with dilute Sulphuric acid to form a gas 'Y'. The gas 'Y' burns with a 'pop' sound. Name 'X' and 'Y'. Write the balanced chemical equation.
4. White silver chloride turns to grey substance 'X' in sunlight. Name 'X' and type of reaction.
5. Compound 'A' is used in fire extinguishers and makes cakes soft and spongy. On heating, it gives sodium carbonate. Identify 'A'.
6. A green salt on heating decomposes to produce a colourless suffocating gas and leaves a reddish brown residue. Name the salt and write the decomposition reaction.
7. A solution of a substance 'X' is used for whitewashing.
   1. Name the substance 'X' and write its formula.
   2. Write the reaction of the substance 'X' named in (a) above with water.

Balance and Identify the type of reaction:

8. $Fe + H_2O \rightarrow Fe_3O_4 + H_2$
9. $MnO_2 + Al \rightarrow Mn + Al_2O_3$ (Thermite Reaction)
10. $Pb(NO_3)_2 \xrightarrow{\Delta} PbO + NO_2 + O_2$
11. Write the balanced equation for: Carbon monoxide reacts with hydrogen gas at 340 atm to form methyl alcohol.
12. Write the balanced equation for: Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.

Give Scientific Reasons:

13. Why does the colour of copper sulphate solution change when an iron nail is dipped in it? Write the equation.
14. Respiration is considered an exothermic reaction. Why?
15. Oil and fat containing food items are flushed with nitrogen. Why?
16. Decomposition reactions are called the opposite of combination reactions. Why?

Case Study: Corrosion

Corrosion is a natural process that converts a refined metal into a more chemically stable form such as oxide, hydroxide, or sulphide. It is the gradual destruction of materials (usually a metal) by chemical and/or electrochemical reaction with their environment. Rusting of iron is the most common example.

17. What is the chemical formula of Rust?
18. Name two factors necessary for rusting of iron.
19. How can we prevent rusting of iron grills? (Two methods)
20. Is rusting an Oxidation or Reduction reaction for Iron?
21. Identify the type of reaction in the following: $Fe_2O_3 + 2Al \rightarrow Al_2O_3 + 2Fe + Heat$. Why is this reaction used in welding railway tracks?