Which of the following oxides produces salt upon reacting with sodium hydroxide?

1. Na₂O
2. CaO
3. NO
4. ZnO

The acidic gas produced when lead nitrate is heated is :

1. PbO
2. NO₂
3. MgO
4. N₂O

When copper metal is added to a solution of silver nitrate, the colour of the solution changes to blue. This shows:

1. copper is less reactive than silver
2. copper is more reactive than silver
3. both are equally reactive
4. none of the above

Which of the following equations represents an endothermic reaction?

1. C + O₂ $\xrightarrow{\text{heat}}$ CO₂
2. CaO + H₂O → Ca(OH)₂
3. NaOH + HCl → NaCl + H₂O
4. CaCO₃ $\xrightarrow{\text{heat}}$ CaO + CO₂