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