Add hydrochloric acid to a concentrated solution of sodium hydroxide taken in a test tube. As the reaction proceeds, does the test tube become hot or cold? 

• What do you observe?
• In which watch glass does the colour of silver bromide change? 
• Which form of energy was responsible for this chemical change? 

Set up the apparatus. What changes do you observe when electricity is passed through the circuit? 
• In the colour of copper sulphate solution: 
• On the carbon rods.
• Does it happen if a carbon rod is simply kept in copper sulphate solution? Which form of energy was responsible for this chemical change? Was the energy liberated or absorbed?

• What do we do with these types of cells after their use?
• Which among these cells can be recharged and reused? 
• Do they cause environmental pollution? 

Assess the chemical reactions given below and answer the questions.

Calcium carbonate $+$ heat $\to$ Calcium oxide $+$ Carbon dioxide 

Calcium oxide $+$ water $\to$ Calcium hydroxide $+$ heat 

a) Mention the reactants and products in each case. 

b) Which among these are endothermic? Which one is exothermic?