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?

When the electric current is passed through copper sulphate solution, the colour of copper sulphate solution fades. Copper will be deposited at the cathode and oxygen gas will be liberated at the anode. There will be no change of colour of copper sulphate solution and no deposition of copper on carbon rod. This is because there is no flow of electricity through the solution. Electrical energy is responsible for this chemical change. Such reactions involving electricity is known as electrochemical reactions. In this reaction, energy is absorbed.

Cell Instrument in which it is used Dry cell Radios Cameras Clocks Toys Mercury cell Watches Calculator Electronic instruments Nickel- cadmium cell Rechargeable torches Cameras Lithium ion cell Mobile phone Laptops 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?

In the given table, some of the cells are rechargeable, some recyclable and some cells cannot be reused. So, after usage recyclable cells and rechargeable cells are reused, whereas some of the cells are disposed. The cells such as Nickel- cadmium cell and Lithium ion cell can be recharged and reused. Yes, cells causes environmental pollution. As these cells are not decomposed easily.

Assess the chemical reactions given below and answer the questions. Calcium carbonate + heat → Calcium oxide + Carbon dioxide Calcium oxide + water → Calcium hydroxide + heat a) Mention the reactants and products in each case. b) Which among these are endothermic? Which one is exothermic?

a) A reactant is a substance that is present at the start of a chemical reaction. A product is a substance that is present at the end of a chemical reaction. So, in the given reactions: Calcium carbonate + heat → calcium oxide + carbon dioxide Calcium carbonate and heat are reactants and calcium oxide and carbon dioxide are products. Calcium oxide + Water → Calcium hydroxide + Heat Calcium oxide and water are reactants and calcium hydroxide and heat are products. b) The chemical reaction in which heat is released is known as exothermic reaction and the chemical reaction in which heat is absorbed is known as endothermic reaction. The reaction between calcium carbonate and heat is known as endothermic reaction because calcium carbonate absorbs heat and decomposes to produce calcium oxide and carbon dioxide. The reaction between calcium oxide and water is an exothermic reaction. Because in this reaction, heat is liberated.

Identify the energy change involved and write down what type of chemical reaction takes place in burning of a candle.

Burning of candle requires heat energy. So, the energy change that involved is heat energy and the type of chemical reaction that takes place is thermochemical reaction.

Identify the energy change involved and write down what type of chemical reaction takes place in glowing of a fire fly.

The energy change that involved in glowing of a fire fly is light energy. Light produced by a fire fly is the result of a chemical reaction that happens in its body. So, the type of chemical reaction that takes place is photochemical reaction.

Identify the energy change involved and write down what type of chemical reaction takes place in plating a copper ring with gold.

The energy change that involved in plating a copper ring with gold is electrical energy. So, the type of chemical reaction that takes place in plating a copper ring with gold is electrochemical reaction.

Identify the energy change involved and write down what type of chemical reaction takes place in reaction between potassium hydroxide and sulphuric acid.

When potassium hydroxide reacts with sulphuric acid, potassium sulphate and water are formed with the release of heat. Since, the reaction takes place with the release of heat energy, the reaction is called exothermic reaction. Therefore, the type of chemical reaction that takes place in reaction between potassium hydroxide and sulphuric acid is known as thermochemical reaction.

Identify the energy change involved and write down what type of chemical reaction takes place in Burning of fuels.

During burning of fuel, fuel chemically combines with oxygen and forms carbon dioxide with the releases of energy in the form of heat and light. As heat is released in this reaction, it is termed as exothermic reaction. So, the energy change involved is heat energy and the type of chemical reaction that takes place is thermochemical reaction.