Heat — Important Questions

The heat is used to break intermolecular bonds and change the state from solid to liquid at constant temperature

Step 1: During change of state, the temperature remains constant — this is the key concept. Step 2: The heat absorbed during melting is called latent heat. Step 3: This latent heat is used to overcome the forces of attraction (intermolecular bonds) between molecules of the solid, converting it to liquid without raising the temperature. Step 4: The first option is wrong — temperature does NOT rise during melting; it stays at 0°C. The third option is wrong — stored potential energy concept is partially true but the heat is not 'released when water boils'. The fourth option is wrong — the heat is

Thermal expansion causes the girder to increase in length, and rollers allow free expansion to prevent structural damage

Step 1: When temperature rises, solids expand — this is called thermal expansion. Step 2: A steel girder fixed at both ends cannot expand freely. The expansion force would cause it to buckle or damage the pillars. Step 3: By placing one end on rollers, the girder can slide freely when it expands in summer or contracts in winter, without exerting force on the support pillars. Step 4: Convection does not move solid girders. Radiation does not cause mass loss in steel. Rollers do not help in heat conduction — they help in movement.

Water is a bad conductor of heat, so heat does not reach the bottom of the test tube

Step 1: In this experiment, the test tube is heated near its mouth (top). Step 2: Water at the top boils, but ice at the bottom remains unmelted. Step 3: This can only happen if heat does not travel easily downward through the water — proving water is a bad conductor of heat. Step 4: Conduction in water is very poor, and convection currents (warm water rising, cold sinking) would not carry heat to the bottom. Convection moves heat upward, not downward. Ice melts at 0°C, not at a temperature higher than 100°C.

Vacuum between walls (prevents conduction and convection), shiny surfaces (prevent radiation), cork stopper (prevents convection through the mouth)

Step 1: Conduction and convection both require a medium (matter). Vacuum has no medium, so it prevents BOTH conduction and convection between the walls. Step 2: Radiation does not need a medium. To reduce radiation, the surfaces of both walls are made shiny — shiny surfaces reflect heat radiation instead of absorbing it. Step 3: The cork stopper prevents hot air from escaping through the mouth, stopping convection at the opening. Step 4: Option A incorrectly assigns vacuum to preventing radiation. Option C incorrectly says shiny surfaces prevent conduction. Option D incorrectly claims a cork a