Assertion (A): When you touch two bodies, the body which is felt warmer must be of higher temperature than the other one.
Reason (R): The heat flows from lower temperature to higher temperature.
Assertion (A) is true; sensation of warmth indicates heat flow to the body from the object. Reason (R) is false; heat always flows from higher to lower temperature. Thus, (A) is true but (R) is false.
Assertion (A): High thermal conductivity of metals is due to presence of free electrons.
Reason (R): Electrons at same temperature have very high average velocity than atoms.
Assertion (A) is true, free electrons are primary carriers of heat in metals. Reason (R) is true;
Electrons have higher velocity due to smaller mass at same kinetic energy. This high velocity enables efficient heat transfer, explaining (A). Both are true and (R) explains (A).
Assertion (A): A hot iron bar placed under a running tap loses heat by convection.
Reason (R): Convection involves flow of matter within a fluid due to unequal temperature of its parts.
Assertion (A) is true; heat loss from a hot bar to running water is a classic example of forced convection. Reason (R) correctly defines convection as heat transfer by bulk fluid movement due to temperature differences. Thus, both are true and (R) correctly explains (A).
Assertion (A): Ice is placed at top of the bucket to cold the water.
Reason (R): Heat transfer by convection takes place in upward direction.
When ice is placed at the top, the water layer near the ice cools, becomes denser, and sinks. Warmer, less dense water from below rises to the top to be cooled by the ice. This sets up convection currents, efficiently cooling the entire volume. Thus, heat is transferred upwards to the ice by the rising warmer water. Both A and R are true, and R correctly explains A.
Assertion (A): Snow is better insulator than ice.
Reason (R): Snow contain air packet and air is bad conductor of heat.
Snow traps air, and air is a very poor conductor of heat. Therefore, snow acts as a better insulator than solid ice. Both assertion and reason are true, and the reason correctly explains the assertion.
Assertion (A): Snow is better insulator than ice.
Reason (R): Snow contain air packet and air is bad conductor of heat.
Snow traps air, making it a poor conductor of heat. Air is a bad conductor, hence snow acts as a better insulator than solid ice.
Assertion (A): Woolen clothes keep the body warm in winter.
Reason (R): Woolen fibres enclose a large amount of air in them & both air and wool are the bad conductors of heat.
Woolen clothes trap air, which is a poor conductor of heat, reducing heat loss from the body. Wool itself is also a poor conductor. Both A and R are true, and R explains A.
Assertion (A): Salt is mixed with ice in an ice-cream box.
Reason (R): Salt lowers the temperature of ice.
Adding salt to ice creates a freezing mixture by lowering the freezing point of water (freezing point depression), allowing temperatures well below \(0^{\circ}\text{C}\). Both A and R are true, and R explains A.
Assertion (A): On the sea shore, cool breeze flows in the evening.
Reason (R): Convection currents are set up from sea to the land since land cools slower than water.
Land heats and cools faster than water. In the evening, land is cooler than the sea. This causes a breeze from land to sea (land breeze), not sea to land. Thus, A is true but R is false.
Assertion (A): Two thin blankets put together are less warmer than a single blanket of double the thickness.
Reason (R): Thickness increases because of vacuum layer enclosed between the two blankets.
Two thin blankets are warmer than one thick one due to trapped air layers acting as insulation. No vacuum forms. Both assertion and reason are incorrect.