CBSE Class 7 Science Chapter 4: Heat NCERT Solutions

Solution: Both clinical and laboratory thermometers are instruments used to measure temperature and work on the same fundamental principle of thermal expansion of mercury. They both consist of a glass tube with a fine capillary bore, enclosing mercury, and are marked with temperature scales (Celsius and Fahrenheit).

Similarities:

• Both thermometers operate based on the expansion and contraction of mercury with temperature changes.
• Both are used for measuring temperature.
• Both have a thick-walled glass tube containing a fine, uniform bore capillary tube.
• Mercury is used as the thermometric substance in both.
• Both typically display temperature readings in both Celsius and Fahrenheit scales.

Differences:

• Usage: A clinical thermometer is specifically designed to measure human body temperature, while a laboratory thermometer is used to measure the temperature of various substances in laboratories and industries.
• Temperature Range: A laboratory thermometer generally has a wider range, typically from -10°C to 110°C, to accommodate various experimental conditions. A clinical thermometer has a much narrower range, usually from 35°C to 42°C, as this covers the normal range of human body temperature and slight variations.
• Constriction: A clinical thermometer has a small constriction (kink) near the mercury bulb. This constriction prevents the mercury level from falling back into the bulb after removal from the body, allowing the temperature to be read easily. Laboratory thermometers usually do not have this constriction.
• Handling: A clinical thermometer can be tilted to read the temperature easily due to the constriction. A laboratory thermometer must be kept upright to ensure an accurate reading.

Solution: Materials can be classified based on their ability to conduct heat. Conductors allow heat to pass through them easily, while insulators resist the flow of heat.

Examples of conductors of heat:

• Copper
• Aluminum
• Iron
• Steel

Examples of insulators of heat:

• Wood
• Water
• Air
• Plastic
• Glass

Solution:

1. The hotness of an object is determined by its temperature.
2. Temperature of boiling water cannot be measured by a clinical thermometer, as its range is too limited for such high temperatures.
3. Temperature is measured in degree Celsius (or Fahrenheit or Kelvin).
4. No medium is required for transfer of heat by the process of radiation. This is how heat from the sun reaches Earth.
5. A cold steel spoon is dipped in a cup of hot milk. It transfers heat to its other end by the process of conduction. This is because metals are good conductors of heat.
6. Clothes of dark colours absorb heat better than clothes of light colours. This is why we prefer dark clothes in winter.

Solution: The matching is as follows:

1. Land breeze blows during (d) night
2. Sea breeze blows during (c) day
3. Dark coloured clothes are preferred during (b) winter
4. Light coloured clothes are preferred during (a) summer

Explanation:

• Land breeze blows from land to sea at night because land cools faster than the sea.
• Sea breeze blows from sea to land during the day because the land heats up faster than the sea, creating an upward air current over land and drawing cooler air from the sea.
• Dark coloured clothes are preferred in winter because they absorb more heat from the surroundings, keeping us warm.
• Light coloured clothes are preferred in summer because they reflect most of the heat that falls on them, helping to keep us cool.

Solution: Wearing multiple layers of clothing during winter helps us stay warmer because of the insulating properties of trapped air. When you wear several thin layers, pockets of air get trapped between each layer and also between the clothing and your body. Air is a poor conductor of heat, meaning it does not transfer heat easily. These trapped air pockets act as an effective barrier, preventing your body heat from escaping into the colder surroundings. A single thick piece of clothing might not trap as much air effectively, making it less efficient at insulation compared to multiple thinner layers.