Exercises
SECTION I
A. Choose the correct option.
1.A cup of hot milk is placed on a table. After some time, the temperature of the milk becomes the same as the surrounding temperature. What is this state called?
a. Melting point
b. Liquefaction point
c. Normal heat
d. Thermal equilibrium
Answer: d. Thermal equilibrium
2.Wet hair drying is an example of which of the following?
a. Boiling
b. Condensation
c. Melting
d. Evaporation
Answer: d. Evaporation
3.When we touch a metal pan which is at a room temperature, we feel cold due to which of the following?
a. Conduction of heat from the hand to the pan
b. Conduction of cold from the pan to the hand.
c. Conduction of cold from the hand to the pan.
d. Conduction of heat from the pan to the hand
Answer: a. Conduction of heat from the hand to the pan
4.Water gets boiled due to which of the following?
a. Conduction
b. Convection
c. Radiation
d. Conduction and convection
Answer: b. Convection
5.Which of the following statements is false?
a. Ventilators work on the principle of conduction.
b. Wood and cork are bad conductors of heat.
c. Metals are good conductors of heat.
d. Land breeze and sea breeze are convection currents in nature.
Answer: a. Ventilators work on the principle of conduction. (Ventilators work on convection, not conduction.)
6. Which of the following statements is correct?
a. Clean snow melts faster than dirty snow as it reflects heat.
b. Clean snow does not melt faster than dirty snow as it reflects heat.
c. Dirty snow does not melt faster than clean snow as it absorbs heat.
d. Dirty snow melts faster than clean snow as it reflects heat.
Answer: b. Clean snow does not melt faster than dirty snow as it reflects heat. (Dirty snow absorbs more heat and melts faster.)
7.Why do shopkeepers cover ice blocks with jute sacks?
a. It is easy to carry ice in a jute sack.
b. Jute sack can transmit heat.
c. Jute sack acts as an insulator.
d. Ice does not get dirty if it is covered with jute sack.
Answer: c. Jute sack acts as an insulator.
8. Vacuum does not allow transmission of heat by which of the following?
a. Conduction
b. Convection
c. Radiation
d. Conduction and convection
Answer: d. Conduction and convection (Radiation can occur through a vacuum.)
B. Assertion and Reasoning questions.
1.Assertion (A): Air is a bad conductor of heat.
Reason (R): Its molecules absorb all the heat.
a. Both A and R are True
b. Both A and R are False
c. A is True and R is False
d. A is False and R is True
Answer: c. A is True and R is False (Air is a bad conductor because its molecules are far apart, not because they absorb all heat.)
2.Assertion (A): The temperature of the water in a polished tea pot increases slower than a dull black tea pot when they are put out in the sun.
Reason (R): The outer polish surface reflects most of the radiation incident on it.
a. Both A and R are True
b. Both A and R are False
c. A is True and R is False
d. A is False and R is True
Answer: a. Both A and R are True (Polished surfaces reflect radiation, reducing heat absorption.)
C. Write T for True and F for False. Correct the false statements.
- Water freezes at 32 °F.
Answer: T - Heat changes the arrangement of particles in a substance when there is a change of state.
Answer: T - Frost is formed due to sublimation.
Answer: F – Frost is formed due to deposition (water vapor changing directly to solid). - Wood and plastic are good conductors of heat, so they are used to make handles of cookware.
Answer: F – Wood and plastic are bad conductors of heat, which is why they are used for handles to prevent heat transfer.
Choose the correct option to fill in the blanks.
- The difference between the freezing point and the boiling point of water in the Kelvin scale is …… (100 equal parts/273 equal parts).
Answer: 100 equal parts - The minimum temperature at which a substance catches fire and starts burning is known as its …… (inflammable temperature/ignition temperature).
Answer: ignition temperature - Freezing involves …… (absorption of energy/release of energy).
Answer: release of energy - Moth balls and camphor are materials that undergo …… (sublimation/vaporization).
Answer: sublimation - Different types of solids and liquids expand by …… (same/different) amounts when given the same amount of heat.
Answer: different
SECTION II
Give reasons for the following.
1.Railway tracks are laid in such a way that there is some space between them.
Answer: To allow for thermal expansion in summer. Without space, the tracks could buckle due to heat.
2.A balloon fixed to the neck of a bottle expands when kept in hot water.
Answer: The air inside the bottle expands when heated, increasing pressure and inflating the balloon.
3. Mud houses remain cool in summer and warm in winter.
Answer: Mud is a bad conductor of heat, so it slows down heat transfer, keeping indoor temperatures stable.
4.A tiled floor feels colder than a wooden floor when you walk on it, even when both are at the same temperature.
Answer: Tiles are better conductors of heat than wood, so they draw heat from feet faster, feeling colder.
- We wear dark-coloured clothes in winter and light-coloured clothes in summer.
Answer: Dark colors absorb more heat radiation, keeping us warm in winter. Light colors reflect heat, keeping us cool in summer.
6.We keep convectors down in a room, while ventilators are kept higher up in the room.
Answer: Hot air rises due to convection, so convectors are placed low to heat rising air, and ventilators are high to release hot air.
Differentiate between the following.
1. Melting and Condensation
| Aspect | Melting | Condensation |
| Definition | Change of state from solid to liquid | Change of state from gas to liquid |
| Heat Change | Absorbs heat (endothermic) | Releases heat (exothermic) |
| Temperature | Occurs at a specific melting point | Occurs when vapor cools below dew point |
| Particle Motion | Particles gain energy and move freely | Particles lose energy and come closer |
| Examples | Ice melting into water | Dew formation from water vapor |
2. Boiling and Evaporation
| Aspect | Boiling | Evaporation |
| Definition | Rapid vaporization at boiling point | Slow vaporization at any temperature |
| Temperature | Occurs at fixed boiling point | Occurs at any temperature below boiling point |
| Location | Occurs throughout the liquid | Occurs only at the surface |
| Bubbles | Involves bubble formation | No bubbles formed |
| Speed | Fast process | Slow process |
3. Good Conductors and Bad Conductors of Heat
| Aspect | Good Conductors | Bad Conductors |
| Heat Transfer | Allow heat to pass easily | Do not allow heat to pass easily |
| Materials | Usually metals (e.g., copper, silver) | Usually non-metals (e.g., wood, plastic) |
| Thermal Conductivity | High thermal conductivity | Low thermal conductivity |
| Electron Movement | Have free electrons that transfer heat | Lack free electrons; heat transfer is slow |
| Uses | Used in cooking utensils, radiators | Used in insulators, handles of cookware |
4. Sea Breeze and Land Breeze
| Aspect | Sea Breeze | Land Breeze |
| Time of Occurrence | During the day | During the night |
| Direction | Blows from sea to land | Blows from land to sea |
| Cause | Land cools faster than the sea; air rises over sea | Land cools faster than the sea; air rises over the sea |
| Temperature | Land heats faster than the sea; air rises over land | Brings cooler air to sea |
| Effect | Brings cooler air to the land | Provides a cooling effect on the sea |
Short answer questions.
1.When you enter a room after being out in the hot sun, you feel cool. But, after being in the cold, if you entered the same room, you would feel warm. What can you infer from this?
Answer: This shows that temperature sensation is relative. The body adapts to external temperatures, so the same room feels different based on prior exposure.
2. How is the temperature of a substance related to the random motion of particles in the substance? What would be the outcome if the random motion were reduced?
Answer: Higher temperature increases the random motion of particles. If random motion reduces, the temperature decreases.
3. How is dew an example of a change of state? Is heat absorbed or given out during this change?
Answer: Dew forms when water vapor condenses onto cool surfaces. Heat is given out during condensation.
4. Liquids do not have to be at the boiling point to change into a gas. Defend this.
Answer: Evaporation can occur at any temperature below the boiling point, as surface molecules gain enough energy to escape as gas.
5. What would you recommend if telephone wires are laid in summer?
Answer: Telephone wires should be laid with slack to allow for contraction in winter, preventing damage from tension.
6 . When water is taken in a test tube and heated at the top of the test tube, the water at the top starts to boil. But the water at the bottom of the test tube is still cold. What do you infer from this?
Answer: Water is a poor conductor of heat, so heat does not reach the bottom quickly, and convection currents are not established.
7. Why are cloudy nights generally warmer than clear nights?
Answer: Clouds trap heat radiated from the Earth, acting as insulation. On clear nights, heat escapes easily, making it colder.
8. How can we use a thermos flask to keep ice from melting and also to keep hot water from becoming cold?
Answer: A thermos flask reduces heat transfer by having a vacuum to prevent conduction and convection, silvered surfaces to reflect radiation, and a cork stopper to reduce conduction. This keeps ice from melting by blocking heat inflow and keeps hot water hot by blocking heat outflow.
Long answer questions.
1.The temperature of a solution is 300 K. What would be its temperature in degree Celsius?
Answer: T(°C) = T(K) – 273 = 300 – 273 = 27°C
2. a. The melting point of iron is 1500 °C. What do you understand by this statement? What will be its melting point in K?
Answer: It means iron changes from solid to liquid at 1500°C. In Kelvin, it is 1500 + 273 = 1773 K.
b. The boiling point of alcohol is 78 °C. What happens to the alcohol if we keep heating even after reaching 78°C?
Answer: At 78°C, alcohol boils and turns to vapor. Further heating keeps the temperature constant until all liquid vaporizes, after which vapor temperature rises.
3.In a mercury or an alcohol thermometer, the level of the liquid rises when its bulb comes in contact with a hot object. What is the reason for this rise in the level of mercury or alcohol?
Answer: The liquid expands due to heat, increasing in volume, so it rises in the narrow capillary tube.
4.What kind of metals should be used in temperature-sensitive devices such as watches and measuring tapes?
Answer: Metals with low thermal expansion coefficients should be used to minimize size changes with temperature, ensuring accuracy.
5.White shining bodies are good reflectors of radiant energy. Give two examples.
Answer: Polished silver surfaces and white painted surfaces.
6.Compare conduction, convection, and radiation of heat in terms of the following: (a) how the transfer of heat takes place in matter, (b) speed of the process, and (c) the direction in which it takes place.
Answer:
(a) Conduction: Heat transfer through direct particle contact. Convection: Heat transfer by fluid movement. Radiation: Heat transfer via electromagnetic waves.
(b) Speed: Radiation is fastest (light speed), convection is moderate, conduction is slowest in solids.
(c) Direction: All transfer heat from hotter to colder regions, but radiation can travel in all directions.
7.Explain how heat transfer is reduced in a thermos flask using the following: (a) the cork stopper, (b) vacuum between the inside and outer wall of the flask, and (c) silver coating on the inner side of the outer wall and the outer side of the inner wall of thermos flasks.
Answer:
(a) Cork stopper reduces conduction as it is a bad conductor.
(b) Vacuum reduces conduction and convection by eliminating a medium.
(c) Silver coating reflects radiation, reducing heat loss.
8.a. Thin strips of the same size of copper, aluminium, and brass with wax at one end of each strip are taken in a cup of hot water. Wax from the copper melts first, then aluminium and then brass. What can you infer from this?
Answer: Copper is the best conductor, followed by aluminium, then brass.
b. If we take wooden strip and plastic strip, along with the metal strips, the wax from wood and plastic fall off last. What do you infer from this?
Answer: Wood and plastic are poor conductors (insulators) compared to metals.