A. Multiple Choice Questions
1. In Fahrenheit scale, the distance between the lower and upper fixed points is divided in how many equal parts?
o Answer: (c) 180
2. The temperature at which a solid changes into a liquid is called:
o Answer: (b) melting point
3. The property of matter that increases its size on heating is called:
o Answer: (c) thermal expansion
4. Boiling point of water at normal atmospheric pressure is:
o Answer: (b) 100 C
5. Which of the following is an insulator?
o Answer: (c) air
6. The only liquid, in which thermal conduction is possible, is:
o Answer: (a) mercury
7. Convection is possible in:
o Answer: (d) both liquids and gases
8. The fastest method of heat transfer from a hot body to a cold body is:
o Answer: (c) radiation
B. Assertion-Reason Questions
1. Question:
o Assertion [A]: Heat is a form of energy which makes a body hot.
o Reason [R]: When we rub our palms, heat energy is generated.
o Answer: (a) Both Assertion [A] and Reason [R] are true.
2. Question:
o Assertion [A]: When a body possesses heat energy, it tends to become heat.
o Reason [R]: When some heat is lost by a body, it becomes hotter than before.
o Answer: (b) Both Assertion [A] and Reason [R] are false.
C. Fill in the Blanks
1. Solids ______ on heating and ______ on cooling.
o Answer: Solids expand on heating and contract on cooling.
2. ______ on Fahrenheit scale, lower fixed point is ______ F and upper fixed point is ______ F
o Answer: On Fahrenheit scale, lower fixed point is 32Fand upper fixed point is 212F.
3. ______ is a shining liquid, hence, its reading can be easily noted in a thermometer.
o Answer: Mercury
4. A newly laid quilt is ______ than an old one because it contains ______.
o Answer: A newly laid quilt is warmer than an old one because it contains trapped air.
5. Sea and land breezes are examples of ______ of heat.
o Answer: convection
6. Air is a ______ conductor of heat.
o Answer: bad / poor
7. Heat can travel even through ______ in radiation.
o Answer: vacuum
8. Good absorbers of heat are ______ radiators but ______ reflectors.
o Answer: Good absorbers of heat are good radiators but poor reflectors.
9. Mud is a ______ conductor of heat.
o Answer: bad / poor
D. State Whether True or False (With Corrections)
1. The degree of hotness of a body cannot be estimated correctly by touching and feeling.
o Answer: True
2. Body temperature of a healthy person is 98.4C
o Answer: False. Normal human body temperature is 98.4F (or 37C).
3. Heat energy can cause change of state for a given substance.
o Answer: True
4. Heat coming from the sun travels to us by convection.
o Answer: False. Heat from the sun travels to us by radiation.
5. Woollen clothes are helpful in keeping us warm because they have air enclosed in them.
o Answer: True
6. Metals are poor conductors of heat.
o Answer: False. Metals are good conductors of heat.
7. The bottom parts of cooking utensils are highly polished.
o Answer: False. The bottom parts of cooking utensils are dull or blackened to absorb more heat.
8. Black colour absorbs maximum amount of heat radiation falling on it.
o Answer: True
9. Thermal radiation can travel through vacuum.
o Answer: True
E. Define These Terms
1. Define Heat.
o Answer: Heat is a form of energy that flows from an object at higher temperature to one at lower temperature, producing the sensation of warmth or cold.
2. Define Thermal expansion.
o Answer: The increase in the dimensions (length, area, or volume) of a substance when it is heated.
3. Define Conduction.
o Answer: The mode of heat transfer from the hotter end to the colder end of a body from particle to neighbouring particle without any bodily movement of the particles themselves.
4. Define Convection.
o Answer: The mode of heat transfer in fluids (liquids and gases) where heat is carried by the actual movement of the fluid molecules.
F. Give One Word for the Following
1. Degree of hotness of a body.
o Answer: Temperature
2. A device used to measure the temperature.
o Answer: Thermometer
3. The common scale of temperature used in a laboratory.
o Answer: Celsius scale
4. A substance which does not allow heat to flow through itself.
o Answer: Insulator (or bad conductor)
5. The mode of transfer of heat from one particle to another in solids.
o Answer: Conduction
6. The mode of heat transfer/flow even through vacuum.
o Answer: Radiation
G. Differentiate Between the Following
1. Differentiate between Heat and Temperature.
o Answer:
§ Heat: A form of energy representing total internal kinetic energy; measured in Joules J or calories.
§ Temperature: A thermal condition measuring the degree of hotness or coldness; measured in C , F or K
2. Differentiate between Conductors and Insulators.
o Answer:
§ Conductors: Substances that allow heat to pass through them easily (e.g., metals like copper and aluminium).
§ Insulators: Substances that resist the passage of heat (e.g., wood, plastic, glass, air).
3. Differentiate between Conduction and Convection.
o Answer:
§ Conduction: Heat transfers without the physical displacement of particles; occurs primarily in solids.
§ Convection: Heat transfers via the physical bulk movement of fluid particles; occurs only in liquids and gases.
4. Differentiate between Thermal Absorber and Reflector.
o Answer:
§ Thermal Absorber: A surface (such as a dull black surface) that captures incident thermal radiation.
§ Thermal Reflector: A surface (such as a smooth, shiny silver/white surface) that bounces incident thermal radiation away.
H. Match the Columns
1. Good thermal conductor (c) Copper
2. Bad conductor of heat (e) Rubber
3. Heat radiation source (a) Fire
4. Heat flow through vacuum (b) Radiation
5. Convection current (f) Ventilation
6. Black body (d) Good absorber
I. Encircle the Odd One Out with Reasons
1. Copper, brass, mercury, rubber and aluminium
o Odd one out: Rubber
o Reason: Rubber is an insulator, whereas copper, brass, mercury, and aluminium are good thermal conductors (metals).
2. Wood, plastic, platinum, glass, paper and water
o Odd one out: Platinum
o Reason: Platinum is a metallic conductor of heat, while all others are thermal insulators.
3. Freezing, melting, boiling, thermal expansion and condensation
o Odd one out: Thermal expansion
o Reason: Thermal expansion refers to a change in volume/dimensions without a phase change, whereas all the others are changes of state.
J. Give Reasons for the Following
1. Why is mercury used in thermometers?
o Answer: Mercury does not stick to glass, is an opaque shining liquid easily visible in thin capillaries, expands uniformly, has a high boiling point 357C and low freezing point -39 and is a good conductor of heat.
2. Why are cooking utensils provided with insulating handles?
o Answer: Handles made of ebonite, Bakelite, or wood do not conduct heat, allowing people to hold hot utensils without burning their hands.
3. Why do we wear woollen clothes in winter?
o Answer: Wool fibres trap pockets of air between them. Because air is a poor conductor of heat, it prevents the escape of body heat into the cold surroundings.
4. Why are two thin blankets taken together warmer than a single thick blanket?
o Answer: A thin layer of air gets trapped between the two blankets. Air acts as an insulator and reduces heat loss significantly.
5. Why does climate remain moderate in coastal areas?
o Answer: Land and sea breezes set up continuous convection currents between the land and the ocean due to differences in their specific heat capacities, moderating the coastal temperature.
6. Why are air conditioners in offices generally fitted at higher positions on walls?
o Answer: Cold air blown from an AC is denser and falls downward, while lighter warm air rises to be cooled, creating continuous convection currents that cool the entire room uniformly.
7. Why are radiators of automobiles painted black?
o Answer: Black surfaces are the most effective radiators of heat, enabling rapid dissipation of excess engine heat.
8. Why are ice slabs covered with gunny bags?
o Answer: Jute gunny bags contain air trapped within their pores. Air is an insulator, shielding the ice from ambient heat and drastically reducing its rate of melting.
K. Answer These Questions
1. State any three effects of heat.
o Answer: (i) Rise in temperature, (ii) Expansion of matter (thermal expansion), and (iii) Change in state of matter.
2. Describe the construction of a mercury thermometer.
o Answer: It consists of a thick-walled capillary glass tube with a uniform narrow bore sealed at one end and terminating in a thin-walled bulb at the other. The bulb is filled with purified mercury, and the stem is calibrated with a temperature scale.
3. How is a laboratory thermometer graduated?
o Answer: By determining two fixed points: the lower fixed point 0 C , the melting point of pure ice) and the upper fixed point 100 C , the boiling point of pure water under standard atmospheric pressure). The length between them is divided into 100 equal intervals, each marking 1 C
4. What is thermal expansion? Give one example.
o Answer: The increase in the dimensions of a substance caused by heating. Example: Small expansion gaps left between sections of railway tracks to accommodate metal expansion during hot summers.
5. What are good conductors of heat? Give examples.
o Answer: Materials that facilitate quick and easy transfer of heat energy through conduction. Examples: Silver, copper, and aluminium.
6. State three applications of heat conductors.
o Answer: (i) Copper or aluminium bases for cooking pans, (ii) Radiator fins in automobile engines, (iii) Metal heating coils and bases in electric irons.
7. Show that water is a bad conductor of heat.
o Answer: Wrap an ice cube in wire gauze and sink it to the bottom of a test tube filled with water. Direct a flame at the upper layer of the water; the water near the top will boil while the ice at the bottom remains frozen for a prolonged period.
8. Describe three important applications of thermal insulators from your daily life.
o Answer: (i) Heat-resistant handles on cookware, (ii) False ceilings made of plaster or fiberboard to insulate rooms from exterior heat/cold, (iii) Woollen garments used to maintain body heat in winter.
9. Give an experiment to show formation of convection currents in liquids.
o Answer: Fill a round-bottom flask with water and drop a potassium permanganate crystal to the bottom. Apply gentle heat directly below the crystal. Colored streams of warm water are seen ascending through the center, spreading out at the top, and cooler water streams sink down along the walls.
10. Give any two applications of thermal convection.
o Answer: (i) Ventilation in rooms (stale warm air escapes through high ventilators while cool fresh air enters through lower windows), (ii) Placement of space heaters on floors to circulate heated air upwards.
11. Give an experiment to show that black surface is a better radiator of heat.
o Answer: Take two identical tin cans—one painted dull black and the other shiny white. Fill both with equal quantities of boiling water at the same temperature and insert thermometers. The temperature of the water in the black can drops significantly faster than that in the white can over the same period.
12. How does a thermos flask minimise heat transfer?
o Answer:
§ Conduction and convection are minimized by the vacuum between the double glass walls and by the insulating stopper.
§ Radiation is minimized by the silvered surfaces of the walls, which reflect radiant heat back.
60/5 = (F−32)/9 → 12 = (F−32)/9 → F − 32 = 108 → F = 140 °F
2. Boiling point of alcohol, 78 °C → Kelvin
78/5 = (K−273)/5 → K − 273 = 78 → K = 351 K
3. Temperature same on both Celsius and Fahrenheit scales
Set C = F = x:
x/5 = (x−32)/9 → 9x = 5x − 160 → 4x = −160 → x = −40° (i.e., −40 °C = −40 °F)
4. Patient's temperature 100.4 °F → Celsius
(C)/5 = (100.4−32)/9 = 68.4/9 = 7.6 → C = 5 × 7.6 → C = 38 °C
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