CBSE Class 7 Science Advanced Physics Heat Very Short Answer Questions Set 01

Very Short Answer Questions

Question 1. Fill in the blank.
The capacity to do work is called ________.
Answer:
energy
In simple words: Anything that can make work happen has energy.

Teacher's Note:
1. Heat is also a form of energy.
2. The SI unit of energy is the joule.

Question 2. Fill in the blank.
If heat energy is given to a substance and its temperature remains constant, then the substance undergoes a change in ________.
Answer:
state. During melting or boiling the heat is used to change the state, so the temperature stays constant.
In simple words: The heat goes into changing solid to liquid or liquid to gas, not into raising the temperature.

Teacher's Note:
1. Look at the flat parts of a heating curve.
2. Ice at 0 °C stays at 0 °C while it melts.

Question 3. Fill in the blank.
The temperature at which a liquid converts to gaseous state is called ________ of a liquid.
Answer:
boiling point
In simple words: The boiling point is the fixed temperature at which a liquid boils.

Teacher's Note:
1. For water it is 100 °C.
2. Do not confuse it with the melting point.

Question 4. Fill in the blank.
________ is the SI unit of heat.
Answer:
joule (J)
In simple words: Heat is energy, so it is measured in joule.

Teacher's Note:
1. 1 cal = 4.2 J.
2. The CGS-type unit is the calorie.

Question 5. Fill in the blank.
The transfer of heat which takes place due to the vibration of particles is ________.
Answer:
conduction
In simple words: In conduction the particles vibrate and pass heat to their neighbours.

Teacher's Note:
1. The printed answer key swaps the answers of Questions 5 and 6; the correct answer here is conduction.
2. Conduction mainly happens in solids.

Question 6. Fill in the blank.
A black body absorbs ________ heat energy than a white body.
Answer:
more
In simple words: Black surfaces take in more heat; white surfaces reflect it.

Teacher's Note:
1. The printed answer key swaps the answers of Questions 5 and 6; the correct answer here is more.
2. This is why dark clothes are worn in winter.

Question 7. Fill in the blank.
Substances through which heat travels easily and quickly are said to be ________ of heat.
Answer:
good conductors
In simple words: Metals such as copper and silver let heat pass quickly.

Teacher's Note:
1. Substances that do not let heat pass are bad conductors or insulators.
2. Cooking vessels are made of good conductors.

Question 8. Fill in the blank.
The process of changing a substance from gaseous state to liquid state is called ________.
Answer:
condensation
In simple words: Steam turning into water drops is condensation.

Teacher's Note:
1. Heat is released during condensation.
2. It is the reverse of evaporation or boiling.

Question 9. Fill in the blank.
Heat energy produced by 150 g of a fuel of calorific value 8000 cal g⁻¹ is ________.
Answer:
12 × 10⁵ cal. Heat produced Q = calorific value × mass = 8000 × 150 = 12,00,000 cal = 12 × 10⁵ cal.
In simple words: Multiply the heat given by 1 g by the number of grams burnt.

Teacher's Note:
1. Q = C × M.
2. 12,00,000 cal is the same as 1200 kcal.

Question 10. Fill in the blank.
When hot water is poured in ordinary glass vessel, it breaks because of ________.
Answer:
uneven expansion. The inner surface of the glass gets hot and expands while the outer surface is still cold, and the uneven expansion cracks the glass.
In simple words: The inside swells before the outside, so the glass cracks.

Teacher's Note:
1. Glass is a bad conductor, so heat does not spread quickly.
2. Pyrex or Borosil glass expands very little and does not break.

Teacher's Note:
1. Check the units first: g⁻¹ alone is calorific value.
2. At 4 °C density is maximum and volume is minimum.

Teacher's Note:
1. Match the definitions you know first.
2. The freezing mixture has salt and ice in the ratio 1 : 3.

Teacher's Note:
1. Aluminium (b) fits Conductor, so use it once.
2. Dark clothes absorb heat and light clothes reflect it.

Question 14. State whether the following statement is true or false.
Heat energy is invisible.
Answer:
True. We cannot see heat; we only feel its effects, such as hotness or coldness.
In simple words: We feel heat but cannot see it.

Teacher's Note:
1. Heat is a form of energy, not a material you can see.
2. Its effects (expansion, melting) can be observed.

Question 15. State whether the following statement is true or false.
For a substance to undergo a change of state, heat must be either given to it or taken away from it.
Answer:
True. Melting and boiling need heat to be given; freezing and condensation need heat to be taken away.
In simple words: No heat given or taken, no change of state.

Teacher's Note:
1. Heat absorbed: solid to liquid to gas.
2. Heat released: gas to liquid to solid.

Question 16. State whether the following statement is true or false.
The heat required to raise the temperature of 1 kg of water by 1 °C is called one calorie.
Answer:
False. The heat needed to raise 1 kg of water by 1 °C is one kilocalorie. One calorie is for 1 g of water.
In simple words: One calorie warms 1 g; one kilocalorie warms 1 kg.

Teacher's Note:
1. 1 kcal = 1000 cal.
2. 1 cal = 4.2 J.

Question 17. State whether the following statement is true or false.
The heat absorbed by a substance decreases with increase in temperature.
Answer:
False. The heat absorbed increases with the rise in temperature (Q = m s Δt). A larger rise in temperature needs more heat.
In simple words: More heat is needed for a bigger rise in temperature.

Teacher's Note:
1. Q is directly proportional to Δt.
2. Doubling Δt doubles Q.

Question 18. State whether the following statement is true or false.
The temperature of boiling water can be measured by a clinical thermometer.
Answer:
False. A clinical thermometer reads only from 35 °C to 43 °C, so it cannot read 100 °C; the mercury would also break the thermometer.
In simple words: Its scale stops far below 100 °C.

Teacher's Note:
1. Use a laboratory thermometer for boiling water.
2. Clinical thermometers are only for body temperature.

Question 19. State whether the following statement is true or false.
Heat lost by radiation depends upon whether the outer surface of the body is black or polished.
Answer:
True. A black surface radiates heat well, while a polished (shiny) surface radiates little.
In simple words: Dull black surfaces lose heat faster than shiny ones.

Teacher's Note:
1. That is why a thermos flask is shiny.
2. Black is a good absorber and a good radiator.