CBSE Class 7 Science Advanced Physics Heat Short Answer Questions Set 02

Short Answer Questions

Question 1. A graph is drawn by taking the rise in temperature on Y-axis and heat supplied on X-axis. Find out the melting point and boiling point of that substance.

Answer:
The horizontal portions of the graph show change of state.
Lower flat part: melting point = 50 °C.
Upper flat part: boiling point = 80 °C.
In simple words: Read the temperature at the two flat parts.

Teacher's Note:
1. Flat means change of state.
2. Lower is melting, upper is boiling.

Question 2. A physics student performed an experiment by taking a beaker with 1 ℓ of water at 30 °C and dropping an iron sphere with temperature 90 °C in it. After sometime, student measured their equilibrium temperature as 48 °C. If the density of iron sphere is 7870 kg m⁻³, then find the volume of the iron sphere. Specific heat of iron is 0.110 cal g⁻¹ °C⁻¹.
Answer:
Mass of 1 ℓ of water = 1 kg = 1000 g; Δt of water = 48 − 30 = 18 °C; Δt of iron = 90 − 48 = 42 °C.
Heat gained by water = heat lost by iron: 1000 × 1 × 18 = m × 0.110 × 42
m = 18000 ÷ 4.62 = 3896 g = 3.896 kg.
Volume = mass ÷ density = 3.896 ÷ 7870 = 4.95 × 10⁻⁴ m³ (about 495 cm³).
In simple words: Use heat lost = heat gained to find the mass of iron, then divide by density.

Teacher's Note:
1. Convert litre to kg of water.
2. Check the units in density.

Question 3. A science student in a science fair demonstrated the existence of water and ice in same container at 0 °C and water and steam in a same closed container at 100 °C. Explain how is it possible to have two different states of matter at the same temperature.
Answer:
When ice is heated, its temperature rises to 0 °C and then stays at 0 °C while the ice melts; the heat supplied is used only to change the state. So ice and water can exist together at 0 °C.
In the same way water at 100 °C stays at 100 °C while it turns to steam, so water and steam can exist together at 100 °C.
In simple words: During melting or boiling the heat changes the state and not the temperature.

Teacher's Note:
1. Two states coexist while the change of state goes on.
2. Temperature stays constant.

Question 4. Ramu took four identical iron balls of temperatures 6 °C, 10 °C, 20 °C and 32 °C. He kept them in physical (thermal) contact with one another. After some time, he measured the equilibrium temperature attained by these four balls. What is the value of the equilibrium temperature?
Answer:
The balls are identical, so they have equal mass and the same specific heat.
The equilibrium temperature is the average: t = (6 + 10 + 20 + 32) ÷ 4 = 68 ÷ 4 = 17 °C.
In simple words: For identical bodies the final temperature is the average of the starting temperatures.

Teacher's Note:
1. Sum then divide by four.
2. It lies between 6 and 32.

Question 5. You have been given with a cup of coffee which is too hot to drink. Which among the following is the best method to cool it? Why?
(1) Placing it in a beaker containing water.
(2) Placing it on the surface of wood.
(3) Placing it on the surface of an iron block.
Answer:
The best method is (1), placing it in a beaker containing water.
Water touches the cup on its whole outer surface, and heat flows out through a large area. On wood (a bad conductor) heat flows slowly, and on an iron block only the bottom touches the metal, so cooling is less.
In simple words: More contact area with water means heat leaves faster.

Teacher's Note:
1. Compare contact area and conductivity.
2. Wood is an insulator.

Question 6. Rajesh took two identical glasses containing the same amount of hot milk, at the same temperature. He kept one glass undisturbed. While the other one, he continuously stirred using a spoon. Then, among them, milk in which glass cools faster? Why?
Answer:
The milk that is stirred cools faster.
Stirring makes the hot milk from the bottom come to the top and the cooler milk from the top go down, so heat is lost to the surroundings faster. Stirring increases the rate of heat transfer by convection.
In simple words: Stirring mixes the milk so heat escapes faster.

Teacher's Note:
1. Stirring adds forced convection.
2. Hot milk at the surface loses heat to the air.

Question 7. A physics student took a tumbler full of dry ice with a thermometer placed in it. He started supplying heat energy at constant rate by placing it on a heating device. The student observed no change in the thermometer reading even after 90 % of ice melted. Explain what happened to the heat energy supplied.
Answer:
While ice melts, its temperature stays constant.
The heat supplied is used to separate the molecules (increase the spacing between them) and to change the state from solid to liquid.
It is not used to raise the temperature, so the thermometer reading does not change.
In simple words: The heat goes into melting the ice, not into raising the temperature.

Teacher's Note:
1. The same happens during boiling.
2. This heat is hidden in the change of state.

Question 8. Water at 10 °C is in liquid form but iron at 10 °C is in solid form. What is the reason?
Answer:
The melting point of water is 0 °C, so water above 0 °C is a liquid, while the melting point of iron is 1535 °C, so at 10 °C it is far below its melting point and remains solid.
In iron the force of attraction between the particles is much greater than in water, so far more heat is needed to melt it.
In simple words: Iron needs a very high temperature to melt, while ice melts at 0 °C.

Teacher's Note:
1. Compare 10 °C with the melting points.
2. Table of melting points.

Question 9. Ram took equal masses of cooking oil and water, respectively, in two different identical beakers and placed them on identical heat engines and supplied same amount of heat to them. With the help of thermometer, he noted their temperatures. Which one of them would be hotter. Why?
Answer:
The cooking oil would be hotter.
Oil has a smaller specific heat capacity than water (water: 1 cal g⁻¹ °C⁻¹). For the same mass and the same heat, Δt = Q ÷ (m s), so the substance with the smaller s shows the larger rise in temperature.
In simple words: Oil warms up more than water for the same heat.

Teacher's Note:
1. Low specific heat means quick temperature rise.
2. Water is slow to heat up.

Question 10. Akbar observed that the things placed inside the refrigerator become cold. He wanted to know how does the heat flow from the inside of the refrigerator to the outside of the refrigerator. Explain.
Answer:
A refrigerant (a special liquid) circulates in the refrigerator.
Inside, it evaporates quickly and takes in heat from the things stored, which makes them cold.
The vapour is carried by copper tubes to the back of the refrigerator, where it is compressed, turns into liquid and gives out the heat to the surroundings.
In simple words: The refrigerant carries heat from the inside to the outside.

Teacher's Note:
1. Evaporation takes heat.
2. Condensation gives heat out.

Question 11. If a uniform metal plate with a hole in it is heated, then explain how the size of the hole gets affected?
Answer:
The size of the hole increases.
On heating, the distance between any two neighbouring particles increases, so every part of the plate expands as if the hole were also made of the same metal; the hole becomes larger.
In simple words: Heating enlarges the whole plate, including the hole.

Teacher's Note:
1. A hole expands like the metal around it.
2. Cooling makes it smaller.

Question 12. Raju was trying very hard to unscrew the metal cap of a bottle. Then he heated the cap slightly and was able to unscrew it easily. Explain.
Answer:
On heating, the metal cap expands more than the glass neck of the bottle, so it becomes loose and can be unscrewed easily.
In simple words: The hot cap becomes slightly bigger and loosens.

Teacher's Note:
1. Metal expands more than glass.
2. Do not heat the bottle itself.

Question 13. Why does a black smith heat the iron block before hammering it to change its shape?
Answer:
On heating, the particles of iron move apart and the force of attraction between them decreases, so the iron becomes soft and its shape is easily changed by hammering.
In simple words: Hot iron is soft and can be shaped.

Teacher's Note:
1. Cold iron is hard.
2. Heating reduces the force between particles.

Question 14. If we touch a piece of steel and wood on a winter day, we feel that the steel is colder than wood. If we touch the steel and the wood on a summer day, we feel that steel is hotter than wood. Why?
Answer:
Steel is a good conductor and wood is a bad conductor.
In winter, heat flows quickly from our hand into the steel, so it feels colder; heat flows slowly into wood, so it feels less cold.
In summer, steel is hotter than our body and heat flows quickly from the steel to our hand, so it feels hotter than wood.
In simple words: Steel moves heat to or from our hand faster than wood does.

Teacher's Note:
1. It is the rate of heat flow we sense.
2. Both are at the same temperature.