Thermal Properties of Matter

15 NEET-UG practice questions on Thermal Properties of Matter, part of Physics. The first 15 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
In a room at 20 °C, a body cools from 70 °C to 50 °C in 5 minutes. Assuming Newton's law of cooling, what is its temperature after a further 5 minutes?
1. A40 °C
2. B35 °C
3. C38 °C
4. D30 °C
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Answer: C · 38 °C
Using average temperatures: 20/5 = k(60 − 20), so k = 0.1 per minute. Then (50 − T)/5 = 0.1[(50 + T)/2 − 20], which gives T = 38 °C. Check: the excess over the room falls by the same ratio in equal times, 50 → 30 → 18, and 20 + 18 = 38 °C.
- A: This assumes the drop simply halves in the second interval.
- B: This uses the rate at 50 °C throughout, not the mean temperature.
- D: This assumes the same 20 °C drop in each interval.
Remember this
Under Newton's law, the excess temperature falls by equal ratios, not equal amounts, in equal times.
Question 2 · Easy
An isotropic solid has a coefficient of linear expansion α. What are its coefficients of area expansion and volume expansion, in that order?
1. A2α and 3α
2. B3α and 2α
3. Cα/2 and α/3
4. Dα² and α³
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Answer: A · 2α and 3α
For small temperature changes, an area grows through two lengths and a volume through three, so β = 2α and γ = 3α.
- B: This interchanges the area and volume coefficients.
- C: This divides by the dimension number instead of multiplying.
- D: This raises α to a power; small changes add instead.
Remember this
α : β : γ = 1 : 2 : 3 for an isotropic solid.
Question 3 · Medium
In a calorimetry experiment, 10 g of ice at 0 °C is added to 100 g of water at 30 °C in an insulated container of negligible heat capacity. Taking the latent heat of fusion of ice as 80 cal/g and the specific heat of water as 1 cal/(g °C), what is the final temperature?
1. A0 °C
2. B27.3 °C
3. C22 °C
4. D20 °C
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Answer: D · 20 °C
Heat lost by the water = heat to melt the ice + heat to warm the melted ice: 100(30 − T) = 10 × 80 + 10T. So 3000 − 800 = 110T and T = 20 °C. Check: the water cools by 10 °C and gives up 1000 cal, of which 800 cal melts the ice and 200 cal warms the 10 g by 20 °C.
- A: This assumes not all the ice melts, although enough heat is available.
- B: This ignores the latent heat of melting.
- C: This forgets to warm the melted ice to the final temperature.
Remember this
Melted ice is water at 0 °C, which must still warm up to the final temperature.
Question 4 · Medium
A 2 kg solid sample is heated steadily, and its temperature is plotted against heat supplied. What is the latent heat of fusion of the material?
Figure: MedExams
1. A20 kJ/kg
2. B120 kJ/kg
3. C200 kJ/kg
4. D100 kJ/kg
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Answer: D · 100 kJ/kg
Melting is the flat part, from 40 kJ to 240 kJ, so 200 kJ melts 2 kg. Therefore L = 200/2 = 100 kJ/kg.
- A: Uses the heat of the sloping solid part, 40 kJ.
- B: Reads the end of the plateau, 240 kJ, instead of its length.
- C: Leaves out division by the mass.
Remember this
Latent heat is the length of the plateau on the heat axis divided by the mass.
Question 5 · Medium
Two slabs of equal thickness L and equal area are joined face to face: conductivity k from x = 0 to L and 2k from x = L to 2L. In steady state the outer faces stay at 100 °C (x = 0) and 40 °C (x = 2L). Which graph shows temperature against x?
Figure: MedExams
1. AGraph A
2. BGraph D
3. CGraph C
4. DGraph B
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Answer: C · Graph C
Equal heat current gives k(100 − T)/L = 2k(T − 40)/L, so T = 60 °C at the joint. Temperature is linear in each slab, with a 40 °C drop in the first and a 20 °C drop in the second, as in Graph C.
- A: Graph A assumes equal conductivities, with 70 °C at the joint.
- B: Graph D is curved, but each uniform slab has a straight profile.
- D: Graph B gives the larger drop to the better conductor.
Remember this
In series, heat current is common, so the poorer conductor takes the larger temperature drop.
Question 6 · Medium
Two slabs of equal thickness and equal area are placed face to face. Slab P has thermal conductivity K and slab Q has 3K. The free face of P is kept at 100 °C and the free face of Q at 0 °C. In the steady state, what is the temperature of the interface?
1. A50 °C
2. B33.3 °C
3. C75 °C
4. D25 °C
Answer and explanation in LearnFree account
Question 7 · Easy
Assertion A: Ice floats on water. Reason R: Water contracts when it freezes. Choose the correct answer.
1. AA is not correct but R is correct
2. BA is correct but R is not correct
3. CBoth A and R are correct and R is the correct explanation of A
4. DBoth A and R are correct but R is not the correct explanation of A
Answer and explanation in LearnFree account
Question 8 · Medium
A 0.2 kg piece of metal at 150 °C is dropped into 0.5 kg of water at 20 °C, and the final common temperature is 25 °C. Neglecting heat taken by the container and lost to the surroundings, what is the specific heat of the metal? (Specific heat of water = 4200 J kg⁻¹ K⁻¹)
1. A2100 J kg⁻¹ K⁻¹
2. B4200 J kg⁻¹ K⁻¹
3. C420 J kg⁻¹ K⁻¹
4. D67.2 J kg⁻¹ K⁻¹
Answer and explanation in LearnFree account
Question 9 · Hard
Two rods of equal length and equal cross-section have thermal conductivities K and 3K. They are first joined end to end (in series) and then placed side by side (in parallel); each time, the same temperature difference is kept between the two ends of the combination. What is the ratio of the heat current in the parallel arrangement to that in the series arrangement?
1. A3 : 16
2. B4 : 3
3. C16 : 3
4. D8 : 3
Answer and explanation in LearnFree account
Question 10 · Easy
As water is heated from 0 °C to 10 °C, its density
1. Adecreases steadily
2. Bincreases steadily
3. Cfirst decreases, reaching a minimum at 4 °C, and then increases
4. Dfirst increases, reaching a maximum at 4 °C, and then decreases
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Question 11 · Medium
A cup of liquid cools from 80 °C to 70 °C in 5 minutes in a room at 15 °C. Using Newton's law of cooling with the average temperature of each interval, how long will it take to cool from 50 °C to 40 °C?
1. A5 min
2. B2.5 min
3. C20 min
4. D10 min
Answer and explanation in LearnFree account
Question 12 · Easy
In which SI unit is the thermal conductivity of a material expressed?
1. AW m K⁻¹
2. BW m⁻¹ K⁻¹
3. CW m⁻² K⁻¹
4. DJ kg⁻¹ K⁻¹
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Question 13 · Hard
Ice of mass 40 g at −10 °C is dropped into 100 g of water at 18 °C in an insulated vessel of negligible heat capacity. Take the specific heat of ice as 0.5 cal g⁻¹ °C⁻¹, of water as 1 cal g⁻¹ °C⁻¹, and the latent heat of fusion of ice as 80 cal/g. What is the final state of the mixture?
1. A0 °C, with 20 g of ice left
2. B0 °C, with 15 g of ice left
3. C0 °C, with all the ice melted
4. D0 °C, with 17.5 g of ice left
Answer and explanation in LearnFree account
Question 14 · Medium
A thin steel plate has a circular hole of diameter 10.0 cm at 20 °C. The coefficient of linear expansion of steel is 1.2 × 10⁻⁵ K⁻¹. How does the diameter of the hole change when the plate is heated uniformly to 120 °C?
1. AIt increases by 0.024 cm
2. BIt increases by 0.012 cm
3. CIt does not change
4. DIt decreases by 0.012 cm
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Question 15 · Medium
Steam of mass 10 g at 100 °C is mixed with 145 g of water at 20 °C in an insulated container of negligible heat capacity. Take the latent heat of vaporisation of water as 540 cal/g and the specific heat of water as 1 cal g⁻¹ °C⁻¹. What is the final temperature, to the nearest degree?
1. A60 °C
2. B25 °C
3. C57 °C
4. D100 °C
Answer and explanation in LearnFree account

Thermal Properties of Matter

In a room at 20 °C, a body cools from 70 °C to 50 °C in 5 minutes. Assuming Newton's law of cooling, what is its temperature after a further 5 minutes?

An isotropic solid has a coefficient of linear expansion α. What are its coefficients of area expansion and volume expansion, in that order?

In a calorimetry experiment, 10 g of ice at 0 °C is added to 100 g of water at 30 °C in an insulated container of negligible heat capacity. Taking the latent heat of fusion of ice as 80 cal/g and the specific heat of water as 1 cal/(g °C), what is the final temperature?

A 2 kg solid sample is heated steadily, and its temperature is plotted against heat supplied. What is the latent heat of fusion of the material?

Two slabs of equal thickness L and equal area are joined face to face: conductivity k from x = 0 to L and 2k from x = L to 2L. In steady state the outer faces stay at 100 °C (x = 0) and 40 °C (x = 2L). Which graph shows temperature against x?

Two slabs of equal thickness and equal area are placed face to face. Slab P has thermal conductivity K and slab Q has 3K. The free face of P is kept at 100 °C and the free face of Q at 0 °C. In the steady state, what is the temperature of the interface?

Assertion A: Ice floats on water. Reason R: Water contracts when it freezes. Choose the correct answer.

A 0.2 kg piece of metal at 150 °C is dropped into 0.5 kg of water at 20 °C, and the final common temperature is 25 °C. Neglecting heat taken by the container and lost to the surroundings, what is the specific heat of the metal? (Specific heat of water = 4200 J kg⁻¹ K⁻¹)

As water is heated from 0 °C to 10 °C, its density

A cup of liquid cools from 80 °C to 70 °C in 5 minutes in a room at 15 °C. Using Newton's law of cooling with the average temperature of each interval, how long will it take to cool from 50 °C to 40 °C?

In which SI unit is the thermal conductivity of a material expressed?

A thin steel plate has a circular hole of diameter 10.0 cm at 20 °C. The coefficient of linear expansion of steel is 1.2 × 10⁻⁵ K⁻¹. How does the diameter of the hole change when the plate is heated uniformly to 120 °C?

Steam of mass 10 g at 100 °C is mixed with 145 g of water at 20 °C in an insulated container of negligible heat capacity. Take the latent heat of vaporisation of water as 540 cal/g and the specific heat of water as 1 cal g⁻¹ °C⁻¹. What is the final temperature, to the nearest degree?