Physics · Properties of bulk matter · chapter 10 of 14

Class 11 Physics Chapter 10: Thermal Properties of Matter MCQs with answers

10 multiple-choice questions on Thermal Properties of Matter, each with its answer and the reason, written from the NCERT chapter. For school exams on the CBSE pattern, JEE Main and NEET, which are set from the same NCERT chapter. Open a question's solution to check your working; then take the free test to find your next chapter.

Question 1
The temperature at which a Celsius thermometer and a Fahrenheit thermometer show the same number is
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Answer: (d) −40°

Put tF = tC = t in tF = (9/5)tC + 32: t = 9t/5 + 32, so −4t/5 = 32 and t = −40. The same number is read at −40 °C = −40 °F.

Question 2
The temperature of a room rises from 18 °C to 43 °C. The rise in temperature on the Kelvin scale is
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Answer: (a) 25 K

The Kelvin and Celsius scales have the same size of unit (T = tC + 273.15), so a rise of 25 °C is a rise of 25 K. 298.15 K and 316.15 K are temperatures, not a rise; 45 is the rise in °F.

Question 3
In a constant-volume gas thermometer filled with a low-density gas, the pressure rises by 0.40 kPa for every 1 °C rise in temperature. Taking absolute zero as −273 °C, the pressure of the gas at 0 °C is
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Answer: (c) 109.2 kPa

At constant volume P ∝ T, and the straight P–t line reaches zero pressure at −273 °C. From −273 °C to 0 °C the pressure rises by 0.40 kPa per degree over 273 degrees: P = 0.40 × 273 = 109.2 kPa.

Question 4
Rods of equal length made of iron (α_l = 1.2 × 10⁻⁵ K⁻¹), aluminium (2.5 × 10⁻⁵ K⁻¹), copper (1.7 × 10⁻⁵ K⁻¹) and brass (1.8 × 10⁻⁵ K⁻¹) are heated through the same rise in temperature. The rod that lengthens the most is the one made of
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Answer: (b) aluminium

Δl = α_l l ΔT, so with l and ΔT the same, the rod with the largest α_l lengthens the most: aluminium, 2.5 × 10⁻⁵ K⁻¹.

Question 5
An aluminium rod 2.0 m long is heated from 20 °C to 120 °C (α_l = 2.5 × 10⁻⁵ K⁻¹). Its length increases by
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Answer: (c) 5.0 mm

Δl = α_l l ΔT = 2.5 × 10⁻⁵ × 2.0 × 100 = 5.0 × 10⁻³ m = 5.0 mm. 2.5 mm forgets the length of 2.0 m; 0.5 mm and 50 mm slip a power of ten.

Question 6
The volume of a metal cube increases by 0.36% when its temperature rises by 100 K. The coefficient of linear expansion of the metal is
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Answer: (a) 1.2 × 10⁻⁵ K⁻¹

α_v = (ΔV/V)/ΔT = 0.0036/100 = 3.6 × 10⁻⁵ K⁻¹. Since α_v = 3α_l, α_l = 1.2 × 10⁻⁵ K⁻¹. 3.6 × 10⁻⁵ K⁻¹ is α_v itself; 1.8 divides by 2 (the area rule); 10.8 multiplies by 3.

Question 7
A beaker of pure water at 0 °C is warmed slowly to 8 °C. Over this range its density
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Answer: (d) first increases, then decreases

Water contracts on heating from 0 °C to 4 °C, so its density rises; above 4 °C it expands and the density falls. Water is densest at 4 °C.

Question 8
In a severe winter the surface of a pond freezes, yet fish survive in the water below. The main reason is that
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Answer: (b) water near 4 °C, being densest, stays at the bottom while ice forms at the top and floats

Water has its maximum density at 4 °C. As the pond cools, water near 4 °C sinks and stays at the bottom; colder, less dense water stays on top and freezes there, so the pond freezes from the top and the water below stays liquid.

Question 9
An ideal gas is heated at constant pressure. At 27 °C its coefficient of volume expansion is about
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Answer: (d) 3.3 × 10⁻³ K⁻¹

For an ideal gas at constant pressure α_v = 1/T, with T in kelvin: 1/300 K = 3.3 × 10⁻³ K⁻¹. 3.7 × 10⁻³ K⁻¹ is the value at 0 °C; 3.7 × 10⁻² K⁻¹ uses 1/27, the Celsius number.

Question 10
A brass rod is 60 cm long. An iron rod is to be cut so that the difference between the two lengths stays the same at every temperature. With α_l(brass) = 1.8 × 10⁻⁵ K⁻¹ and α_l(iron) = 1.2 × 10⁻⁵ K⁻¹, the iron rod must be
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Answer: (c) 90 cm

The difference stays fixed only if both rods lengthen by the same amount for any ΔT: liron α_iron ΔT = lbrass α_brass ΔT, so liron = 60 × 1.8/1.2 = 90 cm. 40 cm inverts the ratio; 150 cm adds the lengths.

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