Question

A liquid cools from 65°C to 55°C in 5 mins. If the surrounding temperature is 35℃, calculate the time taken for temperature to fall from 45°C to 35°C.

(a) 25sec

(b) 25min

(c) 30min

(d) 30sec

I have been asked this question in semester exam.

Origin of the question is Thermal Properties of Matter topic in division Thermal Properties of Matter of Physics – Class 11

Answer 1

The correct answer is (b) 25min

The explanation is: Average temperature of liquid in the first case = 60℃.

Rate of fall of temperature =  65-55 / 5 = 2°C /min.

Which is also equal to k(60 – 35) = 25k, where k is a constant.

∴ 25k = 2 Or k = 2/25

Average temperature in the second case = 40°C

Rate of fall of temp = 45 – 35 / t = k(40-35)

∴ t = 25mins.