Question

Topic: Thermochemistry

Description:

Many people have said that cold water boils faster than hot water. This is not true. In fact, it’s been said so many times that most people believe it to be a fact.

Write a response to the following prompt. 200-250 words.

Prompt:

Postulate a reason for why this may have been thought to be true. Is there any scientific evidence backing this claim at all? Please explain your reasoning.

Answer 1

The heat transfer by conduction can be expressed as follows-

, Where, k is thermal conductivity of the material, t=time, A=area of contact and d is the disctance between the objects.

Now, from the equation it can be said that rate of heat transfer (Q/t) is directly proportional to the temperature difference. This temperature difference will be higher in case of cold water when same sourche of heat is used to heat both types of water. Therefore, cold water will absorb heat faster initially and as temperture of water rises, the heat transfer will slow down. After reaching the temperature of the hot water, it takes just as long to bring it to a boil as the water which was hot initially. Since, cold water comsumes some time to reach the temperature of hot water, it clearly takes longer. So this claim is false.

But, due to initial greater heat absorption in case of cold water, some people believe that it boils faster than water. Psychological effect is also playng some role here, because cold water takes less time than they expected which can be attributed to the heat absorption rate.

This myth may have been introduced to encourage people to cook with cold water, because higher temperature results in higher solubility of different impurities or chemicals. So, hot water tends to contain more impurities than cold water.