Is it easier to boil water than to freeze it?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

05 February 2006

Jimmy Kimani wants to know why it takes longer to cool things than to heat them. He has observed that, while a sufuria of water boils in a few minutes, freezing it can take many hours. He adds, “since the water is at room temperature, why should it be harder to freeze (only 20 degrees down from room temperature) it than to boil it (about 80 degrees up)?”

Jimmy’s argument is correct, but he has applied it in the wrong way. The amount of heat energy lost by the water in cooling from room temperature to freezing is less than that required to heat the same quantity to boiling point.

The reason is that the difference between room temperature and freezing point is only about 20 degrees, while from room temperature to boiling is an 80-degree jump. Therefore, the energy lost by cooling is only one quarter of that required for the heating. So, why does the water boil faster than it cools?

The rate at which the water cools down or heats up is not determined by the final temperature reached but by the temperature of the cooling or heating environment – in this case, the freezer or the fire respectively. The freezer is at about negative 15 degrees celcius. The difference between this and room temperature is only 35 degrees (20 minus negative 15). On the on the hand, the temperature of, say, cooking gas flame is about 1,200 degrees. This is 1,880 degrees above room temperature. Therefore, it will boil faster than it freezes.

But that is only half the story. Another question follows: since a large flame will boil the water faster than a small one, does that mean that the bigger flame is hotter? Of course not! The two flames are burning the same fuel therefore their temperatures are the same. In that case, the difference in the heating rate is determined by the amount of heat energy generated by the flame.

The larger flame burns fuel at a higher rate therefore it generates more energy per second than the smaller fire. Thus the bigger flame will boil the water faster.

The same principle applies to Jimmy’s problem. The average sized gas burner generates about 2,000 watts of heat energy. A freezer, on the other hand removes heat from inside at the rate of about 100 watts. Consequently, the water will boil faster than it freezes.

Now, when you put the water in a freezer which is at, say negative 15 degrees, it will freeze and go down to the temperature of the fridge. However, when the same water is placed on a fire at 1,200 degrees, it goes up to 100 degrees and stops getting any hotter – it never gets to the temperature of the flame. Why is that so? Well, that is a story for another day.