I was having a chat about science with a manager in my office. We were talking about water and he said "you shouldn't boil water from the hot tap because cold water boils faster".
I know about the Mpemba effect which sometimes makes hot water freeze faster than colder water. But I disagreed and said it didn't work to other way.
Does cold water boil faster than hot water?
No, according to Scientific American - Is it true that hot water freezes faster than cold water or that cold water boils faster than hot water? It seems hard to believe, but some people swear that it is so. (emphasize mine):
Cold water does not boil faster than hot water. The rate of heating of a liquid depends on the magnitude of the temperature difference between the liquid and its surroundings (the flame on the stove, for instance). As a result, cold water will be absorbing heat faster while it is still cold; once it gets up to the temperature of hot water, the heating rate slows down and from there it takes just as long to bring it to a boil as the water that was hot to begin with. Because it takes cold water some time to reach the temperature of hot water, cold water clearly takes longer to boil than hot water does. There may be some psychological effect at play; cold water starts boiling sooner than one might expect because of the aforementioned greater heat absorption rate when water is colder.
It even suggests you try it out yourself:
You can readily set up an experiment to learn which freezes earlier: water that is initially hot, or water that is initially cold. Use a given setting on an electric hot plate and clock the time between start and boiling for a given pot containing, say, one quart of water; first start with the water as cold as the tap will provide and then repeat it with the hottest water available from that tap. I'd wager the quart of water initially hot will come to a boil in much less time than the quart of water initially cold.
