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My friend tried this experiment at home:

She put an ice cube into boiling water. The water in the container stops boiling while the ice melts to water.

My question:

Why does the water stop boiling as the ice melts? Shouldn't heat just get transferred to the ice and the water just continues to boil...

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    $\begingroup$ Depends: if you had lots of boiling water, sure. But apparently, the latent heat (you should look this up) which any solid draws upon melting is enough, for the small amount of water you used in your experiment, to lower the temperature of the water below the boiling point. $\endgroup$
    – MariusMatutiae
    Commented Sep 16, 2014 at 15:19
  • $\begingroup$ @MariusMatutiae: The amount of water taken was 500g and the amount of ice was 50g. $\endgroup$
    – Eliza
    Commented Sep 16, 2014 at 15:45

6 Answers 6

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When you put a pot of water on the stove and turn the burner on, the heat source of the stove gets hot relatively fast, but the water takes a while to boil. For multiple reasons it takes a while for the water to heat up*, or in other words for the heat of the burner to be transferred throughout the pot of water.

When you add the ice cube to the boiling water, you very quickly cool off the boiling water which will cause the water to stop boiling. It will then take some time for the heat from the burner to heat the water back up to boiling.

There's a lot of caveats to take into account, for example, if you drop a small ice cube into a massive pot of boiling water, you'll only cool of a small bit of the water near the surface, and maybe only at part of the surface, so you may not even notice the stopped boiling.

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Was the container of boiling water still being heated when she put the ice cube in? If not, the water will stop boiling even without the ice cube as the container loses heat. If so, you could (with enough room around the ice cube) have boiling water near the container wall and non-boiling water near the ice cube. The water will not all be at the same temperatue.

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  • $\begingroup$ The water was still being heated. The ice was 50g and the water was 500g. $\endgroup$
    – Eliza
    Commented Sep 16, 2014 at 15:44
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    $\begingroup$ You now have a competition between the heat supplied to the water from the container and the heat going from the water to melt the ice. It sounds like the ice was absorbing enough heat to stop the boiling. If you looked very close to the edge of the container you might have seen water still boiling. $\endgroup$
    – Ross Millikan
    Commented Sep 16, 2014 at 16:32
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i think,it really depends on the quantity you take!!! In simpler way the heat from water is transferred to ice whic result melting of ice. If u further heat the container the water will boil.

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  • $\begingroup$ The amount of water taken was 500g and the amount of ice was 50g $\endgroup$
    – Eliza
    Commented Sep 16, 2014 at 15:46
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Remember the heat needed to melt ice is higher than that to boil water. At the point when she inserted the ice into the container,the heat absorbed was enough to stop the water from boiling(reduced the latent heat of vaporization of the boiling water)

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When evaporation takes place, it happens only on the surface. However as for boiling, it happens all around, as long as the temperature is or above the boiling point. Ergo if you were to drop an ice cube into boiling water, that immediate region falls below the melting point of water. Even as the colder water sinks, the top will still be cooled down making a convectional current thus creating the illusion that the whole tank stops 'boiling'. On a side note, the change in the state of matter from solid to liquid is an endothermic one as the water molecules are being seperated. This dosen't affect the responses but it may give insight on latent heat and stuffs.

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Here, the boiling point of water is 373K and the temperature of ice is smaller than 273K. When we boil, the water is brought to 373K and when the ice is dropped into the cup (assuming the temperature of ice is 273K), the heat is transferred from the water (The Hotter substance) to the ice (The colder substance) until their temperature is equal, that is 323K (Halfway between them). So as you can see, the water's temperature keeps on decreasing until both the ice's (or water after the ice has melted) and water's temperature are the same, which prevents water from boiling.

NOTE: Temperature is in Kelvin

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  • $\begingroup$ Re, "...until their temperature is equal, that is 323K(That is halfway between them)." You are not accounting for the relative amounts of ice and water (e.g., one tiny ice cube dropped into an enormous vat of water vs. a chunk of ice that is almost as large as the pot.) Also, you are not accounting for the heat of fusion: It takes a lot more energy to turn X grams of ice at 273K to X grams of water at 273K than it take to heat X grams of water from 273K to 274K. $\endgroup$
    – Solomon Slow
    Commented Apr 13, 2016 at 22:18

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