Thursday, May 24, 2012

Is cold water heavier than hot water?

Hot air rises and cold air sinks. So yeah, the same applies with water. Not really. Short answer is No. Long answer: It is not as simple as that. But wait, why is science so complicated?

| f/5.6 | 1/60sec | ISO200 | 135mm | Spot metering |

Cold water is denser than hot water because the individual molecules move more slowly and thus pack together more tightly. This applies only to water at temperature above 4oC. So, a litre of water at 4oC weighs about 4.3% more than a litre of water at just below boiling point. But that's because to fill a litre bottle with water at 4oC, you need to add more molecules of water than you do when the water is at 100oC.

If you take a sealed litre bottle of water at 100oC and let it cool to 4oC, without losing or gaining any molecules, it should weigh the same. Right? Not really. Scientist found that the mass actually drops!

Einstein's equation, E = mc2 means that mass and energy are actually the same thing. By removing energy from a system, you are also taking some of its mass. As the water cools, it loses energy and therefore mass, and the surrounding/atmosphere gains energy (or mass) by the same amount.

The effect is very small, and for each oC that the litre of water cools, it loses 4.7 x 10-14 kg. That is about the weight of two human sperm cells!

Cut the crap, which is heavier - hot water or cold water?

Neither. One kg of cold water weighs the same as one kg of hot water. The difference is their density. Water gets denser as it cools down to 4oC, then starts expanding again (less denser) as it gets colder than 4oC. This is the reason all is not ice in the North Pole. There is water below the iceberg with temperature of about 4oC and fish adapts to this temperature to live there.


©t-Ba said...

So the answer is? Hot or Cold is heavier?

de engineur said...

1kg of lead is no heavier than 1kg of cotton, isn't it. Hehehe