What does salt do to the boiling point of water?

Written by shelley moore
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What does salt do to the boiling point of water?
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Adding salt to water raises the boiling point. This is a scientifically measureable effect, but it takes 56.7gr. of salt to raise the boiling point of 1 litre nearly -16.7 degrees C, so the effect is not noticeable in your average kitchen. Salt added to water also lowers the freezing point.

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Effects

When salt is added to water, the boiling point of the water rises and the freezing point is lowered. From a molecular standpoint, when water temperature rises, the molecules move faster, collide more frequently, and release more vapour gas molecules. Salt chemical ions take up some space, making for fewer collisions among water molecules, so they do not release as many vapour molecules as pure water would. Thus, more energy is required--a higher temperature--for salty water to start to boil. So, raising the boiling point of 1 litre (964gr.) of water by 1 degree C (-16.78 degrees C) requires about 58 grams (2 oz.) of salt.

Types

Salt added to water lowers the freezing point for similar reasons, only in this case, the water molecules are forming crystals when freezing, rather than vapour when boiling. Ions from the salt get in the way of the water molecules, making it harder for them to rearrange into crystals.

Significance

These effects hold true for any non-volatile soluble substance added to water. It's the reason why ethylene glycol, or antifreeze, protects a vehicle radiator from getting to the boiling point in the summer or freezing in the winter. It also explains why distributing salt onto icy sidewalks and roads causes the ice to melt, as the outdoor temperature typically is not low enough to keep the ice frozen with salt on it.

Warning

It's important to be extremely careful if adding salt to water that is already boiling. It will make the water boil much more vigorously for a second or two and can result in spattering. It can even cause the water to explode.

Misconceptions

A prevailing myth states that cold water boils faster than hot water. There is no truth to this theory. However, under some circumstances, hot water freezes faster than cold water. One theory about this phenomenon is that some of the hot water evaporates, leaving a smaller amount of water to be frozen. This effect can only be achieved in specific circumstances and is not easy to replicate at home. Some people add salt to water before cooking pasta, believing salt stops the water from boiling over, or makes the pasta cook faster. In reality, the small amount of salt added makes no significant difference in the boiling point, and adding salt also does not stop water from boiling over.

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