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Q:

Why does salt raise the boiling point of water?

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Salt can be used to retard the effect of evaporation when water is boiled by raising the boiling point. This is most effective in cooking. The salt is absorbed by the water forming a solution the salt is soluble. When the water reaches saturation boiling point remains constant. As soon as any of the salt dissolves in the water, the boiling point of the water will begin to rise -- by about one half degree Celsius for every 58 grams of salt dissolved per kilogram of water. In fact, any non-volatile soluble substance will raise the boiling point of water. That is why antifreeze (ethylene glycol) provides boiling protection in winter as it simultaneously provides freezing protection in the summer. * I did find this online, and am not sure if it's totally true, but willing to give it a try! Bring two pots of plain water to near boiling and then toss salt in one of them. The pot receiving the salt will likely explode into violent boiling because the salt crystals provided nucleation sites ... more
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Salt both lowers the freezing point and raises the boiling point of water. Pure water, at sea level, boils at 100 degrees celsius (212 F). Once water begins to boil, it stops increasing in temperature, after which, all the energy that is trying to heat the water up is going into boiling it. So when you dissolved all that salt into the already boiling water, you raised the boiling point, allowing the water to get hotter than it normally would have without the salt. The same is true for cooling water, salt water can get much colder without freezing into ice than pure water. more
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When you add nonvolatile substance like NaCl (salt) to water, it breaks down into positive sodium ions and negative chloride ions. The ions are nonvolatile (they don't evaporate). The energy needed to break the attractions between water molecules is taken up by these nonvolatile particles and the boiling point rises because of this. Electrolytes (substances that conduct electricity) can have a greater effect because they break into more particles. When you cool a solution of salt and water, it lowers the freezing point because the molecules of water use energy to exclude the sodium and chloride ions and thus freeze at lower temperatures. This is why we use rock salt to make ice cream when we add it to the ice surrounding the ice cream mixture. more
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