Boiling and melting point

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Why does adding something to a liquid, e.g. salt to water increase the boiling temperature, but at the same time decrease the melting temperature?

In: Chemistry

Anonymous 0 Comments

Awesome question!

This can be a little complicated and can vary depending on liquids and solutes, so I’ll just stick to salt and water.

Water loves salt. Just loves it. This is because salt is ionic, meaning it’s made up of a positively charged atom and a negatively charged atom. Water on the other hand, is polar. It’s all the same molecule, and that molecule has a *slight* charge imbalance–one side is more positive and one side is more negative, but it’s not as extreme as the salt ions. Also, it’s important to recognize that a liquid is a bunch of molecules all moving around all the time.

So when you dissolve salt in water, the negative side of water is attracted to the positive sodium ion, and the positive sides of water molecules are attracted to the negative chlorine ion. This is way more enegetically favorable than water just clumping to itself. You basically get these balls of water molecules around ions.

Now when you boil something, you’re putting enough energy into it that molecules start to fly off because their energy overcomes the attraction to other molecules. The more the molecules are attracted to the liquid, the harder it is to boil the liquid. Since adding salt gives something for the water to be even more attracted to, it makes the salt water more attractive to individual molecules. This means it takes more energy (higher heat) to boil.

For freezing, a sort of similar thing happens. When something polar like water freezes, it likes to form neat crystal structures. Water kind of always wants to form these structures, but the molecules have too much energy, so they keep sticking and then breaking away. When it gets cold enough, this sticking, well, sticks, and the water forms nice crystals. The salt disrupts this in a few ways. One is by just getting in the way of the crystal structure, making crystals harder to grow. But also by being more attractive to water than other water molecules, the salt makes the water stay attached to these ions for longer. Balls of water around a salt ions don’t stick well to each other, so they need colder temperatures to lock down.