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Why does salt lower the freezing point and raise the boiling point of water?

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Title:
Why does salt lower the freezing point and raise the boiling point of water?
Topic:
Physics
Question: 

Why does salt lower the freezing point and raise the boiling point of water?

 

Answer: 

When salt (sodium chloride or NaCl) dissolves in water, each molecule separates into sodium (Na+) and chlorine (Cl-) ions, which mix separately among the water molecules. These ions affect the water molecules and their freezing and boiling temperatures in different ways.

Lowered freezing point

Water molecules form crystals when they freeze. But when there is salt in the water, the Na+ and Cl- ions get in the way of the water molecules, making it more difficult for them to rearrange into crystals. This means that, when compared to plain water, salt water remains in a liquid state for a longer amount of time as the temperature decreases.

Raised boiling point

As heat is added to plain water, the temperature rises and the water molecules gain more energy. They move around faster and collide with each other more often. At the boiling point, the water molecules have enough energy to escape the liquid. (In technical terms, the vapor pressure of the water molecules is greater than the surrounding atmospheric pressure.)

Salt water

In salt water, the Na+ and Cl- ions dilute the water, isolating each water molecule somewhat from others. This means that each molecule will need more heat energy to move around and collide with other water molecules than is the case for pure water. In turn, this means that the salt water will have to be at a higher temperature for the water molecules to escape by boiling.

Learn all about water by exploring the articles and activities in the PlanetSEED water theme.


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