MadSci Network: Chemistry

Re: Why does the solubility of a gas decrease with a temperature increase?

Area: Chemistry
Posted By: Lynn Bry, MD/PhD Student, Molecular Microbiology
Date: Wed Dec 25 04:42:52 1996
Message ID: 848119334.Ch

Hi Jennifer,

You've asked a good question. I don't know how much chemistry you've had, so I'll answer in a qualitative fashion.

First, consider the relative 'molecular motion' of molecules in the solid, liquid and gaseous states. As a solid (consider ice), molecules don't vibrate or move around that much. However, as you increase the amount of movement (by adding heat, e.g.) the molecules vibrate more and more until they can no longer stay in the solid state (ice -> water). Add more energy still, and your liquid becomes a gas (water -> steam). The important thing to note is that atoms and molecules in the gaseous state move around a lot while those in the solid state move around considerably less.

Take a solution of sugar (= sucrose) in water. When you add it to the water the motion of the water molecules helps 'dissolve' the sugar crystals. The crystals are constantly bombarded by surrounding molecules of water. This action breaks down the organized structure of the sugar crystal so individual molecules of sucrose can enter the solution. The more energy the water molecules have, the greater the amount of sugar you can get into solution - more energy with which to break down crystal lattices. Molecules in hot water have more energy than molecules in cold water, and indeed you can dissolve more sugar in hot water than you can in cold water.

This same effect seems to work in reverse for gasses. However, keep in mind that a molecule in the gaseous state has far more energy than a molecule in the solid state or in the liquid state. To a gas, cold water is peaceful and calm. With less activity, it's easier for a 'high energy' gas molecule to slip among the water molecules and stay in solution. However as the water is heated, the added bouncing around of the water molecules makes it difficult for a gas molecule to remain in solution. Since it already has a lot of energy, a hit from a passing molecule of water would knock it out of solution into the surrounding air. The same event acting on a molecule in the solid state would serve to knock it 'into solution.'

I hope this answers your question.

-Lynn Bry, MadSci Admin

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