When you drop something (like a book), it falls to the ground because it is attracted to the earth. What is meant by the word "attracted" is that gravity exerts a force which pulls the book to the earth. It doesn't take outside energy for this to happen. The force is supplying the energy necessary.
When a solute dissolves in a solvent, the solute molecules are attracted to the solvent molecules. What is meant by this is a force (called the Van Der Walls force) pulls the molecules towards each other. Just as with the book and the earth, no outside energy is needed to pull the molecules towards each other. The force supplies the energy needed. The molecules move towards each other because it is natural for them to do so. That's what happens when things dissolve.
Don't get confused about how things dissolve and how temperature affects solubility. These are really two different things.
The force that attracts solute to solvent is there regardless of the temperature. However, when you change temperature, you are changing the energy of the individual molecules for both the solvent and the solute.
In the case of a solid dissolving, you are giving the same amount of extra energy to both the solute and the solvent, but the effect of this energy helps the dissolving process. You see, when the temperature of a solid increases, the molecules of the solid start to pull apart. This gives the solvent molecules more room to get in between the solute molecules. Also, the solvent molecules have more energy to fight their way in between the solute molecules. Both of these effects enhance the dissolving process, so solubility increases.
If you increase the temperature when dissolving a gas, however, things are a lot different. Remember, gas molecules are quite far apart. In order to dissolve (and thus become liquid), the gas molecules must get closer together than they normally would. They do this because the solvent molecules exert their attractive force on the solute molecules, pulling them in. However, if you increase temperature, this gives solute and solvent molecules more energy. The extra energy makes the gas molecules want to get even farther apart, just like in the case of the solid. The difference, however, is that the solvent molecules don't want the solute molecules to get farther apart. They are fighting to keep the solute molecules close together. The solvent molecules, then, have a harder time doing their job. So, rather than helping the process of dissolving, an increase in energy harms the dissolving process.
Another way to look at this is that dissolving involves taking the solute and making it a liquid. Well, when you heat a solid, it turns into a liquid. Thus, an increase in temperature increases solubility.
On the other hand, heating a gas makes it even less like a liquid. As a result, heating the gas solute decreases solubility.Tags: Chemistry