Why Do Chemical Compounds Tend to Dissolve More Quickly in Hot Solvent Than in Cold Solvent?

Water is sometimes called the "universal solvent" because many substances (though not all) can dissolve in water.

When a solid dissolves, the individual ions or molecules that compose it break up or dissociate and mix with the solvent. Gases can also dissolve in liquids. The solubility of gases and liquids may vary depending on temperature.

Types

Gases exhibit decreasing solubility with increasing temperature. Most solids, on the other hand, exhibit increasing solubility, although this is not necessarily true for all solid compounds. Some compounds are actually less soluble at high temperatures than they are at lower temperatures. The general trend, however, is towards increasing solubility with increasing temperature, although the same basic principle explains both types of behavior.

Entropy

Entropy is a measure of total disorder in a system. Entropy changes help determine why solubility differs with changing temperature. The laws of thermodynamics dictate that the entropy of the universe can never decrease, so for a process to occur it must increase net entropy (i.e. the total entropy of both the system and its surroundings). Some processes may decrease the entropy of a system and yet still increase the entropy of their surroundings by releasing heat. Chemists often use the Gibbs free energy, or delta G, to measure the change in total entropy; the delta G for a process is equal to the amount of heat absorbed minus the temperature times the change in entropy of the system. If the delta G of solution is negative, a substance will dissolve spontaneously, and the more negative the delta G the more of it will dissolve.

Considerations

Typically, the entropy of the system will increase when a solid dissolves in a liquid, so the higher the temperature of the system, the more negative the second term in the delta G equation, and the more soluble the compound. For some compounds, however, dissolving in a solvent is actually associated with a decrease in entropy, and these compounds will become less soluble as the temperature rises since the delta G becomes more positive. This is especially true for gases, since the level of disorder in a gas is as a general rule greater than the level of disorder of a gas dissolved in a liquid. For most solid compounds, however, entropy will increase when they dissolve.

References

• "Chemical Principles, the Quest for Insight, 4th Edition"; Peter Atkins and Loretta Jones; 2008.