Question

Caffeine is about 10 times as soluble in hot water as in cold water. A chemist puts a hot-water extract of caffeine into an ice bath, and some caffeine crystallizes. Is the remaining solution saturated, unsaturated, or supersaturated?

Short answer

The remaining solution is saturated.

Step-by-step solution

Understand Solubility Concept

First, understand that solubility increases with temperature. That means a substance like caffeine dissolves more readily in hot water than in cold water.

Caffeine Solubility in Hot Water

Recognize that caffeine is highly soluble in hot water, which means it can dissolve in large amounts in hot water.

Effect of Ice Bath

When the hot-water extract of caffeine is placed in an ice bath, the temperature rapidly decreases, reducing the solubility of caffeine.

Crystallization of Caffeine

Due to the reduced solubility in cold water, some of the caffeine crystallizes out of the solution. This implies that the remaining caffeine in the solution is at its maximum solubility limit for cold water.

Determine Saturation Level

Since the remaining caffeine solution can no longer dissolve any more caffeine and some has crystallized out, the solution is saturated.

Key concepts

solubility and temperature

Solubility is the ability of a substance to dissolve in a solvent, like how caffeine dissolves in water. One crucial factor that affects solubility is temperature. Many substances, including caffeine, become more soluble in hot water. This means more caffeine will dissolve in hot water compared to cold water.
When we talk about solubility and temperature, a few points are important:

• For most solids, solubility increases with temperature.
• For gases, solubility usually decreases as temperature increases.

For caffeine, being more soluble in hot water means its molecules move more rapidly and interact with water molecules more efficiently. So, if a hot solution of caffeine is cooled quickly by placing it in an ice bath, the solubility of caffeine decreases, meaning it can no longer stay dissolved at the same level.

crystallization

Crystallization is the process where a solid forms from a solution or gas. When the solubility of a substance in solution is exceeded, it often starts to crystallize out. Here's a simple breakdown:

• When you have a hot solution of caffeine, it's highly soluble and stays dissolved.
• Once you cool down the solution rapidly, like placing it in an ice bath, the solubility decreases.
• As a result, the excess caffeine that can't stay dissolved begins to crystallize out of the solution.

When the solution cools, the caffeine molecules lose energy, slow down, and start to settle out of the solution. This forms caffeine crystals. Crystals are typically purer than the dissolved substances, making crystallization a useful method to purify chemicals.

saturation of solutions

A solution is saturated when it contains the maximum amount of solute that can be dissolved at a specific temperature. Beyond this point, any additional solute will not dissolve and will separate out.
Let's relate this to our caffeine example:

• A hot solution of caffeine can hold more caffeine dissolved because of its higher solubility.
• Upon cooling the solution, its ability to hold caffeine decreases.
• The caffeine that can't remain dissolved crystallizes out, leaving the remaining solution at its maximum capacity for dissolved caffeine at the new, lower temperature.

In the exercise, since the caffeine crystallizes out when the solution is cooled, it indicates that the remaining solution has reached that maximum solubility limit for the lower temperature, making it saturated.