Problem 89 - Chemistry Matter and Change

Question

When solid sugar crystals are dissolved in a glass of water, they form a clear homogeneous solution in which the crystals are not visible. If the beaker is left out at room temperature for a few days, the crystals reappear in the bottom and on the sides of the glass. Is this an example of freezing?

Step-by-Step Solution

Answer

No, it's due to evaporation, not freezing.

Step 1: Understanding Dissolution

When sugar crystals dissolve in water, they break down and disperse evenly throughout the water, forming a homogeneous solution where the individual sugar particles are no longer visible.

Step 2: Analyzing Reappearance of Crystals

As the solution is left at room temperature, water may slowly evaporate over several days. As water leaves the solution, the concentration of sugar increases until it reaches a point where the water can no longer hold all the dissolved sugar, leading to the reappearance of sugar crystals.

Step 3: Distinguishing Evaporation from Freezing

Evaporation occurs when water changes from a liquid state to a gaseous state at temperatures below boiling. This is different from freezing, which involves a liquid turning into a solid upon cooling. The reappearance of sugar crystals is due to evaporation, not freezing.

Key Concepts

Homogeneous Solution

When you dissolve solid sugar crystals in a glass of water, what's happening at a molecular level is quite fascinating! The sugar molecules break apart and mix evenly throughout the water molecules. This forms what we call a homogeneous solution. In a homogeneous solution, all the components are uniformly distributed.

• No particles or different substances can be seen.
• The sugar blends completely with the water.
• The solution is clear and uniform.

This even distribution is key and makes clear liquids like sugar water appear as one single substance. Although sugar might be invisible to the naked eye in this solution, it's there, nestled amongst water molecules.

Evaporation Process

The evaporation process is all about how a liquid becomes a vapor. When your sugar water solution is exposed to air at room temperature, this process begins. Water molecules on the surface of the solution gain enough energy to escape into the air as a gas.

• Evaporation occurs at temperatures below the boiling point of the liquid.
• It's a gradual process that can happen at ambient temperatures.
• Warm, dry conditions can speed up evaporation.

As the water molecules leave, the amount of sugar per unit volume increases, because the water content decreases. With less liquid to hold them, the sugar starts to re-solidify. This is not freezing, as no temperature drop is involved. It's simply the concentration of sugar becoming too high for the reduced volume of water.

Recrystallization

Recrystallization may sound complex, but it's just the process that occurs when dissolved substances start to form solid crystals again. In this context, as water evaporates from the sugar solution:

• The concentration of sugar increases.
• Eventually, the solution becomes supersaturated.
• Sugar can't remain dissolved in such small water quantities.

Thus, sugar molecules begin to form crystals once more. The conditions that allow this crystallization are primarily driven by water loss through evaporation, forcing the sugar out of the solution and back into a solid state.
This process is widely used in chemistry to purify compounds, using the principle of solubility changes with temperature or concentration.