Question

You have a solution of table salt in water. What happens to the salt concentration (increases, decreases, or stays the same) as the solution boils? Draw pictures to explain your answer

Short answer

As the saltwater solution boils, water molecules evaporate, leaving behind the salt molecules. As a result, the salt concentration in the remaining solution increases. This can be visualized in three stages: before boiling, where water and salt molecules are mixed together; during boiling, when water molecules evaporate; and after boiling, when the remaining solution has a higher salt concentration due to the decrease in water molecules.

Step-by-step solution

Understand the boiling process

When a solution is boiled, heat energy is added to it, and the liquid (in this case water) starts to evaporate, turning into vapor. As a result, the amount of liquid in the solution gradually decreases.

Visualize the solution before boiling

To help understand the process better, we can draw a container (e.g., a beaker) containing the saltwater solution before boiling. The beaker shows water molecules (H2O) mixed with salt molecules (NaCl). The salt concentration is represented by the ratio of salt molecules to the overall molecules in the solution. 

Visualize the boiling process

As the solution is heated and starts to boil, water molecules gain kinetic energy and turn into vapor. Water molecules then leave the solution in the form of vapor, while salt molecules remain in the container. The beaker now has fewer water molecules but the same number of salt molecules. 

Visualize the solution after boiling

After some time, the amount of liquid in the solution has decreased, but the salt molecules are still present in the container. The concentration of salt in the remaining solution has increased due to a decrease in the amount of water. 

Conclusion

As the saltwater solution boils, water molecules evaporate, leaving behind the salt molecules. As a result, the salt concentration in the remaining solution increases.

Key concepts

Evaporation

Evaporation is a natural process where a liquid turns into a gas. It happens when molecules in a liquid gain enough energy to become a vapor. In the case of boiling water, this process intensifies, as heating provides the energy needed for water molecules to escape from the liquid into the air.
As evaporation occurs, the liquid level decreases, but the non-volatile solutes like salt generally stay dissolved in the liquid rather than evaporate.
This is because their molecules do not have enough energy to break free and enter the gaseous state along with the water vapor. So, when you have a saltwater solution, the water evaporates but the salt remains.
This plays a critical role when boiling a saltwater solution, as it directly affects the concentration of salt in the remaining liquid.

Solution Concentration

Solution concentration refers to how much solute is dissolved in a given amount of solvent. In this context, the solute is salt and the solvent is water. It's typically measured in units like moles per liter (molarity) or grams per liter.
During boiling, the water component of a saltwater solution evaporates, while the salt remains dissolved in the remaining liquid. As the amount of water decreases, the concentration of salt increases because the same amount of salt is now dissolved in a smaller volume of water.
This concept can be visualized by imagining a beaker filled with saltwater. Before boiling, the beaker might have a certain concentration represented by a balance of salt to water molecules.

• As boiling continues, water evaporates and the concentration of salt increases.
• The concentration rises because there is less water to spread out the salt molecules.
• This results in a more concentrated salt solution the longer the boiling occurs.

Understanding solution concentration is key to predicting and explaining changes in molarity during boiling.

Saltwater Solution

A saltwater solution is a mixture where salt (sodium chloride) is dissolved in water. It’s a homogeneous mixture, meaning the salt is evenly distributed throughout the water, rather than being a separate phase like oil in water.
In a saltwater solution, water acts as the solvent that dissolves salt, the solute. This type of solution is commonly encountered in everyday life and science experiments.
When a saltwater solution boils, it's essential to remember that water, the solvent, evaporates while the salt does not. The unchanged presence of salt molecules in the evaporating water implies greater concentration in the leftover liquid.
This is a basic but important aspect influencing various natural and industrial processes where saltwater is involved. Understanding the behavior of such solutions helps in fields like chemistry, cooking, and even environmental science where saltwater concentration is a crucial variable.