Why is the Salt Solution Heated in Step 6?

Heating the salt solution in step 6 is performed to evaporate the water, leaving the salt behind.

The Purpose of Heating Salt Solutions

When a salt solution is heated, a physical change occurs where the liquid water turns into a gas (water vapor) and escapes into the atmosphere. This process is known as evaporation.

• Evaporation: The conversion of a substance from a liquid state to a gaseous state.
• Non-Evaporating Solute: Salt (sodium chloride, NaCl) is a solid at the temperatures typically used for evaporating water from a solution. It has a very high boiling point compared to water.

According to the provided reference: "When the solution is heated the water is evaporates, and the salt does not evaporate and so it stays behind."

This selective evaporation allows for the separation of the dissolved salt from the water. As heating continues, more and more water evaporates until only the solid salt remains. This is a common technique used to recover solid solutes from a liquid solvent.

How Heating Achieves Separation

Here's a breakdown of why heating is effective:

1. Increases Kinetic Energy: Heating adds thermal energy to the water molecules, increasing their kinetic energy.
2. Overcomes Intermolecular Forces: With increased energy, water molecules can overcome the attractive forces holding them together in the liquid state.
3. Phase Transition: The energetic water molecules escape the liquid surface and become water vapor (gas).
4. Salt Remains: Salt crystals have much stronger ionic bonds holding them together compared to the weak intermolecular forces in water. The temperature required to break these ionic bonds and cause salt to evaporate is significantly higher than the boiling point of water. Therefore, at the temperature where water boils and evaporates, the salt remains as a solid.

Essentially, heating provides the energy needed to transition the water from liquid to gas, facilitating its removal, while the salt's properties ensure it remains in its solid form.

Practical Application

This method, often called evaporation to dryness, is a standard technique in chemistry and various industries for:

• Obtaining solid salts from solutions.
• Desalinating water on a small scale (though large-scale desalination often uses other energy-efficient methods).
• Concentrating solutions.

The process relies on the significant difference in boiling points between water and the dissolved salt.