Evaporation

 

What is evaporation?

Evaporation is when a liquid changes into a gas slowly, without reaching its boiling point.

Evaporation can happen at any temperature, but boiling only happens at a specific temperature for each liquid.

 

 

Using evaporation to separate a solution

Evaporation can be used as a separation method to separate components of a mixture with a dissolved solid in a liquid. The liquid is evaporated, meaning it is converted from its liquid state to gaseous state. This often requires heat. Once the liquid is completely evaporated, the solid is all that is left behind.

 

 

A solution is placed in an evaporating dish and heated with a Bunsen burner.

 

Crystallisation

Crystallisation is a way of separating a solid solute from a solution by letting the solvent (water) evaporate slowly over time.

Crystallisation makes the solute form bigger crystals, because the slower the water evaporates, the more time the solute has to arrange itself into a regular pattern.
These are the steps for crystallisation:

 

• Instead of heating the evaporating basin directly with a Bunsen burner, it is often better to heat it gently and slowly over a beaker of boiling water.
• Heat the solution until only half of the water is left, which makes the solution more concentrated. Leave the solution to cool down and large crystals will grow slowly.
• Put the evaporating basin in a warm place so the rest of the water evaporates.

 

🧪 Knowledge Check: Evaporation

Test your understanding of how evaporation works as a separation technique.

1. Evaporation is a technique used to separate:

• An insoluble solid from a liquid.
• A soluble solid from a liquid.
• Two liquids with different boiling points.

2. During evaporation, what happens to the liquid solvent?

• It turns into a solid.
• It stays in the dish with the solid.
• It turns into a gas and escapes into the air.

3. Why does the evaporating dish have a wide, shallow shape?

• To keep the solution cool.
• To provide a large surface area for faster evaporation.
• To prevent the solid from forming crystals.

4. Which of these is a common substance obtained using evaporation?

• Copper sulphate crystals
• Distilled water
• Sand and water mixture

5. In a school lab, what is usually used to provide the heat for evaporation?

• A refrigerator
• A filter funnel
• A Bunsen burner

Click to Reveal Answers

1. A soluble solid from a liquid (Evaporation targets substances dissolved in a solution).
2. Turns into a gas and escapes (The heat causes the solvent to change state and leave the solid behind).
3. To provide a large surface area (A wider surface allows more molecules to escape as gas at once).
4. Copper sulphate crystals (Crystallisation occurs as the liquid solvent is removed).
5. A Bunsen burner (This provides the thermal energy needed to speed up the process).

 

 

Summary:

 

• Evaporation is when a liquid changes into a gas slowly, without reaching its boiling point.
• Crystallisation is a way of separating a solid solute from a solution by making crystals.
• When a solution is heated, the liquid solvent goes away and the solid solute stays as crystals.