Evaporation and Crystallisation

To understand crystallisation and evaporation, it’s important to know what a solvent, a solute and a solution are.

The process of creating a solution. On the left, a jug containing a blue liquid, labelled as "Solvent", pours its contents into a beaker. Above the beaker, a spoon is shown adding small particles labelled "Solute". Beneath the beaker, the word "Solution" indicates the resultant mixture. On the right side of the image, after an arrow indicating progression, there is another beaker filled with a mixture of blue liquid and dispersed orange and blue particles, again labelled as "Solution". This represents the uniform mixture of solute and solvent.

  • Solute – Substance that dissolves to form a solution
  • Solvent – A substance that dissolves a solute to form a solution
  • Solution – Formed when a solute dissolves in a solvent

Evaporation

Evaporation is the process by which a liquid changes into a gas. This process occurs when molecules of a liquid are given enough energy to escape the surface of the liquid and become a gas. This energy is usually provided by heat.

  • Evaporation is an important process in the formation of crystals

As evaporation changes a liquid to a gas below its boiling point, it’s different from boiling.

Separating a solution

Evaporation can be used to separate a solid solute from a solvent in a solution. When the solution is heated, the solvent (the component present in large amounts) evaporates faster than the solute (the component present in smaller amounts).

This process can be used to obtain a solid substance from a solution. Once the solvent has evaporated, the solute is left behind as a solid.

In the lab, the solution is usually heated using a Bunsen burner and an evaporating basin.

The process of evaporation. A blue liquid containing dispersed bubbles is inside an "Evaporating basin", which is positioned on a metal stand. Directly below the basin is a flame emanating from a purple burner, labelled as "Heat". As a result of the heating, steam or vapour rises from the liquid. An annotation pointing towards the liquid reads, "Solution becomes more concentrated", indicating that as the solvent evaporates, the remaining solution becomes denser.

For example, when a saltwater solution is heated, the water will evaporate more quickly than the salt. This leaves behind a more concentrated solution of salt. This process can be used to purify the salt or to obtain a specific concentration of salt in the solution.

Evaporation is useful for separating mixtures because it’s relatively simple and doesn’t require special equipment.

Crystallisation

Crystallisation is a separation technique used to extract a solvent from a solution, thereby producing solid crystals. When the solution is heated, some of the solvent evaporates, which leaves behind a more concentrated solution.

During crystallisation, the solvent is evaporated, leaving the solute behind. However, crystallisation is different from the evaporation method because it takes place over a longer period of time and produces larger crystals.

Below are the steps involved in the crystallisation process.

1. Gently heat the solution in an evaporating basin to evaporate some of the solvent and leave a more concentrated solution behind.

  • Carefully monitor the temperature and avoid overheating, as this may cause chemical breakdown

2. As the solvent evaporates, you will see crystals start to form along the edge of the basin.

3. Stop heating the solution before all of the solvent evaporates, then leave the solution to cool.

The process of crystallisation. On the left, a shallow dish with a liquid is positioned on a wooden tripod above a Bunsen burner with a blue flame. Steam rises from the dish, indicating it's being heated. A green arrow labelled "Heat" points to a similar setup on the right. Here, the liquid in the dish has evaporated, leaving behind white "Crystallised solute" at the bottom. Both burners are fuelled by red gas taps connected to them.

4. As the solution cools, more crystals will start to form because solutes are more concentrated at lower temperatures.

5. Filter the crystals from the remaining solution using filter paper and a funnel, then collect them in a dry container.

6. Finally, dry the crystals thoroughly by placing them in a warm place, such as an oven, to remove any remaining solvent or water.

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