Question

Give a brief description of the principles of the following techniques taking an example in each case. (a) Crystallisation (b) Distillation (c) Chromatography

Short answer

Crystallisation separates crystals from a solution by cooling, distillation separates substances by boiling points, and chromatography separates components by movement through a medium.

Step-by-step solution

Understanding Crystallisation

Crystallisation is a technique used to separate a pure solid in the form of its crystals from a solution. This process is based on the principle that different substances have different solubilities in a solvent at different temperatures. Typically, the solution is heated to dissolve the solute, and upon cooling, the solubility decreases, and crystals form. *Example:* To purify salt, dissolve impure salt in hot water. As the water cools, pure salt crystals will form, separating from impurities.

Understanding Distillation

Distillation is a process used to separate components of a liquid mixture based on differences in boiling points. The mixture is heated to vaporize the more volatile component, which is then condensed back into liquid and collected. This method is effective for separating substances with significant boiling point differences. *Example:* To separate alcohol from water, the mixture is heated. Since alcohol has a lower boiling point (78°C) than water (100°C), it evaporates first, is then cooled, and collected as a pure liquid.

Understanding Chromatography

Chromatography is a technique used to separate and identify components of a mixture based on differences in their movement through a medium. It involves a stationary phase and a mobile phase that carries the mixture through the stationary phase. Components separate as they move at different speeds. *Example:* In paper chromatography, a drop of ink is placed on paper and submerged in water (mobile phase). Different pigments in the ink travel at different rates on the paper (stationary phase), resulting in separation of the pigments.

Key concepts

Crystallisation

Crystallisation is a fascinating separation technique used to obtain a pure solid from a solution. This process relies heavily on the principle of solubility, particularly how it changes with temperature. When you dissolve a solute in a solvent, the resulting solution can hold only a specific amount of dissolved solute at a given temperature. By changing the temperature, you can manipulate how much solute remains dissolved.
A simple way to understand this is through the purification of salt. If you have some impure salt, you can dissolve it in hot water. Heating the solution increases the solubility of the solute, allowing more to dissolve. As the solution begins to cool, its capacity to hold the dissolved solute diminishes. Consequently, the extra solute crystallises out of the solution.

• Heat the solution to dissolve the impure solute completely.
• Allow it to cool gradually so that pure crystals form.
• The impurities remain dissolved in the solution or form separate crystals that can be filtered out.

Understanding crystallisation gives insights into the microscopic world of crystals and how we can employ temperature changes to purify substances.

Distillation

Distillation is another essential technique used in separating components based on their boiling points. This method is particularly effective when the liquids in a mixture have significantly different boiling points, as it allows one component to vaporise and leave behind the other.
To better comprehend how distillation works, think of separating alcohol and water. Alcohol boils at 78°C, while water boils at 100°C. When you heat a mixture of the two, the alcohol vaporises first due to its lower boiling point. This vapor can then be collected and condensed back into a liquid form in a separate container, leaving the water behind in the original flask.

• Heat the liquid mixture until the more volatile component vaporizes.
• Guide the vapor into a condenser, where it cools and liquefies.
• Collect the separated liquid for further use or analysis.

Distillation is widely used in many industries, such as the production of distilled beverages and in laboratories for purifying chemicals, showcasing its versatility and usefulness.

Chromatography

Chromatography is a complex yet methodically straightforward separation technique used to identify and separate the components of a mixture based on their movement through a stationary phase. The principle behind chromatography is that different substances will move or flow at different rates through a stationary medium depending on their interaction with the medium and the mobile phase.
An excellent example of chromatography is how we can separate pigments in ink using paper chromatography. In this method, a drop of ink is placed on a strip of paper (stationary phase), and the bottom of the paper is submerged in water (mobile phase). As the water climbs up the paper, the different pigments travel at varying speeds, resulting in a separation of colors. This visually shows the diversity of substances within a single drop of ink.

• Apply the mixture to the stationary phase.
• Introduce the mobile phase that carries the mixture through the stationary phase.
• Observe the separation of components as they move differently.

Chromatography is invaluable in scientific research, such as testing the purity of compounds in chemistry or identifying substances in forensic science.