0620 · 12.4Separation and purification

Solubility and choosing a solvent

0:00A beaker containing salt, sand and water presents several different separation problems. Recovering the sand needs one method, obtaining salt crystals needs another, and collecting the water needs a third. The right choice depends on the properties of the substances and the product you want. By the end, you will understand how those choices work and how to check purity. A mixture contains substances that are not chemically combined with one another.

0:23Separation uses differences in their physical properties, without needing to create new substances. Useful differences include solubility and boiling point. Purification means removing unwanted substances from a material, so the desired substance becomes purer. We need three terms before choosing a method. A solvent is a liquid that dissolves a substance, while the dissolved material is called the solute. Together, they form a solution. A soluble substance dissolves in a particular solvent; an insoluble substance does not dissolve appreciably in that liquid.

0:55Dissolving is not the same as melting. When salt dissolves in water, its particles become distributed throughout the solvent. The salt has not disappeared, and it can be recovered. Melting, by contrast, changes a solid into a liquid without requiring a solvent. Suppose we have a dry mixture of sodium chloride and sand. Water is a suitable solvent because the salt dissolves, but the sand does not. Add water and stir to dissolve the sodium chloride.

1:19This changes the mixture into an insoluble solid surrounded by a solution, making filtration possible. A suitable solvent should dissolve the substance you want to extract without dissolving the material you want to leave behind. It should not react chemically with the substances involved. Water is useful here, but it is not automatically suitable for every mixture. Always use the solubility information given in the question.

Filtration and crystallisation

1:42Filtration separates an insoluble solid from a liquid. Pour the mixture through filter paper supported by a funnel. The liquid passes through, while the undissolved solid remains on the paper. The retained solid is called the residue, and the liquid collected underneath is the filtrate. The filter paper has tiny openings that allow liquid and dissolved particles through. Sand grains are too large to pass through those openings. Dissolved sodium chloride travels with the water, so the filtrate is still a salt solution.

2:10Filtration therefore cannot remove dissolved salt from water. If the sand is our desired product, wash the residue with a little clean water. This removes salt solution trapped between the grains. Then dry the sand to remove the remaining water. If we want the sodium chloride instead, keep the filtrate for the next stage. Crystallisation obtains a dissolved solid as crystals from its solution. Begin by heating the solution gently in an evaporating basin to remove some solvent.

2:35As water evaporates, the remaining liquid becomes more concentrated. Stop heating at the point of crystallisation, rather than boiling the mixture completely dry. At the point of crystallisation, a drop taken on a glass rod forms crystals when it cools. This indicates that enough solvent has been removed. Stop heating and leave the concentrated solution to cool. Crystals form because the cooling liquid can no longer keep all the solute dissolved.

2:59For many solids, solubility decreases as temperature falls. Copper(II) sulfate is a useful example of this behaviour. Sodium chloride changes much less in solubility with temperature, so removing water is especially important when recovering it. The same overall sequence still applies: concentrate the solution, cool, filter and dry. Filter off the crystals from the remaining solution. Wash them with a small amount of cold solvent to remove solution clinging to their surfaces.

3:26Too much solvent would dissolve more of the product. Finally, dry the crystals between filter papers or by another suitable gentle method. Crystallisation can also help purify a solid because some soluble impurities remain in the liquid left behind. However, it does not guarantee complete purity for every mixture. Heating to dryness can leave those impurities mixed with the product. Strong heating may also damage some crystals, so controlled concentration is the better approach.

Simple and fractional distillation

3:51If we want water from the salt solution, we use simple distillation. Evaporation alone would let the water escape into the surroundings. Distillation adds a cooling stage so that the vapour becomes liquid again. This allows us to collect the solvent rather than lose it. Heat the salt solution in the flask until the water boils. Water vapour moves through the side arm into the condenser. Cold water flowing through the outer jacket removes heat from the vapour inside.

4:16The vapour condenses into liquid water, which runs into the receiving flask. The collected liquid is called the distillate. Sodium chloride remains in the original flask because it does not vaporise under these conditions. The separation works through a difference in volatility, meaning how readily a substance becomes vapour. Water vaporises readily here, whereas the dissolved salt does not. The thermometer bulb sits at the entrance to the side arm, where it measures the temperature of vapour entering the condenser.

4:42Cooling water enters at the lower connection and leaves at the upper connection. This keeps the cooling jacket filled. The receiving arrangement remains open, so the apparatus is not a sealed heated system. Separating ethanol and water presents a different problem because both liquids can vaporise. They mix together and have different boiling points. Fractional distillation separates mixed liquids using that difference. Its apparatus includes a fractionating column between the heated flask and the condenser.

5:10Ethanol has the lower boiling point, so it vaporises more readily than water. However, heating the mixture does not produce only ethanol vapour. Some water also enters the vapour, so a single evaporation and condensation would give a less effective separation. The column improves the process through repeated condensation and vaporisation. The fractionating column is hotter near the bottom and cooler near the top. As vapour rises, some condenses on the packing.

5:35The condensed liquid can vaporise again as it receives heat from below. During these repeated changes, the rising vapour becomes richer in the lower boiling liquid. Vapour reaching the top enters the condenser, where it becomes liquid and is collected. The first fraction is richer in ethanol. Later, as the composition changes and the vapour temperature rises, a different fraction can be collected separately. Do not assume that the first liquid collected must be completely pure.

6:00Simple distillation is suitable for collecting water from a solution containing a dissolved, nonvolatile solid. Fractional distillation is the appropriate choice for mixed liquids such as ethanol and water. The column matters because both components can enter the vapour. Choosing between these methods requires information about the mixture, not just the word liquid.

Worked separation of salt and sand

6:18Let us work through the salt and sand example as a complete separation. The question asks us to obtain dry sand and sodium chloride crystals from their mixture. Start with the property difference: sodium chloride dissolves in water, whereas sand does not. Add water and stir before attempting filtration. Filter the mixture and keep both products. The sand is the residue, so wash it with clean water and then dry it.

6:41The filtrate contains dissolved sodium chloride, so gently evaporate some water to reach the point of crystallisation. Allow it to cool, filter off the crystals and dry them. Each stage has a specific purpose. Dissolving makes the salt travel with the liquid, while filtration separates the insoluble sand. Crystallisation then recovers the dissolved solid. If the question instead required the water, simple distillation would replace crystallisation because we would need to collect the solvent.

Purity tests and exam techniques

7:06Melting point and boiling point information can help identify a substance and assess its purity. A pure substance has a sharp melting point and a sharp boiling point at a fixed pressure. Sharp means the change occurs at a definite temperature rather than across a broad interval. Compare the measured value with reference information. An impure solid usually melts at a lower temperature and over a range. Its particles no longer form the same uniform arrangement as the pure material.

7:33The impurities disrupt that arrangement, so melting begins more easily. A lowered, broadened melting interval is therefore evidence that a sample is impure. Mark schemes expect you to state that impurities raise the boiling point of a liquid. Dissolved nonvolatile impurities, such as salt in water, make it harder for the solvent to escape into the vapour. Mixtures of volatile liquids need more care. Ethanol and water cannot be treated as water containing a nonvolatile impurity.

7:58Suppose a sample begins melting below the reference temperature and finishes melting several degrees later. That pattern suggests an impure sample rather than a pure substance with a sharp transition. Matching a reference value supports identification, but does not prove identity by itself. Different substances may have similar melting or boiling points. Several common mistakes come from overlooking what remains dissolved. Filtering salt solution does not produce pure water because dissolved particles pass through the paper.

8:25Crystallisation should not be described simply as boiling to dryness. Also, the condenser changes vapour into liquid; it does not perform the initial boiling. Cooling water enters the condenser at the bottom, and the thermometer bulb sits at the side arm, never in the boiling liquid. In written examinations, you may be asked to suggest a suitable technique, describe its steps, or explain why it works. Practical papers can assess apparatus, heating, filtration, crystallisation and interpretation of observations.

8:53A describe question needs the actions in order, while an explain question links each action to the relevant physical property. An exam question might ask you to describe how to obtain pure water from sodium chloride solution. Use simple distillation. Heat the solution so that water vaporises. Pass the vapour through a cooled condenser, where it condenses. Collect the liquid distillate, while sodium chloride remains in the heated flask. The central idea is to match the method to a useful property difference.

9:20A suitable solvent separates substances through solubility, while filtration removes an insoluble solid. Crystallisation recovers a dissolved solid, and distillation collects a liquid through vaporisation followed by condensation. Sharp melting and boiling points support purity, giving you a way to check the result.