Worked examples: Consumer and Industrial Chemistry

Work through each example by covering the answer, attempting it yourself, then checking your method against the solution. Every example flags the slip that most often costs marks. These are original questions in SPM style, not past-year papers. The cleaning-action example (Example 4) is the single most important, so rehearse it until every scoring point is automatic.

Example 1, Oil or fat, and the test for unsaturation

Question. Sample X is a liquid at room temperature and comes from a plant. Sample Y is a solid at room temperature and comes from an animal. State which is an oil and which is a fat, and describe a chemical test to confirm which contains more carbon-to-carbon double bonds.

Solution. Sample X is an oil (liquid, plant source, unsaturated) and sample Y is a fat (solid, animal source, saturated). To test, add bromine water to each. The oil (X) decolourises bromine water faster because its C=C double bonds react with bromine in an addition reaction, showing it is more unsaturated.

Common slip: saying the oil “reacts” without naming bromine water and the colour change. The observation, decolourising bromine water, carries the mark.

Example 2, Saponification

Question. A student boils palm oil with concentrated sodium hydroxide solution. Name this process, name the two products, and explain the purpose of adding sodium chloride at the end.

Solution. The process is saponification. The two products are soap (sodium salt of a fatty acid) and glycerol. Sodium chloride is added to salt out the soap: it lowers the solubility of the soap in the mixture so the soap separates and rises to the surface, where it can be collected.

Common slip: forgetting glycerol as the second product, or saying salt is added “to make it salty” instead of to reduce the soap’s solubility.

Example 3, Soap versus detergent in hard water

Question. Explain why soap does not lather well in hard water but a detergent does.

Solution. Hard water contains calcium and magnesium ions. Soap reacts with these ions to form an insoluble scum, so less soap is left to form lather. A detergent does not form scum because its calcium and magnesium salts are soluble, so it still lathers and cleans well in hard water.

Common slip: stating the result (“soap doesn’t work”) without the reason (scum forms because the calcium/magnesium salt of soap is insoluble). The reason is where the marks are.

Example 4, The cleaning action of soap (the key explanation)

Question. Using the structure of a soap molecule, explain how soap removes greasy dirt from cloth. [Full explanation]

Solution. A soap ion has two ends: a hydrophobic tail (a long hydrocarbon chain that dissolves in grease) and a hydrophilic head (the –COO⁻ end that dissolves in water).

• Soap first lowers the surface tension of the water so the water wets the cloth.
• The hydrophobic tails dissolve into the grease while the hydrophilic heads remain in the water.
• Agitation loosens the grease and breaks it into small droplets; each droplet is surrounded by soap ions, forming an emulsion.
• The droplets carry the same negative charge, so they repel each other and do not redeposit on the cloth.
• Rinsing washes the suspended grease away.

Common slip: naming only one end, or stopping at “the grease is removed” without the emulsion and the like-charge repulsion. Each labelled idea is a separate mark, so include them all.

Example 5, Choosing between soap and detergent

Question. A family lives in an area with very hard water. Recommend whether they should use soap or detergent for washing, and justify your answer with two reasons.

Solution. They should use a detergent. First, a detergent does not form scum with the calcium and magnesium ions in hard water, so it lathers and cleans effectively while soap would be wasted. Second, detergents contain builders (water softeners) that remove hardness ions, further improving cleaning. Therefore the detergent gives better, more economical washing in hard water.

Common slip: giving only one reason, or recommending soap “because it is natural” without addressing the hard-water problem in the question.

Example 6, Identifying a food additive

Question. A bottle of fruit juice lists ascorbic acid on its label. State the type of additive this is, its function, and one benefit to the consumer.

Solution. Ascorbic acid (vitamin C) is an antioxidant. Its function is to slow down oxidation, so the juice does not deteriorate or change colour as quickly. The benefit to the consumer is a longer shelf life and the juice keeping its taste and appearance.

Common slip: calling every additive a “preservative”. An antioxidant works by preventing oxidation specifically; name the correct type and its mechanism.

Example 7, Medicines and safe use

Question. A patient has a bacterial throat infection and is prescribed penicillin. State the class of medicine penicillin belongs to, why it will not help a cold caused by a virus, and one danger of not completing the course.

Solution. Penicillin is an antibiotic. It will not help a viral cold because antibiotics act on bacteria, not viruses. If the course is not completed, some bacteria may survive and multiply, and this can lead to antibiotic resistance, making the infection harder to treat.

Common slip: saying antibiotics “cure all infections”. Be precise: they act on bacteria, and misuse causes resistance.

Example 8, Modern materials, property to use

Question. Explain why a composite material such as fibreglass is chosen to make boat hulls rather than a single traditional material, and give a balanced comment on modern materials.

Solution. Fibreglass is a composite of glass fibres in a plastic (resin). It is chosen because it is strong yet light and does not rust or rot in water, combining the best properties of its two parts, properties that neither glass nor plastic has alone. A balanced comment notes that modern materials give better performance, but some are costly to produce or hard to recycle, so their use should weigh benefits against these drawbacks.

Common slip: listing a property with no link to the use, or giving a one-sided answer. Always connect the property to why it suits the job, and include both a benefit and a limitation when asked to discuss.

Using these examples

Notice that every answer either names the exact substance and its type, justifies a choice with a reason, or completes a mechanism to its final step, the three habits this chapter rewards. The cleaning-action explanation in Example 4 and the property-to-use answers in Examples 5 and 8 are where most marks are won or lost, so practise them until nothing is missing. A one-to-one teacher can check that your cleaning-action answer includes every labelled idea and that your discussions stay balanced, which is exactly the reasoning the SPM Chemistry written papers reward. Once the method feels automatic, move on to the practice questions and mark yourself the same way.