Revision notes: Consumer and Industrial Chemistry

These notes work through the Consumer and Industrial Chemistry chapter one content standard at a time, so you can revise each part and check you have the exact wording the marking scheme expects. This chapter rewards two skills above all: explaining the cleaning action of soap using both ends of the molecule, and giving balanced, property-to-use answers about everyday materials. Keep these notes beside past-paper questions and use them to confirm your answers.

13.1 Oils, fats and their properties

Oils and fats are esters formed from glycerol and long-chain carboxylic acids called fatty acids; each molecule is a triester known as a triglyceride. The exam wants you to separate them clearly.

• Fats are usually solid at room temperature, come mainly from animals, and their fatty acid chains are saturated, they contain only single carbon-to-carbon bonds.
• Oils are usually liquid at room temperature, come mainly from plants, and their chains are unsaturated, they contain one or more carbon-to-carbon double bonds (C=C).

Key idea. Unsaturation lowers the melting point. The double bonds put kinks in the chains, so the molecules pack together less closely, the forces between them are weaker, and less heat is needed to melt them. That is why an unsaturated oil is liquid while a saturated fat is solid.

You should also know the test for unsaturation: add bromine water. An unsaturated oil decolourises bromine water because the C=C bonds react with bromine in an addition reaction, while a saturated fat does not decolourise it (or does so only very slowly). This is a favourite short question.

13.2 Soaps and detergents

Soap is the sodium (or potassium) salt of a long-chain fatty acid, written in general as RCOO⁻Na⁺. It is made by saponification: heating an oil or fat with a hot concentrated alkali such as sodium hydroxide. The products are soap and glycerol. In the laboratory the soap is separated out by adding sodium chloride, this is salting out, which lowers the solubility of the soap so it rises to the surface.

A detergent is the sodium salt of a sulphonic acid derivative, for example a sodium alkyl sulphate or a sodium alkylbenzene sulphonate. It is made from a hydrocarbon from petroleum, not from an oil or fat, and it does not involve saponification.

Soap versus detergent in hard water. Hard water contains calcium and magnesium ions. Soap reacts with these ions to form an insoluble scum, so it lathers poorly and wastes soap. A detergent does not form scum because its calcium and magnesium salts are soluble, so it still cleans well in hard water. This comparison is examined almost every year, learn the reason, not just the result.

Detergents also contain additives with specific jobs, and you should be able to name them and their functions:

• Biological enzyme, breaks down protein or fat stains at lower temperatures.
• Whitening (optical brightener) agent, absorbs ultraviolet light and gives out blue light so white cloth looks whiter.
• Water softener / builder (for example sodium tripolyphosphate), removes calcium and magnesium ions so the detergent works better.
• Suspension agent (for example sodium carboxymethylcellulose), keeps lifted dirt suspended so it is not redeposited on the cloth.
• Drying agent (for example sodium sulphate), keeps the powder dry and free-flowing.
• Fragrance and colouring, make the product pleasant to use.

13.3 Cleaning action of soaps and detergents

This is the most important explanation in the chapter, so learn it as a labelled sequence. A soap or detergent ion has two ends:

• a hydrophobic tail, a long non-polar hydrocarbon chain that dissolves in oil and grease but not in water;
• a hydrophilic head, the ionic end (–COO⁻ for soap, –SO₃⁻ for detergent) that dissolves in water.

The mechanism, step by step.

1. The soap lowers the surface tension of the water so the water wets the greasy surface of the cloth.
2. The hydrophobic tails dissolve into the grease while the hydrophilic heads stay in the water.
3. Agitation (scrubbing or the washing machine) pulls the grease off the cloth and breaks it into small droplets.
4. Each droplet is surrounded by soap ions with their heads pointing outwards, forming an emulsion suspended in the water.
5. The droplets carry the same negative charge, so they repel one another and cannot join back together or redeposit on the cloth.
6. Rinsing carries the suspended grease away with the water.

Every underlined idea, two ends, lower surface tension, emulsion, same charge repelling, is a scoring point. Miss one and you lose a mark; this is exactly where a one-to-one teacher can check your explanation is complete.

13.4 Food additives

A food additive is a substance added to food to improve it in some way. Know the main types with their function and one example each:

• Preservative, slows the growth of microbes so food lasts longer (for example sodium benzoate).
• Antioxidant, slows oxidation so fats and oils do not turn rancid, and cut fruit does not brown (for example ascorbic acid, vitamin C).
• Flavouring / flavour enhancer, improves or strengthens taste (for example monosodium glutamate).
• Colouring (dye), restores or adds colour to make food attractive.
• Stabiliser and thickener, keeps a mixture uniform and gives the right texture.
• Sweetener, adds sweetness, sometimes with fewer calories.

Balanced view. Additives bring real benefits, longer shelf life, safer food, better appearance, but excessive or unsuitable additives may harm health, so their use is regulated. A good exam answer gives both sides, not only the dangers.

13.5 Medicines and cosmetics

A medicine is a substance used to prevent, treat or cure illness. Know the main classes and what each does:

• Analgesics relieve pain, for example aspirin and paracetamol for mild pain, and stronger opiates such as codeine for severe pain.
• Antibiotics kill or stop the growth of bacteria, for example penicillin. They do not work against viruses.
• Psychotherapeutic medicines act on the mind, stimulants, antidepressants and sedatives, and must be taken only as prescribed.
• Traditional medicines from plants and other natural sources are also used, but their doses are less standardised.

Correct use. Medicines must be taken at the right dose and for the right length of time. Misuse, wrong dose, sharing prescriptions, or not finishing a course of antibiotics, can cause harm and resistance. Say this clearly if the question asks about the dangers of misusing medicine.

A cosmetic is a substance applied to the body to clean, protect or improve appearance, such as skin cream, sunscreen, deodorant, shampoo and lipstick. Understand a few functions and the idea that some ingredients can cause side effects: for example, harmful substances such as mercury or unregulated skin-lightening agents can damage skin and health, which is why cosmetics are regulated. Again, a balanced answer names a benefit and a risk.

13.6 Modern materials and nanotechnology

Modern materials are engineered to have properties ordinary materials lack. Know one use that follows from each property:

• Superconductors carry electricity with no resistance when very cold, used in powerful magnets and fast transport.
• Ceramics are hard, heat-resistant and chemically stable, used for cutting tools, engine parts and electrical insulators.
• Composite materials combine two materials to gain the best of both, fibreglass, reinforced concrete and photochromic glass are examples; each is strong, light or responsive in a way its parts alone are not.
• Fibre optics carry data as light, used in fast communication.

Nanotechnology works with matter at the scale of nanometres, where materials behave differently. Nanoparticles have a very large surface area for their size, so they are useful as efficient catalysts, in stronger and lighter materials, in targeted medicine and in self-cleaning or water-repellent coatings. The syllabus expects a balanced view here too: alongside the benefits, note that the health and environmental effects of some nanomaterials are not yet fully understood, so their use should be careful and researched.

How to use these notes

Revise one content standard at a time and test yourself by explaining it aloud. For 13.1, be able to link unsaturation to a lower melting point and to the bromine-water test. For 13.2, write out saponification and the soap-versus-detergent-in-hard-water comparison with reasons. For 13.3, rehearse the cleaning-action sequence until every scoring point comes automatically, this single explanation earns the most marks in the chapter. For 13.4 to 13.6, practise property-to-use answers and always give a balanced, two-sided discussion. Then attempt the practice questions for this chapter and mark your answers against the wording above.

Because so much of this chapter is about explaining and discussing rather than calculating, the marks go to precise terms and complete reasoning. Learn the vocabulary in the key-terms list, keep every safety and health point balanced, and this becomes one of the most reliable scoring chapters in the whole of SPM Chemistry.