Lesson 3.1.3.2

3.1.3.2 Properties of lipids and the emulsion test: AQA Biology, Unit 1

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson 3.1.3.2, Properties of lipids and the emulsion test: 20 multiple choice questions for the AQA Biology (7402), Unit 1: Biological molecules, written with Revision Ninja.

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The 20 questions

  1. Which two substances are used in the emulsion test for lipids, in order?

    • Biuret reagent, then sodium hydroxide
    • Water first, then iodine solution
    • Ethanol first, then water
    • Benedict's solution, then heat

  2. What is a positive result in the emulsion test?

    • A cloudy white emulsion forms
    • The solution turns blue-black
    • The solution turns purple
    • A brick-red precipitate forms

  3. Why is ethanol added before water in the emulsion test?

    • Ethanol reacts with any lipid present to produce a coloured compound that is easy to see by eye
    • Ethanol dissolves any lipid present, so the lipid can then form an emulsion when water is added
    • Ethanol removes the water from the sample, so that any lipid present is concentrated in the solution
    • Ethanol breaks the ester bonds in the lipid, so that the fatty acids are released and can be detected

  4. What is the main property of triglycerides that makes them suitable as waterproofing in animals?

    • They are hydrophobic and insoluble in water
    • They are hydrophilic and dissolve readily in water
    • They are charged and bind to water molecules
    • They are amphipathic and form micelles in water

  5. Triglycerides release more energy per gram than carbohydrates on oxidation. Which statement explains why?

    • Triglycerides contain more water, which releases heat on oxidation
    • Triglycerides have a higher proportion of C-H bonds and less oxygen than carbohydrates
    • Triglycerides contain more phosphate, which releases energy when hydrolysed
    • Triglycerides contain more glucose units, each of which releases more energy

  6. What role do phospholipids play in cells?

    • They carry genetic information from the nucleus to the ribosomes
    • They form the basic bilayer structure of cell-surface and organelle membranes
    • They store energy in large insoluble granules in the cytoplasm
    • They act as enzymes that catalyse the hydrolysis of glycosidic bonds

  7. Why are triglycerides good thermal insulators in animals?

    • They react with cold water to release heat for the body
    • They are hydrolysed at low temperature, which warms the surrounding tissue
    • They dissolve in the blood and carry heat to the skin
    • They are stored in adipose tissue, and their low thermal conductivity slows heat loss

  8. A sample is shaken with ethanol and then water, and the solution stays clear. What conclusion is correct?

    • Starch is present, which reduces cloudiness
    • No lipid is present in the sample
    • A reducing sugar is present in the sample
    • Lipid is present in high concentration

  9. What is the purpose of a control tube containing water in place of ethanol in an emulsion test?

    • To show that the water alone does not produce a cloudy emulsion
    • To increase the rate of reaction between the sample and water
    • To provide a second sample that is used to measure the lipid mass
    • To dissolve the lipid more effectively than ethanol can

  10. A student shakes a plant oil directly with water, without ethanol, and the layers separate quickly. What does this show?

    • Triglycerides are insoluble in water and do not form a stable emulsion without a solvent
    • Triglycerides dissolve completely in water, forming a clear solution that does not separate on standing
    • Triglycerides contain phosphate groups that react strongly with water, releasing a cloudy precipitate
    • Triglycerides are hydrolysed by water at room temperature, so the layers separate as the products form

  11. Why does a cloudy white emulsion form when water is added to an ethanol solution of lipid?

    • Lipid reacts with water to form a white precipitate
    • The water breaks the lipid into small monomers that are white with no exceptions
    • Lipid comes out of solution as tiny droplets that scatter light
    • Ethanol evaporates and leaves a white solid residue

  12. A fat releases about 37 kJ of energy per gram on complete oxidation. How much energy is released from 25 g of this fat?

    • 925 kJ
    • 740 kJ
    • 370 kJ
    • 1480 kJ

  13. Which property of a phospholipid makes it suitable as the main component of a cell membrane?

    • It is amphipathic, with a polar head and nonpolar tails
    • It has a high energy content, so it stores energy in the membrane
    • It is entirely nonpolar, so it does not interact with water
    • It is entirely polar, so it dissolves completely in the cytoplasm

  14. Why does a thick layer of fat give an animal buoyancy in water?

    • Fat reacts with water to produce heat, which lowers the body density
    • Fat dissolves in water and forms a low-density solution around the animal
    • Fat releases gas when hydrolysed, which fills the body with air
    • Fat has a lower density than water and is not dissolved by it

  15. Explain why the emulsion test is more reliable when a clean, dry test tube and equal sample volumes are used.

    • It stops the ethanol from evaporating, so the lipid is dissolved at a higher temperature during the test
    • It makes the water react chemically with the lipid, so that a different and more distinct colour is produced
    • It reduces variables such as contamination and dilution errors, so that any cloudiness reflects only the lipid present
    • It increases the concentration of lipid in the sample, so the emulsion produced becomes much more visible

  16. A student states that the emulsion test proves a sample is pure lipid. Which evaluation is correct?

    • Correct, because all substances that become cloudy when mixed with water are lipids and nothing else
    • Incorrect, because the emulsion test cannot detect any lipid at all, even when lipid is present in the sample
    • Correct, because the cloudy emulsion is only ever produced by a sample that consists of pure lipid alone
    • Incorrect, because the test shows that lipid is present but does not show that the sample is pure lipid

  17. Explain why lipids dissolve in ethanol but not in water.

    • Ethanol is charged and attracts lipid molecules strongly, whereas water repels every type of molecule
    • Ethanol and water are both strongly polar, so lipids cannot dissolve in either of them at all
    • Ethanol contains phosphate groups that bind to lipid molecules, whereas water has no electrical charge at all with no exceptions
    • Ethanol has a small nonpolar region as well as a polar group, whereas water is strongly polar and lipids are nonpolar

  18. Why do lipids produce more metabolic water than carbohydrates when oxidised in respiration?

    • Lipids contain more hydrogen atoms per gram, which combine with oxygen to form water
    • Lipids are hydrolysed to produce water directly as a product
    • Lipids contain more oxygen atoms per gram, which are released as water
    • Lipids contain phosphate groups that release water when broken down

  19. A student repeats the emulsion test on an olive oil sample and gets a cloudy emulsion, but on a second sample of sugar solution gets a clear solution. Which factor should be controlled to ensure the test is valid?

    • Equal volumes of sample and ethanol, and the same shaking time, in each test
    • The number of carbon atoms in the sugar, which must be identical to the oil with no exceptions
    • The colour of the test tube, which must always be red
    • The temperature of the water bath, which must be set above 100 C

  20. Which best explains why a phospholipid bilayer is impermeable to water-soluble ions but permeable to some small lipid-soluble molecules?

    • The fatty acid tails are charged, so they attract only water-soluble ions and let them cross the membrane with no exceptions
    • The nonpolar tails in the centre of the bilayer repel charged particles but allow nonpolar molecules to pass
    • The phosphate heads are nonpolar, so they allow only charged particles to cross the membrane easily
    • The glycerol backbone forms pores that let only lipid-soluble molecules pass through the membrane

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