IGCSE · Practice questions · Food tests and the structure of DNA

Reagent, method, colour.

Six original Cambridge-style questions on the five food tests, comparing vitamin C content with DCPIP and (Extended) the double helix and its base pairs.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Score every mark

Name the reagent, the method and both colours.

Starch: iodine, orange-brown to blue-black. Protein: biuret, blue to purple.
Reducing sugar: Benedict's, heat in a water bath, blue to green, yellow, orange or brick red.
Fat: shake with ethanol, pour into water, cloudy white emulsion. Vitamin C: DCPIP, blue to colourless; fewer drops means more vitamin C.
Extended DNA: two strands, double helix, bases, bonds between base pairs, A with T and C with G.

Recall

[4 marks]

Describe how you would test a sample of fruit juice for reducing sugar, and state the result you would see if a lot of reducing sugar were present.

Add an equal volume of Benedict's solution (blue) to the juice in a test tube. ✓
Heat the tube in a hot water bath (about 80 °C) for a few minutes. ✓
A lot of reducing sugar gives a brick red colour (precipitate). ✓
Smaller amounts give green, yellow or orange; with no reducing sugar it stays blue. ✓

Recall

[3 marks]

Describe the ethanol emulsion test for fats and oils and state a positive result. Give one safety precaution.

Shake (or crush) the food with ethanol so that any fat dissolves. ✓
Pour the ethanol into water (or add water to it): a cloudy white emulsion shows that fat is present. ✓
Ethanol is flammable, so keep it away from any flame (and wear eye protection). ✓

Data

[4 marks]

A student tested three foods. The table shows the results.

Food	Iodine	Benedict's (heated)	Biuret	Ethanol emulsion
X	blue-black	blue	blue	clear
Y	orange-brown	brick red	blue	clear
Z	orange-brown	yellow	purple	cloudy white

State which nutrients each food contains and suggest which food could be milk.

X contains starch only (blue-black with iodine; all other tests negative). ✓
Y contains a lot of reducing sugar only (brick red with Benedict's). ✓
Z contains some reducing sugar (yellow), protein (purple) and fat (cloudy white emulsion), but no starch. ✓
Z could be milk, which contains a sugar, protein and fat. ✓

Calculation

[3 marks]

A 0.1% vitamin C solution decolourised 1 cm³ of DCPIP after 6 drops. A sample of pineapple juice decolourised 1 cm³ of the same DCPIP after 15 drops. Calculate the vitamin C concentration of the pineapple juice.

Concentration of juice = 0.1% × (drops of standard ÷ drops of juice). ✓
= 0.1 × 6 ÷ 15 ✓
= 0.04%. ✓

Explain

[3 marks]

A student found that freshly squeezed orange juice decolourised DCPIP after 6 drops, but the same juice after boiling for ten minutes needed 18 drops. Explain this result and state two variables the student should keep the same.

Boiling destroys (breaks down) vitamin C, so each drop of boiled juice contains less vitamin C and more drops are needed. ✓
Keep the same volume of DCPIP (1 cm³) and the same concentration of DCPIP. ✓
Also acceptable: the same size of drop (same pipette), shaking after each drop, the same end point (blue just disappears). ✓

Analysis

[4 marks]

Extended. (a) Describe the structure of a DNA molecule. (b) In a sample of DNA, 30% of the bases are A. State the percentage of the bases that are T and the percentage that are C.

(a) Two strands coiled together to form a double helix; each strand contains bases. ✓
Bonds between pairs of bases hold the two strands together; A always pairs with T and C with G. ✓
(b) Every A is paired with a T, so T = 30%. ✓
That leaves 100 − 60 = 40% for C and G, which are equal in number, so C = 20%. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.