CCEA GCSE Biology 1.3 Nutrition and Food Tests Exam Questions

About CCEA GCSE Biology 1.3 Nutrition and Food Tests

Nutrition and Food Tests is a short but highly examined topic in CCEA GCSE Biology Unit 1. You must know the reagents, methods and positive results for the main food tests, and how large biological molecules are built from smaller units. The topic also includes the prescribed practical in which food is burned to heat water so its energy content can be estimated. It links to enzymes and digestion, where large molecules are broken into small soluble ones, and to photosynthesis, where starch is stored. Examiners frequently ask for colour changes, energy calculations and evaluation of why the burning food method underestimates the true energy content.

What you need to know

• Starch test Add a few drops of iodine solution. A blue-black colour shows starch is present; if not, the solution stays orange-brown.
• Reducing sugar test Add Benedict's solution and heat in a water bath at about 80 °C. A positive result changes from blue to green, yellow, orange or brick-red depending on how much reducing sugar (such as glucose) is present.
• Protein and fat tests Biuret test: add biuret reagent (sodium hydroxide and copper sulfate); a purple colour shows protein. Ethanol emulsion test: shake the food with ethanol, then add water; a cloudy white emulsion shows fat.
• Biological molecules Carbohydrates such as starch, glycogen and cellulose are made of many glucose units. Proteins are made of chains of amino acids. Fats (lipids) are made of fatty acids and glycerol. These large molecules must be digested before absorption.
• Prescribed practical: energy in food Burn a weighed food sample under a boiling tube containing a measured volume of water and record the temperature rise. Energy (J) = mass of water (g) x 4.2 x temperature rise (°C), using 1 cm3 of water = 1 g. Divide by the mass of food to get energy per gram.
• Evaluating the method The value is lower than the true energy content because heat is lost to the air and the boiling tube, and the food may not burn completely. Repeating, shielding the apparatus and keeping the distance between food and tube constant improve reliability.

Common mistakes in 1.3 Nutrition and Food Tests

• Forgetting to heat the Benedict's test. It needs a hot water bath; the biuret test and iodine test do not.
• Calling the positive Benedict's colour red or pink. Use brick-red (or orange, yellow or green for smaller amounts).
• Saying the iodine result is black. The correct description is blue-black.
• Using the mass of food instead of the mass of water in the energy equation. The 4.2 J needed per gram per °C refers to the water being heated.
• Explaining the low result only as human error. Give the real reasons: heat loss to the surroundings and incomplete burning.

Try three 1.3 Nutrition and Food Tests exam-style questions