AQA GCSE Biology (8461) — Biological Organisation
Have you ever wondered how nutritionists know exactly how much sugar is in your favourite snack? They use specific chemical tests to find out what nutrients are hiding inside.
A qualitative test is a chemical test used to identify the presence of a substance, rather than its exact numerical amount. Before performing these tests, you must prepare a liquid food sample.
First, break the food up using a pestle and mortar. Next, mix it with distilled water and stir with a glass rod to dissolve the nutrients. Finally, for most tests, filter the mixture using a funnel and filter paper to remove solid bits.
Crucially, you must not filter the mixture for lipid tests. Lipid molecules are large and can get trapped by the filter paper, which would lead to a false negative result.
To test for a reducing sugar (such as glucose), add about 10 drops of Benedict's reagent to 5 cm³ of your filtered sample. You must heat this in a water bath set to 75°C - 80°C for 5 minutes. The blue solution will change through green, yellow, and orange, eventually forming a brick-red precipitate (a solid that settles out of a liquid) if a high concentration of sugar is present.
To test for starch, add a few drops of iodine solution to the filtered sample. If starch is present, the initial orange-brown colour will turn blue-black.
To test for proteins, add an equal volume of Biuret reagent (a mixture of sodium hydroxide and copper(II) sulfate) to your sample at room temperature. A positive result changes the solution from blue to a purple, lilac, or mauve colour.
There are two ways to test for lipids. For the ethanol test, add 2 cm³ of ethanol to the unfiltered sample, shake to dissolve the lipids, and pour it into an equal volume of distilled water. A positive result forms a cloudy, milky-white emulsion (a mixture of immiscible liquids). Alternatively, add Sudan III stain to the unfiltered sample; if lipids are present, the mixture separates and the top layer is stained bright red.
If your stomach acid backed up into your mouth, your saliva would completely stop working! This is because the enzymes in your saliva only function correctly at a very specific pH.
Amylase is a digestive enzyme that catalyses the breakdown of starch into maltose. It has an optimum pH (approximately pH 7 for salivary amylase) where it works fastest.
If the pH is too high or too low, the change in hydrogen ions affects the charges on the amino acids, disrupting the hydrogen and ionic bonds in the protein's structure. This alters the shape of the active site, preventing the substrate from binding. The enzyme has undergone denaturation and the reaction stops.
To investigate how pH affects amylase, you use a continuous sampling method. First, set up a water bath at 35°C–37°C so all solutions can reach the same temperature before mixing.
Mix the amylase and starch with a specific buffer solution (a liquid that resists changes in pH) to keep the environment stable. Start a stopwatch as soon as they are mixed.
Every 30 seconds, take a drop of the reaction mixture and add it to a drop of iodine solution in a spotting tile. The reaction is complete when the iodine remains orange-brown and no longer turns blue-black. Remember to rinse your glass rod between every sample to prevent cross-contamination.
How do we turn a recorded time into a measurable speed of reaction? We calculate the rate!
The faster an enzyme works, the shorter the time it takes for the starch to disappear. Therefore, rate is inversely proportional to time.
Question: In an experiment at pH 7, the iodine solution remained orange-brown after 120 seconds. Calculate the rate of reaction.
Step 1: Write down the formula for rate.
Step 2: Substitute the known value. Ensure time is always in seconds.
Step 3: Calculate the final answer with units.
(Note: You can also use to produce easier-to-plot arbitrary units: )
Students often say that extreme pH or high temperatures 'kill' enzymes. Enzymes are proteins, not living organisms, so you must use the scientific term 'denatured'.
In 6-mark practical methodology questions for food tests, examiners expect you to state the initial colours of the reagents (e.g., 'Benedict's starts blue and turns brick-red') to secure full marks.
Do not filter the food sample when testing for lipids, as the large lipid molecules will get trapped in the filter paper and cause a false negative result.
Always specify 'iodine solution' rather than just 'iodine' in your exam answers.
Always ensure your time is converted into seconds before calculating the rate of reaction using the 1/time formula.
Qualitative test
A chemical test used to identify the presence of a substance rather than its exact numerical amount.
Reducing sugar
A sugar that can donate electrons to Benedict's reagent, causing a color change and the formation of a precipitate.
Benedict's reagent
A blue chemical reagent used to test for the presence of reducing sugars, which requires heating.
Precipitate
A solid that forms and settles out of a liquid mixture during a chemical reaction.
Iodine solution
An orange-brown solution used to test for the presence of starch.
Biuret reagent
A blue mixture of sodium hydroxide and copper(II) sulfate used to test for proteins.
Emulsion
A mixture of immiscible liquids where one is dispersed as droplets in the other, such as fat in water.
Sudan III stain
A fat-soluble red dye used to test for the presence of lipids.
Amylase
A digestive enzyme produced in salivary glands and the pancreas that breaks down starch into maltose.
Optimum pH
The specific pH level at which an enzyme's activity is at its highest.
Active site
The region of an enzyme that is complementary in shape to a specific substrate where the reaction occurs.
Denaturation
A permanent change in the shape of an enzyme's active site (due to extreme pH or temperature) that prevents the substrate from binding.
Continuous sampling
A method where samples are taken from a reaction mixture at fixed time intervals to monitor the progress of a reaction.
Buffer solution
A solution that resists changes in pH, used to keep the environment stable during an enzyme experiment.
Put your knowledge into practice — try past paper questions for Biology
Qualitative test
A chemical test used to identify the presence of a substance rather than its exact numerical amount.
Reducing sugar
A sugar that can donate electrons to Benedict's reagent, causing a color change and the formation of a precipitate.
Benedict's reagent
A blue chemical reagent used to test for the presence of reducing sugars, which requires heating.
Precipitate
A solid that forms and settles out of a liquid mixture during a chemical reaction.
Iodine solution
An orange-brown solution used to test for the presence of starch.
Biuret reagent
A blue mixture of sodium hydroxide and copper(II) sulfate used to test for proteins.
Emulsion
A mixture of immiscible liquids where one is dispersed as droplets in the other, such as fat in water.
Sudan III stain
A fat-soluble red dye used to test for the presence of lipids.
Amylase
A digestive enzyme produced in salivary glands and the pancreas that breaks down starch into maltose.
Optimum pH
The specific pH level at which an enzyme's activity is at its highest.
Active site
The region of an enzyme that is complementary in shape to a specific substrate where the reaction occurs.
Denaturation
A permanent change in the shape of an enzyme's active site (due to extreme pH or temperature) that prevents the substrate from binding.
Continuous sampling
A method where samples are taken from a reaction mixture at fixed time intervals to monitor the progress of a reaction.
Buffer solution
A solution that resists changes in pH, used to keep the environment stable during an enzyme experiment.
