2a, 2b, 2c & 2d – Testing for the Presence or Absence of Substances (LC 2027) (Leaving Cert Biology): Revision Notes

2a, 2b, 2c & 2d – Testing for the Presence or Absence of Substances

Introduction to qualitative testing

Qualitative testing in biology means checking whether a particular substance is present or absent in a sample. These tests are essential for identifying the chemical composition of foods and biological materials. Each test produces a characteristic colour change that indicates a positive or negative result.

Investigation 2a - Testing for starch

The iodine test for starch

The iodine test is the standard method for detecting starch in biological samples. This test works because iodine molecules fit into the spiral structure of starch, creating a distinctive blue-black complex.

Method and procedure

1. Prepare your sample: Use a dropper to add 2-3 drops of dilute iodine solution to either a liquid sample or a solid food sample
2. For liquid samples: Add the iodine directly to fruit juice, soft drinks, or other liquids in a test tube
3. For solid samples: Place the food item (bread, potato, rice, biscuit) on a protective surface like aluminium foil or plastic plate, then add the iodine drops
4. Control test: Always test water with iodine solution as a negative control

Results interpretation

• Positive result: The iodine solution changes from red-yellow to blue-black, indicating starch is present

• Negative result: The iodine solution remains red-yellow, indicating no starch is present

• Control: Water with iodine should remain red-yellow

  

Investigation 2b - Testing for reducing sugars

Benedict's or Fehling's test

This test detects reducing sugars like glucose, fructose, and maltose. The test works because these sugars can reduce copper ions in the reagent, causing a colour change when heated.

Method and procedure

1. Dissolve the sample: Place the food sample in water in a test tube
2. Add Benedict's solution: Mix equal volumes of the sample solution and Benedict's (or Fehling's) qualitative solution, which appears blue initially
3. Prepare control: In a second test tube, mix equal volumes of water and Benedict's solution
4. Heat carefully: Place both test tubes in a very hot water bath for 5 minutes
5. Observe results: Note any colour changes that occur during heating

Results interpretation

• Positive result: Solution changes from blue to red (often called brick-red), indicating reducing sugars are present
• Negative result: Solution remains blue, indicating no reducing sugars present
• Control: Water and Benedict's solution should remain blue

Investigation 2c - Testing for protein

Biuret test for protein

The Biuret reagent contains sodium hydroxide and copper sulphate. When protein is present, the reagent forms a complex with peptide bonds, producing a characteristic colour change.

Method and procedure

1. Dissolve the sample: Place the food sample in water in a test tube
2. Add Biuret reagent: Add an equal volume of Biuret reagent (containing sodium hydroxide and copper sulphate) - this appears blue initially
3. Prepare control: Mix equal volumes of water and Biuret reagent in a second test tube
4. Observe results: Look for colour changes without heating

Results interpretation

• Positive result: Solution turns purple-violet, indicating protein is present
• Negative result: Solution remains blue, indicating protein is absent
• Control: Water and Biuret reagent should remain blue

Investigation 2d - Testing for lipids

Brown paper test for lipids

This simple test works because lipids (fats and oils) create permanent, translucent stains on paper that don't disappear when the paper dries.

Method and procedure

1. Prepare samples: Take a piece of brown paper for each sample you want to test
2. Apply sample: Rub the food sample (like butter, oil, cheese) onto the brown paper
3. Control test: Rub a few drops of water on a second piece of brown paper
4. Allow to dry: Leave the pieces of paper to dry completely
5. Check results: Hold the papers up to the light to examine them

Results interpretation

• Positive result: A permanent translucent stain remains on the paper, indicating lipids are present
• Negative result: No stain remains on the paper, indicating lipids are absent
• Control: Water should leave no permanent stain