9.2 Qualitative testing for biological molecules - lipids

Aims

• To identify the presence of lipids in liquid and solid samples

• To observe formation of a milky white emulsion as a positive result

Independent variable (what you change)

Type of sample (e.g. water, vegetable oil, sucrose solution, protein, cheese, bread, etc.)

Dependent variable (what you measure)

Appearance after test
(clear → cloudy/milky emulsion if lipid present)

Control variables (what you keep the same)

• Volume of distilled water (2 cm³)

• Amount of sample used

• Shaking method and time

Limitations

Subjective observation → “cloudy” vs “clear” can be interpreted differently

Contamination risk → glass rod or pipettes may transfer lipids between samples

Emulsion instability → may separate over time, affecting observation

Improvements

• Standardise sample size and mass (especially for solids)

• Grind solids more thoroughly for better lipid extraction

• Repeat tests and calculate a mean result

Ethanol

Dissolves lipids (lipids are soluble in ethanol)

Distilled water

• Causes lipids to precipitate out of ethanol

• Forms the milky emulsion if lipids are present

Black paper

Provides contrast to make the milky emulsion easier to see

Paper towels

Used to clean equipment and prevent contamination