Which of the following mixtures is an example of a colloid?

1. Analyze the question You're asked to pick the mixture that is a colloid. So we need to know how a colloid differs from a solution and a suspension.

2. Explain the concept

• Solution (true solution): Particles are very small (less than 1 nm). They are completely dissolved, do not settle, and do not scatter light. The mixture is clear and homogeneous. Examples: saltwater, sugar in water.
• Colloid: Particles are intermediate in size (about 1 nm to 1000 nm). They do not settle on standing, cannot be separated by ordinary filtration, and scatter light (the Tyndall effect). Examples: milk, fog, smoke, blood, paint, starch paste, mayonnaise.
• Suspension: Particles are large (above 1000 nm). They settle on standing and can be filtered. Example: muddy water, chalk in water.

3. Solve step by step

• C. Saltwater: The ions of NaCl dissolve completely. This is a true solution, so it is not a colloid.
• D. Sugar dissolved in water: The sugar molecules disperse fully. This is also a true solution, so it is not a colloid.
• B. Orange juice: Freshly squeezed juice contains pulp, which makes it partly a suspension. Its sugars and acids are dissolved. It is generally treated as a solution/suspension mixture and is not the standard textbook colloid example.
• A. Milk: Tiny droplets of fat and particles of protein (casein) are dispersed in water. They are too small to settle out or be filtered, but large enough to scatter light, which is why milk looks cloudy. Milk is an emulsion, a type of colloid (liquid dispersed in liquid).

4. Answer The correct option is A. Milk.

5. Exam tip Remember the common colloid examples: milk, blood, fog, smoke, paint, mayonnaise, starch. If the mixture is clear and the solute completely disappears (salt, sugar), it is a solution. If it is cloudy but does not settle, it is likely a colloid. Also remember the Tyndall effect: a colloid scatters a beam of light, while a true solution does not.