Solution, colloid or suspension: it depends on particle size

The problem

Stir salt into one glass of water, starch into a second and chalk into a third. All three are mixtures of a solid in water. Leave them for an hour and pour each through filter paper, and they behave in three different ways.

True solution: the solute particles are completely homogenised in the solvent, as when sodium chloride or copper sulphate dissolves in water. You cannot see the particles, and they pass through the pores of filter paper, leaving no residue.

Suspension: the particles do not dissolve. You can see them, they settle down if the mixture is left standing, and filter paper holds them back as a residue. Chalk in water is a suspension.

Colloidal solution: the third form. The particles do not homogenise with the solvent; they are a little bigger than those of a true solution but still too small to see. They do not settle, and they pass through filter paper. Starch solution and the white of an egg are colloidal solutions.

Particle size sets the type. Smallest: true solution. In between: colloidal solution. Large enough to see: suspension, which settles and leaves a residue.

Key terms

True solution

Solute completely homogenised in the solvent: invisible, never settles, no residue.

Colloidal solution

Particles slightly bigger than a true solution's: invisible, do not settle, pass through filter paper.

Suspension

Particles that do not dissolve: visible, settle on standing, caught by filter paper.

Residue

The solid collected when particles are too big to pass through the paper's pores.

Short questions with model answers

1. Q1. Chalk is stirred into water. Classify the mixture and give two observations that support your answer.

   • Chalk does not dissolve, and its particles can be seen.
   • Left to stand, the chalk settles; filtered, it stays on the paper as a residue.

   A suspension: it settles on standing and leaves a residue on filtering.

2. Q2. Starch solution looks clear, never settles and passes through filter paper. Why is it still not a true solution?

   • Its particles do not homogenise with the water.
   • The particles are a little bigger than in a true solution, though still too small to see.

   It is a colloidal solution: its particles are bigger than a true solution's and not homogenised.

3. Q3. Three flasks hold copper sulphate solution, egg white in water and chalk in water. Each is filtered. Which leaves a residue?

   • Copper sulphate solution is a true solution, and egg white is a colloid.
   • Chalk in water is a suspension.

   Only the chalk in water.

Common mistakes

• ✗ Writing that the particles of a colloidal solution settle on standing.

  ✓ They do not settle. Only a suspension settles.

• ✗ Saying filter paper separates a colloid from water.

  ✓ Colloid particles pass through filter paper, just like a true solution's.

• ✗ Saying chalk dissolves in water because it spreads through the water when stirred.

  ✓ Spreading is not dissolving. The particles stay visible and settle, so it is a suspension.

MCQs

1. 1. Which liquid among the following is a colloidal solution?

   1. (a) Vinegar solution
   2. (b) Mixture of AgCl in water
   3. (c) Milk
   4. (d) Slaked lime used for whitewash
   Show answer

   (c) Milk is the book's answer (MCQ v, p. 14): its particles stay spread out and do not settle, like starch and egg white.

2. 2. When tiny visible particles of a substance are dispersed through a medium, the mixture is called:

   1. (a) Suspension
   2. (b) True solution
   3. (c) Colloid
   4. (d) Saturated solution
   Show answer

   (a) Visible is the key word: only suspension particles can be seen (MCQ vii, p. 14).

3. 3. Which mixture leaves a residue on filter paper?

   1. (a) Copper sulphate in water
   2. (b) Starch solution
   3. (c) Egg white in water
   4. (d) Chalk in water
   Show answer

   (d) Chalk forms a suspension, whose particles cannot pass through the pores. The other three pass through.

4. 4. If a colloidal solution is left standing for some time, its particles:

   1. (a) Settle at the bottom
   2. (b) Do not settle
   3. (c) Float to the top
   4. (d) Turn into a residue
   Show answer

   (b) Your book: the particles of a colloidal solution do not settle down (p. 10).

5. 5. In a true solution, the solute particles:

   1. (a) Can be seen with the naked eye
   2. (b) Are held back by filter paper
   3. (c) Are completely homogenised in the solvent
   4. (d) Settle at the bottom
   Show answer

   (c) That is the book's definition (p. 10). The other three describe a suspension.

Quick revision

• True solution: completely homogenised, invisible particles, no settling, no residue (salt or copper sulphate in water).
• Colloidal solution: slightly bigger particles, still invisible, no settling, pass through filter paper (starch, egg white, milk).
• Suspension: visible particles that settle and leave a residue (chalk in water).