Mixtures, Solutions and Colloids: Quick Revision

A brief note on Mixtures, Solutions and Colloids. Read it before a test. Try each question at the end before you open its answer.

The big ideas

• A pure substance has one kind of matter and fixed properties, such as a sharp melting point. A mixture has two or more, in any amounts.
• A homogeneous mixture (a solution, an alloy, air) is the same all through. A heterogeneous one is not.
• A solution has a solute dissolved in a solvent. Its concentration says how much solute there is.
• A saturated solution can dissolve no more at that temperature. Solubility is the most that dissolves in $100\ \text{g}$ of water.
• Particle size decides between a solution, a colloid and a suspension.

The three kinds of mixture at a glance

A worked example

At $20\ ^\circ\text{C}$, $36\ \text{g}$ of salt dissolves in $100\ \text{g}$ of water. Find the concentration of $26\ \text{g}$ of salt in $174\ \text{g}$ of water. Then say what happens to $60\ \text{g}$ of salt stirred into $150\ \text{g}$ of water.

1. The concentration. The solution is $26\ \text{g} + 174\ \text{g} = 200\ \text{g}$, so $\dfrac{26\ \text{g}}{200\ \text{g}} \times 100 = 13\ \%$.
2. The most that dissolves in $150\ \text{g}$. $36\ \text{g} \times \dfrac{150\ \text{g}}{100\ \text{g}} = 54\ \text{g}$.
3. Left over. $60\ \text{g} - 54\ \text{g} = 6\ \text{g}$ stays undissolved: the solution is saturated.

Practice with new numbers

A program to try

The program works out the worked example.

Mistakes to avoid

• Dividing by the solvent alone. A percentage is out of the whole solution.
• Calling a clear liquid pure. Salt water is clear, and it is a mixture.
• Calling milk a solution. It is a colloid: a torch beam shows in it.
• Forgetting the temperature. A solution saturated when cold is not saturated when hot.

Check yourself

1. Is a steel spoon a pure substance?

2. $9\ \text{g}$ of sugar is dissolved in $141\ \text{g}$ of water. What is the mass by mass percentage?

3. Why does the fat in milk not settle, while mud in water does?

4. How can you show with a torch that a clear liquid is a colloid and not a solution?