Look at a glass of milk, a spoon of sugar and a piece of copper wire. To a chemist these are not all the same kind of thing. Matter is first sorted into pure substances and mixtures. A pure substance is made of only one kind of particle and has a fixed composition — for example, distilled water, pure gold or table salt (sodium chloride). A mixture contains two or more pure substances that are simply mixed together, not chemically combined, and that keep their own properties.
Homogeneous and heterogeneous mixtures
A homogeneous mixture has a uniform composition throughout — you cannot see the separate parts even under a microscope. Sugar dissolved in water, air and brass (copper + zinc) are homogeneous. A heterogeneous mixture has a non-uniform composition with visibly different parts, such as a mixture of sand and iron filings, or oil floating on water.
Solutions, solute and solvent
A solution is a homogeneous mixture of two or more substances. The component present in the larger amount is the solvent; the component dissolved in it is the solute. In sugar solution, water is the solvent and sugar is the solute. In a true solution the particles are smaller than 1 nm, so they never settle, cannot be filtered out and do not scatter light.
Concentration
The amount of solute in a given amount of solution is the concentration. Two common ways to express it are:
- Mass percentage = (mass of solute ÷ mass of solution) × 100.
- Volume percentage = (volume of solute ÷ volume of solution) × 100.
Saturation and solubility
A saturated solution is one in which no more solute can dissolve at a given temperature; an unsaturated solution can still dissolve more. The maximum mass of solute that dissolves in 100 g of solvent at a given temperature is its solubility. Solubility of most solids rises with temperature.
Suspensions and colloids
A suspension is a heterogeneous mixture with particles larger than 100 nm — they settle on standing, can be filtered and scatter light (e.g. chalk in water). A colloid has particles between 1 nm and 100 nm; it looks homogeneous but is actually heterogeneous (e.g. milk, fog). Colloid particles scatter a beam of light — the visible path of light is the Tyndall effect, seen when sunlight enters a dusty room. Colloids are named by the state of the dispersed phase and dispersion medium: an aerosol (liquid in gas, e.g. fog), foam (gas in liquid), emulsion (liquid in liquid, e.g. milk), sol (solid in liquid) and gel (liquid in solid, e.g. jelly).
A solution is made by dissolving 25 g of common salt in 100 g of water. Find the mass percentage of the salt in the solution.
Solution- Mass of solute (salt) = 25 g; mass of solvent (water) = 100 g.
- Mass of solution = solute + solvent = 25 + 100 = 125 g.
- Mass % = (mass of solute ÷ mass of solution) × 100 = (25 ÷ 125) × 100.
- = 20%.
Answer: The mass percentage of salt is 20%.
Calculate the mass of glucose and water needed to prepare 250 g of a 4% (by mass) glucose solution.
Solution- Mass % = (mass of solute ÷ mass of solution) × 100, so mass of glucose = (4 ÷ 100) × 250.
- = 10 g of glucose.
- Mass of water = mass of solution − mass of solute = 250 − 10.
- = 240 g of water.
Answer: 10 g of glucose dissolved in 240 g of water.
40 mL of ethanol is mixed with water to make 200 mL of solution. Find the volume percentage of ethanol.
Solution- Volume of solute (ethanol) = 40 mL; volume of solution = 200 mL.
- Volume % = (volume of solute ÷ volume of solution) × 100.
- = (40 ÷ 200) × 100 = 20%.
Answer: The volume percentage of ethanol is 20%.
At 20 °C, 36 g of potassium nitrate dissolves in 100 g of water to give a saturated solution. State the solubility of potassium nitrate at this temperature, and decide whether adding 5 g more at the same temperature will dissolve.
Solution- Solubility = maximum mass of solute per 100 g of solvent at that temperature = 36 g per 100 g water.
- The solution is already saturated, so it can hold no more at 20 °C.
- The extra 5 g will not dissolve and will settle at the bottom (unless the temperature is raised).
Answer: Solubility = 36 g per 100 g water; the extra 5 g stays undissolved.
Classify each as a true solution, suspension or colloid: (a) salt dissolved in water, (b) muddy river water, (c) milk.
Solution- Salt in water is clear, does not settle, cannot be filtered — particles below 1 nm, so it is a true solution.
- Muddy water has large visible particles that settle and can be filtered — a suspension.
- Milk looks uniform but shows the Tyndall effect and particles are 1–100 nm — a colloid (emulsion).
Answer: (a) true solution, (b) suspension, (c) colloid.
Why is a beam of light visible when it passes through a colloid but not through a true solution?
Solution- Colloidal particles are large enough (1–100 nm) to scatter the rays of light passing through.
- This scattering makes the path of the beam visible — the Tyndall effect.
- In a true solution the particles are far too small (below 1 nm) to scatter light, so no path is seen.
Answer: Colloid particles scatter light (Tyndall effect); true-solution particles are too small to scatter it.