STUDENT SPACE · TOPIC 1

Solutions and purity

Distinguish solutions from suspensions and use melting data to assess purity.

The essentials

A solute dissolves in a solvent to form a solution.
A pure substance contains one substance; a mixture contains more than one.
At a stated pressure, a pure substance has a characteristic melting or boiling point. Impurities usually lower and broaden the melting range.

Try each question before opening its hint or answer. Use paper for calculations and diagrams.

A solid melts gradually from 61 °C to 66 °C. What does this suggest about its purity?

Need a hint for Question 1?

Compare a sharp melting point with a melting range.

Check Question 1

The melting range suggests the solid is impure. A pure solid normally melts sharply at its characteristic melting point.

Name the solute and solvent in aqueous copper(II) sulfate. Explain why the solution is a mixture.

Need a hint for Question 2?

Identify what dissolves and what does the dissolving.

Check Question 2

Solute: copper(II) sulfate. Solvent: water. The two substances are mixed without being chemically combined.

The reference melting point is 120 °C. Sample P melts sharply at 120 °C and Q melts at 112–116 °C. Which is more likely to be pure? Give two reasons.

Need a hint for Question 3?

Compare both the sharpness and temperature of melting.

Check Question 3

P. It melts sharply and matches the reference. Q melts over a lower temperature range.

Use Lesson 1 in your worksheet for written practice. Your teacher will discuss those answers in class.

PREDICT · TRY · EXPLAIN · 3–5 MINUTES

Choose a method before revealing the outcome. Explain which physical property makes it work.

Your task: collect dry sand.

Mixture
sand + water

→

Predict the collected substance

Explain: does your method depend on solubility, volatility or different boiling points? Schematic decision model; no real apparatus is operated.