Testing the Purity of Water

Introduction

Food labels often call orange juice or spring water "pure", meaning nothing has been added. A chemist means something much stricter: a pure substance contains only one element or compound. Tap water, mineral water and even rain contain dissolved substances, so none of them is chemically pure. How can we test whether a water sample really is pure? Pure substances have sharp, fixed melting and boiling points, and this gives us simple, reliable tests.

Core explanation

Fixed points for pure water. At standard atmospheric pressure (101 kPa), pure water:

- boils at exactly 100 °C ; - freezes and melts at exactly 0 °C ; - boils and melts at a single, sharp temperature that stays constant while the change of state is happening.

How impurities change the results. Dissolving a substance such as salt in water has two effects:

- the boiling point rises above 100 °C; - the freezing point falls below 0 °C.

The more solute dissolved, the bigger the change. An impure sample also tends to boil or melt over a range of temperatures rather than at one sharp value, because the solution becomes more concentrated as water boils away or as ice forms.

Sample Boiling behaviour Freezing behaviour --- --- --- Pure (distilled) water sharp, 100 °C sharp, 0 °C Tap water very slightly above 100 °C very slightly below 0 °C Seawater about 100.6 °C, rising as it concentrates about −1.9 °C

Evaporating to dryness. A different test looks directly for dissolved solids. A small volume of the sample is heated gently in a clean evaporating basin until all the water has gone. Pure water leaves no residue . Tap water leaves a faint white ring; seawater leaves a thick crust of salt. Weighing the basin before and after can measure how much dissolved solid was present. Heating is done gently, towards the end over a water bath or with a low flame, so that the solid does not spit out, and eye protection is worn.

Pressure matters. Boiling points depend on air pressure. High in the mountains, pure water boils below 100 °C because the air pressure is lower. A fair purity test therefore compares the boiling point with the expected value at the local pressure.

Step-by-step reasoning

To decide whether a water sample is pure:

1. Measure its boiling point with a thermometer placed in the vapour and liquid as it boils. 2. If it boils sharply at 100 °C (at standard pressure), it is likely to be pure. 3. If it boils above 100 °C or over a range, dissolved impurities are present. 4. Confirm by evaporating a sample to dryness and checking for a residue.

Visual explanation

Picture a heating curve for water: temperature on the vertical axis, time on the horizontal. For pure water, the line rises, then becomes a perfectly flat plateau at 100 °C while it boils. For salt water, the line rises to just above 100 °C and then keeps creeping upwards, instead of staying flat.

Real-world analogy

A pure substance is like a well-trained choir that hits exactly the same note together every time. An impure sample is like a choir with some members singing slightly different notes — the sound spreads over a range instead of being one sharp tone.

Real-world example

Salt is spread on icy roads in winter because salt water freezes below 0 °C. Adding salt lowers the freezing point, so ice melts at road temperatures a few degrees below zero. The same effect is used in reverse by cooks who add salt to cooking water, although the rise in boiling point from the salt used in a kitchen is tiny.

Why?

Why does dissolved salt lower the freezing point? For water to freeze, water molecules must line up into the regular pattern of ice. Dissolved ions get in the way and reduce the proportion of water molecules at the surface of the liquid ready to join the solid, so the water must be cooled further before freezing can happen.

Common misconception

"Clear, colourless water is pure." Many dissolved substances, including salt, are invisible. Seawater filtered to remove sand is perfectly clear but contains about 35 g of salt per kilogram. Appearance alone never proves purity.

Worked example

Question: Three colourless samples are tested at standard pressure. A boils at 100 °C and leaves no residue. B boils at 100.4 °C and leaves a white residue. C boils at 78 °C and leaves no residue. Identify which is pure water.

Reasoning: A matches the fixed boiling point of pure water and contains no dissolved solid. B contains dissolved solids. C boils far too low to be water — it may be ethanol.

Answer: Sample A is pure water.

Quick check

1. What is the melting point of pure ice at standard pressure? Answer: 0 °C.

Exam focus

When asked how to show that a liquid is pure water, state "it boils at exactly 100 °C" or "it melts/freezes at exactly 0 °C", and mention standard pressure if possible. Do not rely on the anhydrous copper(II) sulfate test — it shows only that water is present. Remember that impurities raise boiling point and lower melting point.

Advanced insight

The size of the boiling-point rise and freezing-point fall depends on the number of dissolved particles, not their identity. One mole of sodium chloride splits into two moles of ions, so it lowers the freezing point about twice as much as one mole of sugar. Chemists use this to find the molar masses of unknown solutes.

Summary

In chemistry, pure water contains nothing but H₂O. At standard pressure it boils sharply at 100 °C and melts sharply at 0 °C. Dissolved impurities raise the boiling point, lower the freezing point and make the change happen over a range of temperatures. Evaporating a sample to dryness shows dissolved solids as a residue; pure water leaves nothing behind.

Practice questions

1. How does dissolving salt in water affect its boiling point and its freezing point? Answer: The boiling point rises above 100 °C and the freezing point falls below 0 °C. 2. A water sample is evaporated to dryness and leaves no residue. Does this prove the water is pure? Explain. Answer: Not completely; it shows there are no dissolved solids, but dissolved gases or volatile liquids would not leave a residue. A boiling-point test is also needed. 3. Why might pure water boil at 90 °C on a high mountain? Answer: Air pressure is lower at high altitude, so water boils at a lower temperature. 4. Why do councils spread salt on roads in icy weather? Answer: Salt dissolves in water and lowers its freezing point, so ice melts even when the temperature is slightly below 0 °C.