Explain that distilled water is used in practical chemistry rather than tap water because it contains fewer chemical impurities

Chemistry of the Environment – Water

Why Distilled Water? 🧪

Tap water is like a messy kitchen counter – it has all sorts of extra stuff on it.

In a chemistry lab we want a clean “bench” so that the reactions we observe are

only due to the chemicals we add.

Distilled water is produced by boiling water and then condensing the steam back

into liquid. This removes most of the dissolved solids (like calcium, magnesium,

chloride and sulphate ions) that are present in tap water.

Because of this, distilled water is the preferred solvent in many experiments

such as titrations, spectrophotometry and when preparing standard solutions.

Common Impurities in Tap Water 💧

  • Calcium ions – \$Ca^{2+}\$ (hardness)

  • Magnesium ions – \$Mg^{2+}\$ (hardness)

  • Chloride ions – \$Cl^-\$ (from disinfectants)

  • Sulphate ions – \$SO_4^{2-}\$ (from natural sources)

  • Trace metals – \$Fe^{3+}\$, \$Al^{3+}\$, etc.

Distillation Process

  1. Heat the tap water to its boiling point (\$100^\circ\$C at sea level).

  2. Steam rises and carries away volatile impurities.

  3. Steam condenses in a cooler part of the apparatus.

  4. Condensed liquid is collected – this is distilled water.

Comparison Table

ComponentTap Water (ppm)Distilled Water (ppm)
\$Ca^{2+}\$50–200< 1
\$Mg^{2+}\$20–100< 1
\$Cl^-\$10–50< 1
\$SO_4^{2-}\$5–50< 1

Exam Tips 📚

• Remember that distilled water is used to avoid interference from ions that can change the colour of indicators or alter the pH.

• When answering questions about titration accuracy, mention that using tap water can give a higher apparent concentration of the titrant.

• In your notes, write a quick mnemonic: Distilled = Devoid of Dirty ions.

• Practice drawing a simple distillation apparatus diagram (you can sketch it on paper) and label the key parts: boiler, condenser, collection flask.