Why Distilled Water Does Not Conduct Electricity | Class 10

Quick answer: A liquid conducts electricity only if it contains free ions. Distilled water has almost no ions, so it practically does not conduct. Rainwater dissolves gases such as $\mathrm{CO_2}$ from the air, forming acids that release ions, so it conducts. Glucose and alcohol contain hydrogen but do not release $\mathrm{H^+}$ ions in water, so their solutions do not conduct and are not acidic.

What makes a liquid conduct electricity?

Key idea: A solution conducts electricity only when it contains ions that can move towards the electrodes. No ions means no current, and the bulb in the circuit does not glow.

Why distilled water does not conduct electricity

• Distilled water is made of neutral $\mathrm{H_2O}$ molecules and contains no dissolved salts or acids.
• Water does split into ions very slightly ($\mathrm{H_2O \rightleftharpoons H^+ + OH^-}$), but the amount is tiny: in pure water at 25 °C, only about 1 molecule in 550 million is ionised.
• So distilled water has almost no free ions and is a very poor conductor. In the school bulb experiment, the bulb does not glow.

Why rainwater conducts electricity

As raindrops fall, they dissolve gases from the air, mainly carbon dioxide (and, in polluted air, sulphur dioxide and nitrogen oxides). These form weak acids.

Step 1: Carbon dioxide dissolves to form carbonic acid:

$\mathrm{CO_2(g) + H_2O(l) \rightleftharpoons H_2CO_3(aq)}$

Step 2: Carbonic acid releases ions:

$\mathrm{H_2CO_3(aq) \rightleftharpoons H^+(aq) + HCO_3^-(aq)}$

Because rainwater contains these ions (and often some dissolved salts), it conducts electricity. The conduction is weak compared with a strong acid, but it is much more than distilled water.

Why glucose and alcohol are not acids

Glucose ($\mathrm{C_6H_{12}O_6}$) and ethanol ($\mathrm{C_2H_5OH}$) both contain hydrogen. Yet:

1. Their hydrogen atoms are held by covalent bonds and are not released as $\mathrm{H^+}$ ions in water.
2. They dissolve as whole, neutral molecules, so their solutions contain no ions.
3. So their solutions do not conduct electricity and do not turn blue litmus red.

Remember: All acids contain hydrogen, but not every compound containing hydrogen is an acid. An acid must produce $\mathrm{H^+}$ ions (as $\mathrm{H_3O^+}$) in water.

Dry HCl gas vs hydrochloric acid

Dry HCl gas does not change the colour of dry blue litmus paper, but it turns moist blue litmus paper red.

• Hydrogen chloride is a covalent gas. Without water it does not form ions.
• In water, it forms hydronium ions:

$\mathrm{HCl + H_2O \longrightarrow H_3O^+ + Cl^-}$

• Acidic behaviour needs $\mathrm{H^+}$ ions, and these exist in solution only as $\mathrm{H_3O^+}$. So acids show their properties only in the presence of water.

 Dry HCl + dry litmus   → no change
 Dry HCl + moist litmus → turns red

Summary table

Exam tips

• For "why does distilled water not conduct but rainwater does?", give two points: distilled water has no ions; rainwater has dissolved acids or salts that form ions.
• Write the equation $\mathrm{CO_2 + H_2O \to H_2CO_3}$ for rainwater.
• For the glucose/alcohol question, say clearly that they do not ionise to give $\mathrm{H^+}$ ions.
• For dry HCl, write the hydronium ion equation and say acids need water to show acidic behaviour.

Common mistakes

• Saying distilled water conducts well. It has almost no ions.
• Writing that glucose is acidic because it contains hydrogen.
• Writing $\mathrm{H_2CO_3 \to 2H^+ + CO_3^{2-}}$ as the main reaction. Carbonic acid mainly gives $\mathrm{H^+}$ and $\mathrm{HCO_3^-}$.
• Forgetting that dry litmus and dry HCl gas show no change; moisture is needed.

Frequently Asked Questions

Why does distilled water not conduct electricity?

Distilled water contains almost no ions. Without free ions there is nothing to carry the electric charge, so it practically does not conduct.

Why does rainwater conduct electricity?

Rainwater dissolves carbon dioxide and other gases from the air. These form acids that release ions, and the ions carry the current.

Why do glucose and alcohol not show acidic character?

Although they contain hydrogen, glucose and alcohol do not release H⁺ ions in water. Without H⁺ ions their solutions are not acidic and do not conduct electricity.

Why does dry HCl gas not change the colour of dry litmus paper?

Dry HCl gas does not form H⁺ ions without water. Acidic behaviour needs H⁺ (as H₃O⁺) ions, which form only when HCl dissolves in water.

What is a hydronium ion?

A hydronium ion, H₃O⁺, forms when an H⁺ ion joins a water molecule. H⁺ ions cannot exist on their own in water, so they are present as H₃O⁺.