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Investigation of Acids X & Y

IGCSE Chemistry · Virtual Lab
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IGCSE Chemistry · Investigation of Acids X & Y
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🔵 Acid X (Strong)

🟠 Acid Y (Weak)

🧪 X1 Drops: 0 🧪 Y1 Drops: 0
📋 Observation Table
Test Acid X Observations Acid Y Observations
Universal indicator
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+ Na₂CO₃ (aq)
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+ NaOH (aq)
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+ Mg ribbon
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Methyl orange
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Blue litmus
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📊 Neutralisation Chart

Plot the drops of Na₂CO₃ required to neutralize the acids.

🧠 IGCSE Concept Check
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What this experiment shows

Strength and concentration are different ideas, and this is the experiment that separates them. A strong acid is fully ionised in water; a weak acid is only partly ionised. Take two acids of the same concentration and the strong one has more hydrogen ions, so it has a lower pH, conducts better and reacts faster with magnesium - even though the same quantity of each will eventually neutralise the same amount of alkali.

HCl → H+ + Cl-    CH3COOH ⇌ H+ + CH3COO-

What you need

  • Hydrochloric acid and ethanoic acid, both 0.1 mol/dm³
  • Universal indicator or a pH meter
  • Magnesium ribbon
  • A conductivity meter or a lamp-and-electrode circuit
  • A stopclock

How it is done

  1. Check both acids are at the same concentration - that is the whole point.
  2. Measure the pH of each.
  3. Measure the electrical conductivity of each.
  4. Add the same length of magnesium ribbon to each and time how long it takes to disappear.
  5. Titrate equal volumes of each against the same alkali and compare the volumes needed.

What you should see

  • Hydrochloric acid has the lower pH.
  • Hydrochloric acid conducts better, because it has more ions.
  • Magnesium fizzes faster in the hydrochloric acid.
  • Both need the same volume of alkali to neutralise - strength does not change that.

Where marks are lost

  • Saying a weak acid is dilute. Strength is about ionisation; concentration is about amount.
  • Expecting the weak acid to need less alkali. It does not.
  • Comparing acids at different concentrations, which tests nothing.
  • Forgetting the reversible arrow in the equation for a weak acid.

Questions students ask

What is the difference between strong and dilute?
Strong means fully ionised in water. Dilute means there is little acid in a lot of water. An acid can be strong and dilute at the same time.
Why do both acids need the same volume of alkali?
Titration measures how much acid is present, not how ionised it is. As the hydrogen ions are used up, a weak acid ionises further to replace them.
Why does the strong acid conduct better?
Conductivity depends on the number of free ions. A fully ionised acid puts far more ions into solution at the same concentration.

Try these next

Written for Cambridge IGCSE Chemistry 0620 and CBSE Class 9-12 by Ajay Shekhawat, founder of MrChemCoach. Run the simulator above, then check yourself against the questions.