🧪 Chemistry Lab
SPM Salts: Acids & Alkalis
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ACIDS, BASES & SALTS

Small strip. Big discovery.

Are all salts neutral? Test three salt solutions with litmus paper and discover what makes a solution acidic or alkaline.

TRY IT YOURSELF

The litmus experiment

1. Choose a fresh strip

2. Dip it into a solution
Click a beaker or drag a strip onto it. Try both colours in every solution.

SOLUTION A Not tested

Ammonium chloride

Blue: untested Red: untested
SOLUTION B Not tested

Sodium chloride

Blue: untested Red: untested
SOLUTION C Not tested

Sodium carbonate

Blue: untested Red: untested
Blue litmus selected. Choose a solution to start your experiment.
0 of 6 tests recorded 0 of 6
Your observation notebook
Test both colours to complete each row.
Salt solutionBlue litmusRed litmusConclusion
Ammonium chloride Not tested Not tested Test both colours
Sodium chloride Not tested Not tested Test both colours
Sodium carbonate Not tested Not tested Test both colours

A fresh strip is used for every dip. This is a simplified simulation: litmus indicates acidity or alkalinity, but cannot measure an exact pH.

BEHIND THE COLOUR CHANGE

It comes down to the ions.

pH guide at 25 °C
pH < 7

Acidic

More hydrogen ions, H+(aq), than hydroxide ions, OH−(aq). In water, H+ is carried as hydronium, H3O+.

Blue litmus turns red.
pH = 7

Neutral

Equal concentrations of H+(aq) and OH−(aq). Neutral solutions still contain both types of ion.

Neither litmus colour changes.
pH > 7

Alkaline

More hydroxide ions, OH−(aq), than hydrogen ions, H+(aq). An alkali is a base that dissolves in water.

Red litmus turns blue.
Why can a salt solution be acidic or alkaline?

Some salt ions react with water and change the balance of H3O+ and OH−. This is called hydrolysis.

  • Ammonium chloride: ammonium ions react with water to form hydronium ions, making the solution acidic.
  • Sodium chloride: sodium and chloride ions do not appreciably react with water, so the solution is neutral.
  • Sodium carbonate: carbonate ions react with water to form hydroxide ions, making the solution alkaline.

PUT IT INTO PRACTICE

What makes it acidic or alkaline?

6 questions

Use your observations and what you know about ions. Choose one answer for each question, then check your understanding.

0 of 6 answered

CHEMISTRY IN EVERYDAY LIFE

What would you do?

3 situations · 6 questions

A bee sting, an upset stomach, and a mouth ulcer. Make a choice, discover the chemistry, and find out when neutralisation really applies.

Question 1 of 6 0 of 6 0 correct

Explore further: Royal Society of Chemistry: litmus tests Purdue University: salt solutions