Chemistry · Acids and bases · pH scale
pH Explorer
Test six liquids, from lemon juice to limewater, with indicator and a pH meter; find the acids, the bases and the neutral ones yourself.
Decide how acidic a liquid is by eye and by measurement, and see what happens when an acid and a base mix.
20 minDifficulty: Easy
Learning goals
- Decide by measurement whether a liquid is acidic, neutral or basic.
- Compare the indicator colour with the pH meter reading.
- Measure how pH changes when an acid and a base are mixed.
- Observe baking soda fizzing with an acid and explain why.
Materials and tools
- Lemon juice (Citric acid ≈ 0.3 M)
- Vinegar (Acetic acid CH₃COOH ≈ 0.8 M)
- Milk
- Distilled water (H₂O)
- Baking soda solution (NaHCO₃ 0.1 M)
- Limewater (Ca(OH)₂ 0.02 M)
- Universal indicator
- Lemon juice bottle
- Vinegar bottle (5%)
- Milk bottle
- Distilled water bottle
- Baking soda solution bottle (NaHCO₃)
- Limewater bottle (Ca(OH)₂)
- Universal indicator bottle
- Graduated pipette 2 mL
- Tube rack and 8 test tubes (15 mL)
- Beaker 100 mL
- Glass stirring rod
- pH meter
Steps
- Put different liquids into at least three tubes and add a few drops of indicator to each.
- Measure the pH of four different liquids with the pH meter and record it.
- Measure at least one acidic, one neutral and one basic liquid.
- Mix an acid with a base, measure the pH of the mixture, then look at the table and graph.
Scientific explanation
Acids taste sour and have a pH below 7, like lemon juice and vinegar. Bases feel slippery and have a pH above 7, like baking soda and limewater. Distilled water is neutral: its pH is 7.
The pH scale is logarithmic: a liquid with pH 2 is ten times more acidic than one with pH 3. That is why diluting vinegar ten times raises its pH only a little and never above 7.
When baking soda meets an acid, carbon dioxide gas is released: that is the fizzing. The baking powder that makes a cake rise works with the same reaction.