Acid base reactions
To demonstrate the chief properties of acids and bases, and to introduce the concept of acids and bases and some indicators.
What students learn
The science behind this experiment
Acids and bases comprise two important groups of compounds, commonly defined respectively as donors or acceptors of protons. They are easily detected by their interaction with pH indicators, which change colour according to the presence of an acid or a base.
According to the Brönsted definition, acidic substances are donors of hydrogen ions and in aqueous solutions they form oxonium ions.
HCl + H2O → Cl– + H+ + H2O → H3O+ + Cl–.
Bases react to give hydroxide ions in aqueous solution either by simple solution, in the case of alkali metal hydroxides.
Another definition of acids and bases was given by Lewis who defined an acid as a substance that can accept a lone pair of electrons and a base as a substance which donates a lone pair of electrons.
The acidity or alkalinity of solutions is measured by the pH scale that is pH= – log10[H3O+].
Some organic substances undergo reversible changes with acids or bases and change colour as they do so. For this.
Equipment used
- Litmus paper — 8884.02
- Beaker 100 ml — 1118.20
- Glass rod — 1444.10
- Dropping pipette — 1441.50+5448.00
- Test tubes — 1473.60
- Test tube rack — 1680.06
- Dissecting set — 8267.15
- Tripod stand — 5533.10
- Spirit burner — 5514.10
- Iron stand — 5404.52
- Wire gauze — 5534.25
Chemicals and reagents
- Distilled water
- Hydrochloric acid — R:34-37 S:26-45
- Sodium hydroxide — R:35 S:26-37/39-45
- Sodium chloride
- Ammonia — R:34-50 S:26-36/37/39-45-61
- Magnesium oxide
- Magnesium ribbon
- Acetic acid
- Vinegar
- Fresh lemon juice
- Red cabbage juice
Safety notes
- This experiment must be carried out in a fume cupboard or outdoors.
- Do not leave the bottles of hydrochloric acid and ammonia unstoppered.
Questions for students
- Record your observations of what happens.
- Record your observations and compare the results with those obtained using the indicator paper.
- Compare the results of all of these tests.
What to expect
<p>The litmus paper changes its colour in the following way: Hydrochloric acid: Red.</p><p>Sodium chloride: doesn’t change colour = NEUTRAL Ammonia solution: Blue.</p><p>Magnesium Oxide: Blue Acetic acid: Red Vinegar: Red.</p><p>Mg + 2 H+ → Mg2+ + H2.</p><p>Red cabbage juice is a natural pH indicator. It contains chemicals called anthocyanins, which are pigments that react in a different way to acids and bases.</p><p>Chemical substances that change colour in solutions of different pH values are contained in many flowers, fruits, and vegetables.</p><p>The pH values and colours of red cabbage extract.</p>
Disposal
Dispose of solutions into the liquid wastes container.
Run this experiment in your classroom
This experiment is part of the ATP Mobile Lab — a self-contained laboratory that turns an ordinary classroom into a working science lab, with over 200 experiments in physics, chemistry, biology, robotics and engineering. Gali, the AI tutor built into ATP Connect, guides students through each step and answers their questions at the bench.
See the Mobile LabTalk to our team