Lesson 5: SOILS (Again!) pH & Plant Nutrients
Class Room Work

1. Acid Neutral & Alkaline

First of all we asked the class if they knew what was and acid and what was a base. Several students were able to give examples of acidic substances, however nobody could give me an example of an alkali. Explaining the terms acid and alkali to primary students can be a bit daunting the following is what we came up with.

In high school a lot of the chemistry involves reactions between acids and alkalis (also known as bases).

In primary school the easiest way to define and explain these chemical terms is by going back to the oldest definitions:

An early definition of and acid and base was given by the scientist Arrhenius

acids - have a sour taste (Latin acidus- sour)
alkalis - substances that can reverse or neutralise the action of acids (Arabic al kali - ashes of a plant)

[A more modern definition - An acid can donate a proton to another molecule, a base tends to accept protons. It was decided not to get into this type of explanation at this level of school.]

Acids and alkalis can be strong or weak, half way between is called neutral. pH refers to a number scale that is used to represent the relative strengths of acids and alkalis

Figure 10: pH Scale

2. Classroom experiment

What you need:

glass beakers
litmus paper
vinegar
bicarb soda
wood ashes in some water

We demonstrated to the class how it was possible to test for acids and alkalis using litmus paper

Figure 11: Using litmus paper to identify acid, neutral and alkali

What we found was that bicarb soda could make vinegar fizz - CHEMICAL REACTION However, bicarb soda is a quite a strong alkali and vinegar is only a weak acid so it was difficult to get neutral by combining the two.

WARNING We talked about strong acids and alkalis and how they could both BURN. We also discussed the treatment of chemical burns by applying the opposite type of chemical to neutralise the skin.