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Subject knowledge: Irreversible reactions

The primary science curriculum requires children to explain that some changes result in the formation of new materials, and that this kind of change is not usually reversible. This includes changes associated with burning and the action of acid on bicarbonate of soda.

An irreversible change is one where a chemical change has taken place and a new material/materials have been formed. The original substances cannot be got back easily using separation techniques.

One of the most well known examples of this is adding bicarbonate of soda to vinegar and observing the change. Here the solid sodium bicarbonate (bicarbonate of soda) reacts with acetic acid (vinegar) to produce carbon dioxide, water and sodium acetate.

Reversible changes usually involve a physical change, rather than a chemical one. This could be a change of state or it could involve mixing two or more things together. In this case, it is possible to get the original materials back using separation techniques, such as filtration, sieving or evaporation.

Sorting task

Sort these examples into reversible and irreversible changes:

A. A candle burning
B. Metal rusting
C. Frying an egg
D. Vitamin C tablet and water
E. Making chocolate cake
F. Melting chocolate

As in the previous step, select one that you had to consider most and put your justification as to whether it is reversible or irreversible in the comments below.

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This article is from the free online course:

Teaching Primary Science: Chemistry

National STEM Learning Centre