Science 7–10 · Year 8

Physical or chemical change: evidence from seven small changes

Change

PracticalLow risk

The idea

A chemical change makes a new substance, shown by a new colour, a gas, a solid or an energy change that cannot be undone by a physical method, while after a physical change the same substance remains in a new form.

Safety card

Low riskAn adult supervises

Hazards

sodium hydroxide 0.4 mol/L and hydrochloric acid 0.5 mol/L are irritants to eyes and skin
copper(II) sulfate causes eye damage and is toxic to aquatic life

Controls

eye protection throughout
microscale drops only; the sheet wiped with a paper towel into a labelled copper-waste container
wash hands after the activity

Note

NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1.7 (risk assessment before use) and Volume 2 Appendix D (generic assessment advice, user codes and DoE categories). Complete a RiskAssess or CSIS site-specific risk assessment before the lesson.

What you need

clear plastic sheet over a printed seven-box grid, dropping pipettes, hand lens, cotton buds
copper(II) sulfate 0.2 mol/L, sodium hydroxide 0.4 mol/L, iron(II) sulfate 0.2 mol/L (fresh), hydrochloric acid 0.5 mol/L, a few drops each
solids: a few grains of sodium hydrogencarbonate, a zinc granule, a few crystals of sodium chloride, a small ice chip, one drop of food colouring
deionised water

How to do it

Box 1: 2 drops of copper(II) sulfate and 2 drops of sodium hydroxide. Box 2: 2 drops of iron(II) sulfate and 2 drops of sodium hydroxide; look again after 10 minutes.
Box 3: 2 drops of hydrochloric acid on a few grains of sodium hydrogencarbonate. Box 4: 2 drops of copper(II) sulfate on a zinc granule; look again after 10 minutes.
Box 5: a few salt crystals in 2 drops of water, then left to dry. Box 6: an ice chip left to melt. Box 7: a drop of food colouring added to 3 drops of water.
For each box record the properties before and after (colour, state, gas, solid, whether it can be got back unchanged by a physical method such as evaporation or cooling).
Classify each change as physical or chemical and give the evidence.

What you should see

Box 1 forms a pale blue solid (copper(II) hydroxide). Box 2 forms a green solid that turns orange-brown at the edges as air oxidises it. Box 3 fizzes as carbon dioxide forms. Box 4: the zinc turns red-brown with copper and the blue fades. These four are chemical changes: new substances with new properties appear. Box 5 gives the salt back as crystals when the drop dries, Box 6 is still water and Box 7 is still the same dye spread through water: physical changes. The learner knows it worked when every classification is backed by a before-and-after property and the evaporation in Box 5 returns the same white crystals.

What changes

This activity lists no variables to change, measure and keep the same.

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Every colour change is a chemical change.
Dissolving is a chemical change because the solid disappears.
A change that can be reversed must be physical.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7–10 Syllabus (2023)SC4-CHG-01 SC4-WS-06
Australian Curriculum v9AC9S8U07AC9S8I05

Sources

The pages the author read to write this activity.

All Concept Studio activities