C. Matter and Energy · Properties of and Changes in Matter

Physical or Chemical? Investigating Changes

C2.4 C2.5 A1.1 A1.2 A1.5About 45 minutes

Learning goals

I can sort changes into physical and chemical changes and explain my thinking.
I can plan a fair test about a chemical change, with a question, a prediction and a control.
I can record results and draw a conclusion.

Words to know

investigation
A careful way of answering a science question by observing, testing and recording.

prediction
What you think will happen, and why.

variable
Something that can change in an experiment.

control
The part of a test where you change nothing, so you can compare.

conclusion
What you learned from your results, answering your question.

Detective questions

You now know two kinds of changes. When you see something change, ask these detective questions:

Is it still the same substance? If yes → physical change.
Did a new substance form? If yes → chemical change.
What is my evidence? Look for signs: gas without heating, colour change, new smell, heat or light given off, a solid forming from liquids, can't be reversed.
Check for tricks! Bubbles from boiling and colour from mixing paints are physical.

The detective question in one picture: if it is still the same substance, it is physical; if a new substance formed, it is chemical. Check the evidence, and watch for tricks.

Change	Physical or chemical?	Why
breaking a glass	physical	still glass; only the form changed
a firework exploding	chemical	light, heat, sound, smoke: new substances
snow melting	physical	still water; change of state
a banana getting brown spots	chemical	colour change and new smell as it ripens
dissolving drink crystals	physical	the crystals are still there in the water
frying an egg	chemical	the egg turns white and firm and can't go back
sawing wood	physical	still wood
burning wood	chemical	ash, smoke, heat and light

Sorting changes. Notice that wood can have a physical change (sawing) or a chemical change (burning).

Worked example

Let's sort: cutting an apple vs. a browning apple

A grown-up cuts an apple into slices. After an hour on the counter, the slices are brown. How many changes happened, and what kind?

Change 1, cutting: still apple, only the form changed → physical.
Change 2, browning: the colour changed, because substances in the apple reacted with oxygen in the air to make new brown substances → chemical.

Two changes: cutting (physical) and browning (chemical).

A plate of apple slices: some are still pale on the cut side and others have turned brown, with a whole red apple beside the plate. — Cutting the apple was a physical change. The slices that turned brown went through a chemical change when they reacted with oxygen in the air. AI-generated image

Worked example

Let's plan an investigation

Question: Does lemon juice slow down how fast apple slices turn brown?

Research first: ask a grown-up or look in a cookbook. Many cooks brush lemon juice on cut apples.
Predict: "I think the slice with lemon juice will stay light the longest, because cooks use lemon juice to keep fruit from browning."
Plan a fair test: use 3 slices from the same apple, cut at the same time, the same size. Slice A: brush with lemon juice. Slice B: brush with water. Slice C: nothing (the control). Put them side by side on the same plate.
What changes: only what is brushed on. What stays the same: apple, size, time, place.
Observe and record every 15 minutes for an hour. Use a colour scale: 0 = not brown, 1 = a little brown, 2 = brown, 3 = very brown.
Conclude by comparing each slice to the control.

A good plan has a question, a prediction with a reason, one variable changed, a control, and a way to record results.

Time	A: lemon juice	B: water	C: nothing (control)
0 min	0	0	0
15 min	0	1	1
30 min	0	1	2
60 min	1	2	3

Sample results from one family's test (0 = not brown, 3 = very brown). Your results may be a little different.

The sample results as a line graph. The control (C) browns fastest and the lemon juice slice (A) browns slowest, so lemon juice slowed the chemical change.

Just for fun

What's the difference between a wood carver and a campfire?

The carver makes a physical change to wood. The campfire makes a chemical change. Only one of them can toast your marshmallows!

Hands-on activity

The apple browning investigation

You need: 1 apple, a grown-up with a knife and cutting board, lemon juice, water, 2 small bowls, a clean pastry brush or 2 cotton swabs, a plate, a marker, a clock, paper and pencil.

Write your question and prediction.
A grown-up cuts 3 slices the same size from the same apple.
Label spots on the plate A, B and C. Brush A with lemon juice, B with water, and leave C alone (control).
Every 15 minutes for 1 hour, give each slice a browning score from 0 to 3 and record it in a table.
Conclude: Did lemon juice slow the chemical change? Was your prediction right?
Share: tell or show your family your results table. Then sort 10 changes you find at home that day into physical and chemical, and explain each.

What to look for: the lemon slice usually browns the least; the control browns the most. The water slice is often in between. Results can vary between apples, which is why scientists repeat tests.

Safety: only a grown-up uses the knife. Lemon juice stings cuts and eyes. You may eat apple slices that were only handled with clean hands and kitchen ingredients, if your grown-up says so.

Your turn

What is the best question to ask to decide if a change is physical or chemical?

Physical or chemical: breaking a glass?

Physical or chemical: a firework exploding?

Physical or chemical: dissolving drink crystals?

In the apple investigation, why is slice C important?

Wood can have both kinds of change. Give one example of each.

More practice: warm-up

Physical or chemical: snow melting?

Physical or chemical: frying an egg?

Physical or chemical: crumpling foil?

Physical or chemical: a bike chain rusting?

Physical or chemical: water boiling?

More practice: on your own

Look at the sample results. At 60 minutes, which slice was most brown? Least brown?

Write a conclusion for the sample results.

Why should all three slices come from the same apple?

A banana gets brown spots and smells stronger as it ripens. Physical or chemical? Why?

Mia says, "Dissolving is chemical, because the sugar disappears." How would you correct her?

Challenge

Why do scientists repeat an investigation, like the apple test, more than once?

Making a campfire: list one physical change and one chemical change that happen.

Plan a fair test: Does a nail rust faster in tap water or in salty water? Give the variable you change, two things you keep the same, and what you'll observe.

Check your understanding

Quiz

Which is a physical change?

Which is a chemical change?

In an investigation, the control is...

Water bubbling as it boils is a...

In a fair test, you should change...

A cut apple turns brown because...

For parents and teachers

Big idea: This lesson brings together C2.4 (physical changes) and C2.5 (chemical changes) and practises the scientific research process (A1.1) and experimentation process (A1.2): question, prediction, fair test with a control, recording, conclusion, sharing (A1.5).

How to help: Let your child plan the apple test themselves, then ask "What is your control? What stays the same?" Encourage scoring browning honestly, even if it doesn't match the prediction.

Common difficulties:

Deciding by "can it be undone?" instead of "is there a new substance?"
Forgetting the control.
Changing two things at once.
Writing a conclusion without data. Encourage "I know because... (numbers from the table)".

Safety: you do all cutting. Lemon juice stings eyes and cuts.