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

Chemical Changes and Their Signs

C2.5 A1.2 A1.4About 40 minutes

Learning goals

I can explain that a chemical change makes one or more new substances.
I can list signs that a chemical change has happened.
I can give examples of chemical changes in everyday life.
I can follow safety rules when I investigate changes, and explain why we never mix cleaning products.

Words to know

chemical change
A change that makes one or more new substances with new properties.

new substance
A different kind of matter that wasn't there before, like the rust that forms on iron.

evidence
Clues you can observe that support an idea.

rust
A crumbly, reddish-brown substance that forms when iron or steel reacts with oxygen and water.

carbon dioxide
A colourless gas. It makes the bubbles when baking soda and vinegar mix.

Batter goes in, cake comes out

When you pour runny cake batter into a pan and bake it, you get a soft, spongy cake. Can you get the eggs, flour and milk back out of the cake? No way! Something brand new was made. That is a chemical change.

What is a chemical change?

A chemical change happens when substances change into one or more new substances. The new substances have different properties from what you started with.

Everyday chemical changes:

baking a cake or bread
cooking an egg (the clear part turns white and firm)
toasting bread (the brown, crunchy part is new)
burning wood or a candle wick (ash, smoke and gases are made)
rusting: iron or steel, like an old bike chain or a nail, slowly turns into crumbly, reddish-brown rust
an apple slice turning brown in the air
milk going sour
baking soda and vinegar fizzing
your body digesting food

Signs of a chemical change

You can't see particles, so scientists look for evidence. These are signs that a chemical change may have happened:

A gas is given off (bubbles or fizzing), without heating.
A colour change, like bread turning brown or iron turning reddish-brown.
A new smell, like milk going sour or toast.
Heat or light is given off, like a burning candle. (Some chemical changes take in thermal energy and feel cold instead.)
A solid forms when two liquids are mixed.
It is very hard or impossible to reverse. You can't un-bake a cake.

An old bicycle chain covered in reddish-brown rust lies on grey wooden boards. — Rust is a new substance. The iron in the chain reacted with oxygen and water, and the colour change to reddish-brown is a sign of a chemical change. AI-generated image

The six signs that a chemical change may have happened. Look for more than one sign, and ask: is there a new substance?

Careful, detective!

One sign alone is not proof. Boiling water makes bubbles, but it is still water (physical change). Mixing paints changes the colour, but each paint is still there (physical change). Look for more than one sign, and ask: Is there a new substance?

Chemical change	Signs you can observe	New substance
baking a cake	colour change, new smell, can't reverse	cake
burning wood	heat, light, smoke, new smell, ash left, can't reverse	ash, smoke, gases
iron rusting	colour change (reddish-brown), crumbly texture	rust
baking soda + vinegar	bubbles of gas, the mixture feels cooler	carbon dioxide gas (and other new substances)
milk going sour	new smell, lumps form	sour milk

Evidence of chemical changes.

Worked example

Let's look for evidence: baking soda and vinegar

Theo puts 1 teaspoon of baking soda in a cup and adds 2 tablespoons of vinegar. Is this a chemical change? What is the evidence?

Observe: it fizzes and foams with lots of bubbles. Nothing was heated, so the bubbles are not boiling.
The bubbles are a new gas: carbon dioxide.
Feel the side of the cup: it feels a little cooler. The change took in thermal energy.
When the fizzing stops, the baking soda is gone. You can't get it back.

Yes, a chemical change. Evidence: gas bubbles (carbon dioxide, a new substance), a temperature change, and it can't be reversed.

Worked example

Let's look at a candle: physical or chemical?

When a candle burns, what changes do you see?

The wax near the flame melts into a liquid. Melting is a physical change: it is still wax, and it hardens again when it cools.
The wick and some wax burn in the flame. Burning gives off heat and light and makes new substances (gases like carbon dioxide and water vapour, and sometimes soot).
Burning is a chemical change. Over time the candle gets shorter because the wax is turned into gases.

A burning candle shows both: melting wax (physical) and burning (chemical).

A burning candle shows both kinds of change: the wax melting (physical) and the wax burning in the flame (chemical).

Safety: why we NEVER mix cleaning products

Some chemical changes are dangerous. Some cleaning products, like bleach and ammonia, make toxic gases if they are mixed. Breathing these gases can hurt your lungs.

That is why:

Never mix cleaning products, not even "just to see what happens".
Only use kitchen materials like baking soda and vinegar for experiments, with a grown-up's OK.
Read and follow safety symbols on product labels (you learned these in Grade 2).

Just for fun

I was going to tell you a joke about baking soda and vinegar...

...but I was worried I wouldn't get a reaction.

Hands-on activity

Fizz detective

You need: a tray or baking sheet, 2 clear cups, baking soda, white vinegar, a teaspoon, a measuring spoon (tablespoon), water, paper and pencil. Safety glasses if you have them.

Put the cups on the tray. Put 1 teaspoon of baking soda in each cup.
Cup 1: add 2 tablespoons of water. Observe. Feel the side of the cup.
Cup 2: add 2 tablespoons of vinegar. Observe. Feel the side of the cup.
Record the signs you see in each cup.
Conclude: In which cup did a chemical change happen? What is your evidence? Why was Cup 1 useful?
Rust hunt: with a grown-up, look around your home, garage or yard for rust (an old tool, a nail, a bike chain). Look, don't touch rough edges. Where was it? Was it in a damp place?

What to look for: Cup 1 just gets cloudy (the baking soda mixes with water; no fizzing). Cup 2 fizzes and foams and feels a bit cooler: a new gas, carbon dioxide, was made. Cup 1 is the control: it shows that water alone doesn't make bubbles. Rust is usually found where iron or steel gets wet.

Safety: keep vinegar away from your eyes and face (it stings). Never put the mixture in a closed bottle (the gas can build up and pop the lid). Don't taste anything. Never mix cleaning products. Wash your hands afterward. Rusty metal can have sharp edges: look, don't touch.

Your turn

What is a chemical change?

List four signs that a chemical change may have happened.

What gas is made when baking soda and vinegar mix?

Is cooking an egg a chemical change? Give your evidence.

What new substance forms when iron is left in damp air?

Why must you never mix cleaning products?

More practice: warm-up

Chemical change? Burning a log in a campfire.

Chemical change? Toasting bread.

Chemical change? Melting butter.

Chemical change? An apple slice turning brown.

Is fizzing (bubbles) a sign of a chemical change?

More practice: on your own

Water bubbles when it boils. Why isn't that a chemical change?

In the fizz detective activity, why was the cup with water important?

Name two signs that milk has gone sour.

Explain why a burning candle shows both a physical and a chemical change.

Mixing yellow and blue paint makes green. Is this a chemical change? Explain.

Challenge

Why do bikes left outside all winter often have rusty chains, while bikes kept in a dry garage do not?

Theo uses twice as much vinegar in a second test and gets more foam. Is this still a fair test of which liquid makes baking soda fizz? What should he do?

Bread dough rises because yeast makes a gas. When the bread bakes, it turns brown and smells delicious. List the signs of chemical change.

Check your understanding

Quiz

A chemical change...

Which is a chemical change?

Which is a sign of a chemical change?

When baking soda and vinegar mix, the bubbles are...

Why should you never mix cleaning products?

When a candle burns, the wax melting is...

For parents and teachers

Big idea: Ontario Grade 5 expectation C2.5 asks children to describe chemical changes as changes that result in different substances, and to identify signs that a chemical change has occurred. The activity uses a control (A1.2) and safety procedures (A1.4).

How to help: Cooking is full of chemical changes: toast, pancakes, eggs, baking. Ask "What signs do you see? Is there a new substance?" Point out rust on old outdoor tools.

Common difficulties:

Thinking every bubble or colour change is chemical. Boiling and paint mixing are physical. Look for more than one sign and a new substance.
Thinking chemical changes are always dangerous or happen only in labs. Most happen in the kitchen.
Not knowing a candle shows both kinds of change.

Safety: please supervise. Vinegar stings eyes; never seal the fizzing mixture in a bottle. Make it a firm family rule that cleaning products are never mixed, and that children don't experiment with them. Store cleaning products out of reach.