Chemical Changes
Understand the difference between physical and chemical changes, and learn to spot the signs of a new substance being formed.
Physical vs Chemical Changes
When matter changes, it can be either a physical change or a chemical change. The key difference is whether a new substance is formed.
Physical Change
The substance stays the same. Only its shape, size, or state changes. Usually reversible.
- ●Cutting paper (still paper, just smaller)
- ●Melting ice (still water, just liquid)
- ●Dissolving sugar in water (sugar is still there)
- ●Bending a wire (still the same metal)
Chemical Change
A new substance is formed. The original material is changed permanently. Usually irreversible.
- ●Burning wood (creates ash, smoke, gas)
- ●Rusting iron (creates iron oxide)
- ●Cooking an egg (cannot un-cook it!)
- ●Baking a cake (raw ingredients become cake)
Inquiry Question: Think about making toast. Is it a physical change or a chemical change? What clues tell you? Could you turn toast back into bread?
Signs of a Chemical Change
How can you tell if a chemical change has happened? Look for these clues:
Gas Produced
Bubbles, fizzing, or a gas escaping (e.g. vinegar + baking soda)
Colour Change
The substance changes colour permanently (e.g. iron turning red-brown when rusting)
Heat or Light
Heat or light is given off or absorbed (e.g. burning produces heat and light)
New Substance
A completely new material forms that is different from what you started with
Reversible vs Irreversible Changes
| Feature | Reversible | Irreversible |
|---|---|---|
| Can you change it back? | Yes | No |
| New substance formed? | No (same substance) | Yes (new substance) |
| Type of change | Usually physical | Usually chemical |
| Examples | Melting ice, dissolving salt, folding paper | Burning wood, cooking food, rusting |
Everyday Examples
Cooking an Egg
Chemical change. Heat changes the proteins permanently. You cannot un-cook an egg!
Rusting Iron
Chemical change. Iron reacts with oxygen and water to form iron oxide (rust) -- a new, flaky brown substance.
Burning Paper
Chemical change. Paper reacts with oxygen, producing ash, smoke, and heat. You cannot turn ash back into paper.
Key Vocabulary
Chemical Change
A change that produces a new substance. Usually cannot be undone.
Physical Change
A change in shape, size, or state, but the substance stays the same.
Reversible
A change that can be undone, returning the material to its original form.
Irreversible
A change that cannot be undone. The original substance is gone forever.
Combustion
The scientific word for burning. A chemical reaction that produces heat and light.
Oxidation
A chemical reaction involving oxygen. Rusting is a type of slow oxidation.
Worked Examples
You dissolve sugar in warm water. Is this a physical or chemical change?
Step 1: Has a new substance been made? The sugar is still sugar -- you can taste it in the water.
Step 2: Can you get the sugar back? Yes -- if you evaporate the water, sugar crystals remain.
Answer: This is a physical change. No new substance is formed, and it is reversible.
You mix vinegar and baking soda and it fizzes. Physical or chemical change?
Step 1: Look for signs of chemical change: gas is produced (the bubbles/fizzing).
Step 2: Can you un-fizz it and get back the vinegar and baking soda? No.
Answer: This is a chemical change. A new gas (carbon dioxide) is produced, and the original substances are gone.
A metal gate has turned reddish-brown. What happened?
Step 1: The iron in the gate has reacted with oxygen and water in the air.
Step 2: A new substance -- iron oxide (rust) -- has formed on the surface.
Answer: This is a chemical change called rusting. The colour change and new substance (rust) are clear signs.
Knowledge Check
Select the correct answer for each question. Click "Check Answer" to see if you are right.
Question 1
Which of these is a chemical change?
Question 2
Which of these is a sign that a chemical change has occurred?
Question 3
You melt butter in a pan, then cool it back into a solid. Is this reversible or irreversible?
Question 4
A nail left outside turns reddish-brown over time. What type of chemical change is this?
Question 5
Which statement is TRUE about physical changes?
Key Concepts Summary
- ●Physical changes alter the form of a substance but do not create a new substance (e.g. melting, cutting, dissolving).
- ●Chemical changes create a new substance that is different from the original (e.g. burning, rusting, cooking).
- ●Signs of a chemical change include gas production, colour change, heat/light, or a new substance.
- ●Reversible changes can be undone; irreversible changes cannot.
- ●Most physical changes are reversible; most chemical changes are irreversible.