Hello. In the previous lesson, you distinguished physical properties from chemical properties by asking whether identifying the property requires a substance to form something new. This lesson uses the same central idea for an event: when matter changes, did it remain the same substance, or did new substances form?
By the end of this lesson, you will be able to classify an observed event as a physical change or a chemical change, and justify your decision using the evidence given. This is an important skill for the Matter and Chemical Change Review module, and it will lead naturally into reading particle diagrams in the next lesson.
The deciding idea: identity versus appearance
A physical change changes a substance’s form, size, state, or arrangement without changing what the substance is chemically.
A chemical change, also called a chemical reaction, forms one or more new substances. The starting substances are called reactants; the substances made are products.
The question to keep at the centre of every classification is:
After the event, is the material still the same substance, or has a new substance formed?
For example:
- Ice melting is a physical change. Solid water becomes liquid water, but it is still water.
- A sheet of aluminum being bent is a physical change. Its shape changes, but it remains aluminum.
- Wood burning is a chemical change. Ash, smoke, and gases form; these are not wood.
- Iron rusting is a chemical change. Iron reacts with oxygen to form iron oxide, a new substance.
At the particle level, physical changes generally alter the spacing or movement of particles, while chemical changes rearrange atoms into different substances. The atoms themselves are not created or destroyed; what changes is how they are connected.
Chemical changes | Middle school chemistry | Khan Academy
Watch Chemical changes from Khan Academy for a concise contrast between changes that preserve a material's identity and reactions that make new substances. Its final section is especially important because it explains why evidence must be interpreted carefully.
Watch the core distinction between physical and chemical changes. Then continue with common signs of chemical change and the important caution: a visible sign alone does not prove that a chemical reaction happened. Notice the distinction between observing bubbles and knowing that a new gas was produced.
Physical changes: the substance remains identifiable
Physical changes can look dramatic, but appearance alone does not determine the classification. A material may change size, shape, state, or be mixed with another material while keeping its chemical identity.
Common physical changes include:
| Observed event | Why it is physical |
|---|---|
| Ice melts or water freezes | The substance remains water; only the state changes. |
| Water boils | The gas produced is water vapour, not a new substance. |
| Paper is cut or torn | Smaller pieces are made, but each piece is still paper. |
| Glass shatters | The shape changes, but the fragments remain glass. |
| A metal is bent or crushed | The metal’s form changes, not its composition. |
| Sugar dissolves in water | Sugar particles spread through the water but remain sugar; the sugar can be recovered by evaporating the water. |
| Ingredients are mixed into a salad | The ingredients are combined, not transformed into a new substance. |
A physical change is often reversible, such as freezing melted water. However, reversibility is not the rule used for classification. Shattered glass is difficult to restore, yet it is still a physical change because no new substance formed.
Similarly, heating does not automatically mean that a change is chemical. Heating ice until it melts or water until it boils causes physical changes. Heating wood until it burns causes a chemical change. The deciding factor remains the formation of new substances.
Chemical changes: evidence that new substances formed
Because we cannot always see individual particles or directly test every product, we often use observations as evidence that a chemical reaction may have occurred. Important evidence includes:
- an unexpected and lasting colour change
- a new or noticeably different odour
- production of a gas
- formation of a new solid in a liquid mixture, called a precipitate
- an observable energy change, such as unexpected heating, cooling, light, or sometimes sound
Read Signs of Chemical Reactions from CK-12 to consolidate the observable clues scientists use when deciding whether a chemical reaction has occurred.
In the subsection “Signs of Chemical Reactions,” read the opening explanation and the evidence list. Focus on what each observation could tell you about the possible formation of a new substance, especially the meaning of “precipitate.”
These signs are useful, but they are not automatic proof. The evidence must fit the situation.
| Observation | Why it can suggest a chemical change | Physical explanation that is also possible |
|---|---|---|
| Colour changes | A new coloured substance may have formed, as when grey iron develops reddish-brown rust. | Mixing blue and yellow paint makes green paint without creating a new substance. |
| Bubbles appear | A chemical reaction may have produced a gas. | Water can bubble because it is boiling; carbonated water releases gas that was already dissolved. |
| Temperature changes | A reaction may release or absorb energy. | Heating or cooling from an external source can change temperature physically. |
| A solid appears after liquids mix | A new insoluble substance may have formed. | Freezing turns a liquid into a solid, but it does not make a new substance. |
| Light or sound is produced | Some reactions release energy as light or sound. | A light bulb glows because of electrical energy, not necessarily a chemical reaction. |
The precise wording in a science question matters:
- “Bubbles appeared as the water boiled” supports a physical change, because the bubbles are water vapour.
- “A gas was produced when vinegar was mixed with baking soda” supports a chemical change, because the observation says a new gas formed.
- “Two clear liquids formed a cloudy solid when mixed” supports a chemical change, because a precipitate is likely a new substance.
The most convincing conclusion is always: a new substance was formed. The signs help you infer that conclusion when direct proof is not available.
A reliable method for classifying observations
When you are given a short description, do not classify it from one word such as heat, gas, or colour. Instead, reason through the evidence.
1. Identify what actually changed
Look for facts about state, shape, size, colour, odour, bubbles, energy, or solids appearing in a mixture.
For instance, “a clear liquid became a gas” describes a state change. “A clear liquid and a white powder fizzed and produced gas” provides stronger evidence of a reaction.
2. Decide whether the observation indicates a new substance
State changes, cutting, crushing, bending, and mixing generally preserve the substances already present.
Evidence such as rust, ash, an identified new gas, or a precipitate supports the conclusion that new substances formed.
3. State the classification and connect it to the evidence
A strong answer has three parts:
Claim: This is a physical/chemical change.
Evidence: The observation shows that…
Reasoning: Therefore, no new substance formed / a new substance formed.
For a physical change:
“This is a physical change because the water changes from liquid to gas, but the gas is still water vapour. No new substance forms.”
For a chemical change:
“This is a chemical change because a gas and new solid products form during burning. These products are different substances from the original wood.”
Applying the method to observations
Consider how the method works in several common situations.
Melting ice
Observation: A cube of ice becomes a puddle of liquid water.
The state changes from solid to liquid, but both samples are water. Since no new substance forms, melting is a physical change.
Crushing an aluminum can
Observation: A can is compressed into a wrinkled shape.
The can’s size and shape change. It remains aluminum, so this is a physical change.
Boiling water
Observation: Water is heated and bubbles form. Steam rises above the container.
This can be mistaken for a chemical change because a gas appears. However, the steam is water vapour: it is the same substance as the liquid water. Boiling is therefore a physical change.
Burning wood
Observation: A log burns, giving off light and heat. Smoke and ash remain.
Several observations support a reaction: energy is released, gases form, and ash is produced. Ash is chemically different from wood, so burning is a chemical change.
Rusting iron
Observation: A shiny grey iron nail gradually develops a reddish-brown coating in moist air.
The coating is rust, or iron oxide, which is a new substance with different properties from iron. This is a chemical change. The colour change is useful evidence, but the key conclusion is that iron oxide formed.
Mixing vinegar and baking soda
Observation: A powder and liquid are mixed, followed by rapid fizzing and bubbling.
The fizzing indicates that a gas is being produced. In this familiar reaction, the gas is carbon dioxide, a new substance. The event is a chemical change.
Making a salt solution
Observation: Salt disappears when stirred into water, leaving a clear solution.
The salt has dissolved and spread through the water. In the usual classroom classification, this is a physical change: the salt and water have formed a mixture rather than a new substance. The salt can be recovered by allowing the water to evaporate.
Avoiding the most common classification errors
A colour change always means chemical change
Not necessarily. Colour change is a clue, not proof. Paint mixing and food colouring dissolving in water can produce new colours physically. A colour change is stronger evidence when it occurs alongside other signs or when you know a new substance, such as rust, has formed.
Bubbles always mean chemical change
Not necessarily. Boiling water creates bubbles of water vapour, and opening a fizzy drink releases previously dissolved carbon dioxide. Ask whether the gas is a new product of a reaction.
Any change caused by heating is chemical
Heating may cause melting, boiling, or evaporation, all of which are physical changes. Burning is chemical because the heated fuel reacts with oxygen and produces new substances.
A chemical change is just a mixture
No. A mixture contains substances together without necessarily changing them chemically. In a chemical reaction, the original substances are transformed into different substances with different properties.
If something cannot be reversed, it must be chemical
No. Tearing paper and breaking glass are generally irreversible in practice, but they are physical changes. “Can it be reversed?” can sometimes be a helpful clue, but it is not the deciding test.
Key takeaways
- A physical change changes form, state, size, or arrangement while the substance remains the same.
- A chemical change forms one or more new substances.
- Possible signs of chemical change include colour or odour changes, gas production, precipitate formation, and energy changes.
- These signs are evidence, not guaranteed proof. Interpret them in context.
- The strongest justification is always based on chemical identity: Did a new substance form?
- A strong response names the type of change, cites the observation, and explains what that evidence means.
Next, you will move from everyday observations to particle diagrams. You will use the arrangement and bonding of particles before and after a change to decide whether the change is physical or chemical.
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