Classifying Physical and Chemical Changes

Hello. In the previous lesson, you learned that a formula such as $H_{2}O$ identifies a substance by showing its atoms. That idea now becomes useful in a new way: a chemical reaction changes substances into new substances, while a physical change leaves the substance itself unchanged.

This lesson will help you classify a described change by using observations carefully. You will learn the strongest evidence for chemical change, recognise common physical changes, and avoid traps such as assuming that every bubble or colour change must be a reaction.

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The central test: has a new substance formed?

A physical change changes a substance’s appearance, size, shape, or state, but does not change what the substance is.

Examples include:

• ice melting into liquid water;
• water freezing;
• paper being torn;
• sugar dissolving in water;
• water boiling into water vapour.

In each case, the substance is still the same substance. Ice and liquid water may look very different, but both are $H_{2}O$.

A chemical change, also called a chemical reaction, forms one or more new substances. Atoms are not created or destroyed; instead, they are rearranged into different particles with different properties.

For example, when iron rusts, the silvery iron reacts with substances in air and water to form rust, a reddish-brown material. Rust is not simply iron with a new appearance: it is a new substance.

A useful first rule is:

Physical change: same substance, different form.
Chemical change: new substance or substances form.

The difficulty is that, in most everyday situations, we cannot directly see atoms rearranging. Instead, we look for evidence.

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Observable signs that a chemical reaction may have occurred

When a description says that a new substance formed, classification is straightforward: it is a chemical change. More often, a description gives observations. The following signs are strong evidence that a chemical reaction may have taken place.

1. A gas is produced

A reaction may produce a gas, seen as persistent fizzing, bubbles, foam, or fumes. For example, lemon juice and baking soda react to make bubbles of carbon dioxide gas.

However, bubbles alone are not enough. When water boils, its bubbles are water vapour: liquid $H_{2}O$ is becoming gaseous $H_{2}O$. No new substance is made, so boiling is a physical change.

Look for the distinction:

Observation	Likely classification	Why
Water heated until it bubbles and turns to steam	Physical	Water changes state from liquid to gas
Baking soda mixed with vinegar produces fizz	Chemical	A new gas, carbon dioxide, is produced

2. A lasting colour change occurs

A colour change can indicate that a new substance has formed. Iron changing from shiny grey to reddish-brown rust is a familiar example.

But colour change is not conclusive by itself. Mixing blue and yellow paints gives green paint, but this is a mixture of coloured materials rather than evidence that a new chemical substance has formed. Similarly, food colouring spreading through water is a physical mixing process.

3. Temperature changes without continued heating or cooling

Some reactions release thermal energy, making their surroundings warmer. Others absorb thermal energy, making their surroundings cooler. Burning fuel gives out heat, for instance.

Be precise with the wording of a description. A substance becoming hot because it is placed on a stove is not, by itself, evidence of a chemical reaction. The stove supplies energy from outside. A more persuasive sign would be that two substances are mixed and the container becomes noticeably warm or cold without external heating.

4. Light is emitted

Flames, fireworks, glow sticks, and some living organisms such as fireflies emit light because chemical processes are occurring. Burning wood also produces heat, smoke, gases, ash, and a colour change: several pieces of evidence support the classification.

Light from an ordinary lamp is not a chemical reaction in the bulb. The key is whether the observed light comes from a reaction that is forming new substances.

5. A precipitate forms

A precipitate is a solid that forms when two liquids or solutions are mixed. It is different from a solid that was already present.

Imagine two clear solutions being poured together. If a cloudy solid appears and settles at the bottom, the solid is evidence of a newly formed substance. That is a chemical change.

Do not confuse this with stirring sand into water. Sand does not dissolve, but it was already sand before mixing. It settles because of a physical mixture, not because a new solid formed.

Other useful evidence: a new odour

Food rotting or milk souring can produce a new smell because new substances form. In a laboratory, though, never identify chemicals by deliberately smelling them. Scientists use safe methods and follow teacher instructions.

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Evidence is not the same as certainty

The most important scientific habit in this topic is to avoid jumping from one observation to a conclusion.

A colour change, bubbles, or a temperature change is evidence, not absolute proof. The secure conclusion is always based on whether the observations support the formation of a new substance.

A good conclusion therefore has two parts:

1. Classify the change as physical or chemical.
2. Cite the observation and explain what it suggests.

For example:

• “This is a physical change because the ice melted into liquid water, but the substance remained water.”
• “This is a chemical change because a solid appeared when two clear solutions were mixed, suggesting a new substance formed.”
• “This is probably a chemical change because the mixture warmed and released a gas; together, these observations strongly suggest new products formed.”

Notice the careful word probably in the third statement. If the question only provides observations, scientifically cautious language is appropriate. If it explicitly tells you that a new substance formed, you can state confidently that it is chemical.

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A method for classifying written descriptions

For a Year 9 question, use this short routine.

Step 1: Identify what changed

Look for descriptions of:

• state: melting, freezing, boiling, condensing;
• shape or size: cutting, crushing, bending;
• mixing or dissolving;
• burning, rusting, cooking, spoiling, or reacting.

Words such as melted, froze, cut, and crushed often point to physical change. Words such as burned, rusted, fizzed when mixed, and formed a solid often point to chemical change—but still read the evidence.

Step 2: Search for evidence of new substances

Underline or note observations such as:

• a gas produced by a reaction;
• a new solid in a solution;
• an unexpected temperature change;
• light emitted;
• a lasting colour or odour change.

One sign may be ambiguous. Several signs together make the chemical interpretation much stronger.

Step 3: Rule out common physical explanations

Before deciding, test the description against the most common traps:

• Is the “gas” simply steam from boiling?
• Is the colour change just dye, paint, or two substances mixing?
• Did a solid merely fail to dissolve, rather than form during mixing?
• Did the temperature change because an external heater or refrigerator was used?

Step 4: Write an evidence-based conclusion

Avoid answers such as “chemical because it changed.” Both physical and chemical changes involve change.

Instead, specify the nature of the change:

Description	Classification	Evidence-based explanation
A copper wire is bent into a coil.	Physical	Its shape changes, but it remains copper.
A puddle disappears on a warm day.	Physical	Liquid water evaporates into water vapour; no new substance is described.
A nail becomes coated with reddish-brown rust over time.	Chemical	The colour and material change because rust, a new substance, forms.
Two clear liquids are mixed and a yellow solid appears.	Chemical	A precipitate forms, indicating a new solid substance.
Blue food colouring is stirred into water.	Physical	The dye disperses through the water; this is mixing, not evidence of a new substance.
A cake batter is baked and becomes a cake with a new texture, colour, and smell.	Chemical	Heating causes reactions that form new substances; it cannot simply be changed back into batter.

Reversibility can support an answer, but it is not the main test. Melting ice is easy to reverse, which supports the physical classification. Baking a cake is difficult to reverse, which supports the chemical classification. Yet the better explanation is always whether a new substance formed.

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Connecting observations to particles

At the particle level, a physical change does not alter the identity of the particles. During melting, water particles gain energy and move more freely, but they remain water particles.

In a chemical reaction, atoms become connected in new arrangements. This is why the products can have new properties. Iron and rust differ in colour, texture, and chemical behaviour because rust contains particles different from those in pure iron.

This also explains why a chemical change cannot normally be undone by a simple physical action. You can refreeze liquid water because it is still $H_{2}O$. But you cannot turn ash back into the original piece of wood merely by cooling it or reshaping it: burning has produced new substances.

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Key takeaways

A physical change changes form, state, size, or shape while the substance remains the same. Melting, freezing, boiling, crushing, and dissolving are common physical changes.

A chemical change forms one or more new substances. Evidence can include:

• gas produced;
• a new solid precipitate;
• a lasting colour or odour change;
• light emitted;
• temperature change caused by the process itself.

No single observation is guaranteed proof. Boiling water makes bubbles without creating a new substance, and mixing paints changes colour without necessarily causing a reaction. The strongest conclusion comes from asking: What evidence suggests that a new substance has formed?

Next, you will use this idea to write word equations, naming the starting substances as reactants and the new substances as products.