Physical or Chemical? Deciding Between Them

Introduction

Some changes are easy to classify: melting ice is physical, burning paper is chemical. Others are harder. Is a firework's bang physical? Is the fading of a pair of jeans chemical? What about mixing two colourless solutions? Scientists need a reliable way to decide, not just a feeling. This page sets out the new-substance test and shows how to support a decision with evidence.

Core explanation

The one real test. A change is chemical if a new substance is formed, and physical if it is not. Every other clue — colour, bubbles, heat, reversibility — is useful only because it helps you decide whether a new substance has appeared.

Gathering evidence. To apply the test, compare the material before and after, looking at its characteristic properties :

- Melting and boiling points. A new substance usually has different ones. - Colour, smell and state at room temperature. - Behaviour , such as whether it dissolves, conducts electricity, is magnetic or reacts with acid. - Reversibility. Physical changes are often easily reversed by simple means such as cooling or evaporating. Chemical changes usually are not.

A decision table.

Change New substance? Decision --- --- --- Ice melting No — still water Physical Salt dissolving No — salt returns on evaporation Physical Candle wax melting No — still wax Physical Candle wick burning Yes — carbon dioxide and water Chemical Bread toasting Yes — brown, bitter substances Chemical Iron rusting Yes — iron oxide Chemical

Misleading appearances. Watch out for these traps:

- Colour change without reaction. Dissolving blue copper sulfate crystals makes a blue solution, but no new substance forms; the colour is simply spread out. - Bubbles without reaction. Boiling water makes bubbles of steam, and opening a fizzy drink releases dissolved carbon dioxide. Both are physical. - Heat without reaction. Dissolving some solids makes water warmer or colder, but that alone does not prove a reaction. - Irreversible but physical. Shattering a glass cannot be undone, yet the pieces are still glass.

Weigh several clues. One clue on its own can mislead. Two or three together, especially a change in properties such as melting point or the formation of a solid with a different colour, give strong evidence of a chemical change.

Step-by-step reasoning

A reliable method:

1. State what you start with. 2. Observe carefully what you end with. 3. Test or compare a characteristic property. 4. Ask: is this a new substance? Use at least one piece of evidence to support your answer. 5. Write the decision with a reason.

Visual explanation

Think of a flowchart. The first box asks "Is there a new substance with new properties?" A "no" arrow leads to "physical change"; a "yes" arrow leads to "chemical change". Side boxes labelled colour, gas, heat and reversibility all feed evidence into that first question rather than bypassing it.

Real-world analogy

A detective does not convict someone because they look guilty. They collect several clues — fingerprints, witnesses, timing — and ask whether together they prove the case. Classifying a change works the same way: the clues must point to a new substance.

Real-world example

Food scientists must know which changes are chemical. Freezing strawberries is physical, so thawed strawberries are still strawberries, though softer. Cooking them into jam involves chemical changes in the sugars and fruit, which is why jam tastes different and cannot be turned back into fresh fruit.

Why?

Why is reversibility only a clue and not the rule? Because some physical changes, such as smashing a plate, are impossible to reverse in practice, while some chemical reactions can be reversed under the right conditions. Only the presence of a new substance settles the question.

Common misconception

"If the change gives out heat, it must be chemical." When water vapour condenses or liquid water freezes, heat is released, yet both are physical changes. Heat is evidence to weigh, not proof.

Worked example

Question: A white powder is heated. It melts at 801 °C and, on cooling, sets back into a white solid that also melts at 801 °C. Is the change physical or chemical?

Reasoning: The solid after cooling has the same appearance and the same melting point as the starting powder, so it is the same substance.

Answer: Physical change; no new substance has been formed.

Quick check

1. Opening a bottle of fizzy water produces bubbles. Is this physical or chemical? Answer: Physical, because the bubbles are carbon dioxide that was already dissolved in the water; no new substance forms.

Exam focus

When asked to classify a change, always state the decision and the reason. Credit is given for "a new substance is formed" or "no new substance is formed", ideally backed by one piece of evidence such as a changed property.

Advanced insight

Chemists confirm new substances with instruments rather than appearance. Measuring a sharp melting point, or using techniques such as chromatography and spectroscopy, can show whether the particles present after a change are the same as before. These methods apply the same new-substance test with much greater certainty.

Summary

The key question is whether a new substance has been formed. Evidence comes from comparing properties such as melting point, colour, smell and behaviour, and from reversibility. Single clues like colour, bubbles or heat can mislead, so combine several. Always state the classification together with the reason.

Practice questions

1. What is the single test for deciding whether a change is chemical? Answer: Whether a new substance with different properties has been formed. 2. Explain why dissolving blue copper sulfate in water is physical even though the water turns blue. Answer: The copper sulfate is only spread through the water; it can be recovered by evaporation, so no new substance forms. 3. A piece of toast is scraped and the black layer tastes bitter and does not melt. Physical or chemical? Explain. Answer: Chemical, because the black layer is a new substance with different properties from bread. 4. Give an example of a physical change that is difficult to reverse. Answer: For example, breaking a glass or cutting hair; the material is unchanged but cannot easily be put back.