Physical Changes

Introduction

When you snap a pencil, crumple a can or leave an ice lolly in the sun, the material looks different afterwards, but it is still the same stuff. These are physical changes . They happen all the time in kitchens, factories and nature. In this page we look at the main kinds of physical change and at the key idea that links them: the substance keeps its identity from start to finish.

Core explanation

Definition. A physical change is a change in which no new substance is formed . The substance may look very different, but its particles are the same particles as before.

Types of physical change.

- Change of shape. Bending a copper wire, moulding clay, stretching an elastic band. The copper is still copper. - Change of size. Crushing rock salt into powder, sawing wood, grinding coffee beans. Each tiny piece is still the same substance as the large piece. - Change of state. Melting, freezing, boiling, evaporating, condensing and sublimation. Ice, liquid water and steam are all water, made of H₂O molecules. - Mixing and separating. Stirring sand into water, dissolving sugar in tea, filtering, distilling. The substances in a mixture keep their own properties and can be separated again.

Properties stay the same. Each pure substance has a set of characteristic properties , such as its melting point, boiling point and density. After a physical change these are unchanged. Pure water still freezes at 0 °C and boils at 100 °C (at normal atmospheric pressure), whether it came from melted ice or from condensed steam. Iron filings are still attracted to a magnet however finely they are ground.

Appearance can change. Some properties that depend on the size or arrangement of the pieces do alter. Powdered sugar looks whiter and duller than large crystals; a crumpled sheet of foil looks less shiny. These are changes of appearance, not of identity, because the substance itself is the same.

Often easy to reverse. Many physical changes can be reversed quite easily: ice can be melted and refrozen, and salt can be recovered from salt water by evaporating the water. Some cannot be undone in practice — you cannot un-cut a sheet of paper — but they are still physical because no new substance appears. Reversibility is a helpful clue, not the definition.

Energy in physical changes. Changes of state involve energy. Melting and boiling take in energy; freezing and condensing give it out. This energy moves particles apart or lets them come together; it does not break up the particles themselves.

Step-by-step reasoning

To show that a change is physical:

1. Identify the substance before the change. 2. Identify what is present afterwards. 3. Check a characteristic property, such as melting point or magnetism. 4. If the property is unchanged, the substance is the same and the change is physical.

Visual explanation

Picture a jar of identical marbles. Tip them into a tray and they spread out; pour them back and they stack up; shake them and they rattle about. Their arrangement changes each time, but every marble is exactly the same marble. That is how particles behave in a physical change.

Real-world analogy

A physical change is like rearranging the furniture in a room. The sofa might move to a new wall and the chairs might be stacked, but no furniture has been swapped for something else. Every piece is still the same object.

Real-world example

Glass recycling depends on physical changes. Bottles are crushed into small pieces called cullet, which are then melted and moulded into new bottles. The glass changes size, then state, then shape, but it is still glass throughout, which is why it can be recycled again and again.

Why?

Why does the melting point of a substance not change during a physical change? Melting point depends on the particles and the forces between them. A physical change does not change the particles, so the melting point stays the same.

Common misconception

"Dissolving makes a substance disappear, so it must be chemical." The dissolved sugar is still sugar; it has spread out into particles too small to see. You can taste it, and evaporating the water brings the sugar back. No new substance forms.

Worked example

Question: A block of wax is heated until it melts, then poured into a mould and left to set into a candle. Is this physical or chemical? Explain.

Reasoning: The wax changes state from solid to liquid and back, and changes shape. At the end it is still wax, with the same melting point.

Answer: Physical change, because no new substance is formed.

Quick check

1. Name two physical changes that involve a change of size. Answer: For example, crushing salt crystals and sawing a plank of wood.

Exam focus

Be ready to list types of physical change and give an example of each. When asked to explain, always write "no new substance is formed" and, where possible, mention that properties such as melting point stay the same.

Advanced insight

Some physical changes need a lot of energy even though no new substance forms. Melting 1 kg of ice at 0 °C takes about 334 kJ, while boiling 1 kg of water at 100 °C takes about 2260 kJ. These large values show that separating particles against the forces between them requires energy, even without any chemical reaction.

Summary

A physical change produces no new substance. It may change shape, size or state, or involve mixing and separating. The characteristic properties of the substance, such as melting point and density, stay the same. Many physical changes are easy to reverse, but reversibility is only a clue; the real test is whether a new substance has formed.

Practice questions

1. Define a physical change. Answer: A change in which no new substance is formed. 2. Give one example each of a change of shape, a change of size and a change of state. Answer: For example, bending a wire (shape), crushing a stone (size) and melting ice (state). 3. Tearing paper cannot be undone. Explain why it is still a physical change. Answer: The torn pieces are still paper; no new substance has been made, so the change is physical even though it is not easily reversed. 4. How could you show that iron filings are still iron after grinding them finer? Answer: Test a property of iron, for example show that the filings are still attracted to a magnet.