Changes Around Us: Physical and Chemical · Lesson 1 of 6
Observing and Identifying Physical Changes
“Learn how to observe everyday changes and decide whether the material itself has changed.”
• Identify physical changes in everyday life using changes in size, shape, temperature, and state. • Explain why melting ice, folding paper, and crushing chalk do not form new substances. • Compare observations before and after a change without confusing appearance with substance identity. • Explain why some physical changes can be reversed while others cannot.
Begin by noticing what changed
An ice cube left on a plate becomes a puddle. A rosebud opens into a flower. Cold water becomes warmer, and a banana develops brown patches and a different smell. Each event is a change, but the word “change” does not yet tell us what happened to the material. We begin by comparing what was present before with what is present afterward.
An observation is something you notice: a change of shape, a new smell, a sound, a warmer surface, or a different state. An explanation tells you why it happened. For example, “the clothes are less wet” is an observation; “water has evaporated into the air” explains that observation. Keeping the two separate helps us avoid guessing too quickly.
A feature such as size, shape, colour, temperature, or state that we can describe without changing the substance into another substance.
Our senses help us notice changes. Sight shows a smaller ice cube, touch can reveal a temperature difference, and hearing reveals the popping of corn. Smell and taste also help us recognise familiar food changes, but substances in an investigation must never be tasted, and unknown substances should not be smelled directly. Record safe observations before deciding how to classify a change.
| Everyday change | Observation to record |
|---|---|
| Ice melts | The solid becomes liquid water. |
| Vegetables are chopped | Large pieces become smaller pieces. |
| Water boils | Bubbles form and water changes into vapour. |
| Corn becomes popcorn | The kernels expand and split; popping sounds are heard. |
| Paper is cut | The sheet becomes pieces with different shapes. |
| Beetroot extract is added to water | The colour spreads through the water. |
| Wood burns | Heat, light, smoke, and a solid residue may be observed. |
| Wet clothes dry | The clothes lose water and become less wet. |
| Dough is divided into balls | One lump becomes several smaller rounded portions. |
| Dough balls are rolled into chapatis | The same dough becomes flatter and wider. |
This observation table includes several kinds of change. Do not put everything that looks different into one category. Rolling dough and cooking that dough are different processes. Similarly, the appearance of bubbles does not by itself tell us that a new substance formed. We need to investigate what happens to the material.
The same material can have a different shape
Fold a sheet of paper into a boat. Its outline is different, but it is still paper. You can unfold it to recover the sheet, although some creases may remain. This gives us a useful question to ask after any change: has the material simply taken a new form, or has another substance been produced?
A change in physical properties, such as size, shape, or state, in which no new substance is formed.
Now compare two balloon changes. When air enters an intact balloon, the rubber stretches. If the air is released, the balloon shrinks again. If an inflated balloon is punctured, the rubber tears and the air escapes, but the rubber does not become a different substance. Use a teacher’s demonstration rather than handling a sharp pin yourself.
Crushing chalk gives another useful comparison. The original chalk stick becomes powder. Its pieces are much smaller, and pressing the powder together will not restore the original stick in an ordinary classroom activity. Nevertheless, each grain is still chalk. The loss of the original shape is different from the formation of a new substance.
Problem
A chalk stick breaks into three pieces. Is the change physical?
- 1.First identify the starting material: chalk.
- 2.After breaking, the three pieces are still chalk; only size and shape have changed.
- 3.No new substance has formed, so the change is physical. Difficulty restoring the stick does not change this classification.
Problem
A dough ball is rolled into a thin circle. What changed?
- 1.The ball became wider and flatter, so its shape and thickness changed.
- 2.The material remains dough; rolling alone does not cook it or form another substance.
- 3.Rolling the dough is a physical change. Cooking it is a separate change that must be examined separately.
Water changes state while remaining water
Water is a clear example of a material whose appearance changes greatly without becoming a new substance. Ice is solid water, liquid water flows, and water vapour is water in the gaseous state. Moving between these states changes how water behaves and how its particles are arranged, but the substance remains water.
A change between solid, liquid, and gaseous forms of a substance.
Melting changes ice into liquid water. Freezing changes liquid water back into ice. Evaporation and boiling turn liquid water into water vapour, while condensation turns water vapour into liquid water. The cloudy mist near hot water consists of tiny liquid droplets; the gaseous water vapour itself is invisible.
Problem
A wet shirt becomes dry in the sun. Has its water changed into a new substance?
- 1.The shirt originally held liquid water between its fibres.
- 2.Water evaporates and enters the surrounding air as water vapour.
- 3.The water changes state and location, so drying by evaporation is a physical change.
It is tempting to identify a physical change by asking whether you can undo it. That shortcut fails for cut paper, punctured rubber, and crushed chalk. Use the new-substance question first. Reversibility is a separate question about restoring the original form.
Use evidence to make a classification
For a physical change, describe the physical property that changed and explain why the substance remained the same. “It looks different” is too weak because both physical and chemical changes can alter appearance. A stronger explanation is “the chalk became smaller pieces, but every piece remained chalk.”
A change in colour also needs careful thought. Beetroot colour spreading through water can happen when coloured material mixes with the water; a different colour is not automatically proof of a chemical reaction. Later investigations will show how we look for stronger evidence that a new substance has formed.
Quiz
What makes a change physical?
Why is crushing chalk a physical change?
Which description separates observation from explanation correctly?
Which process changes liquid water into gaseous water?
An inflated balloon is punctured. Which conclusion is best?
Which pair consists of physical changes?
Practice Problems
- Describe one observation for each: a warming bottle of water, an opening flower bud, and a browning banana. Do not use the observation alone to claim a type of change.
- Choose three entries from the observation table. State exactly which property changes in each.
- Explain why folding and cutting paper are both physical changes even though they differ in reversibility.
- Compare balloon inflation, deflation, and puncturing. What happens to the rubber in each case?
- Explain why ice, liquid water, and water vapour are forms of the same substance.
- A learner says that bubbles always show a chemical change. Use boiling water to challenge this claim.
- Describe the separate changes involved in dividing dough into balls and rolling one ball flat.
Key Takeaways
• A physical change alters properties such as size, shape, temperature, or state without producing a new substance. • Observe what changed before explaining why it changed. • Paper, rubber, chalk, and water can change appearance while remaining the same material. • Some physical changes are easily reversed; others are not. • Appearance, colour, and bubbles alone do not establish that a new substance has formed.
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Next · Lesson 2
How Chemical Changes Form New Substances