How Matter Changes
FoundationIce melting, sugar dissolving and paper folding make no new substance — physical changes. Iron rusting, paper burning and food rotting create new substances — chemical changes.
- Physical changes alter the look, create no new substance, and can usually be undone.
- Chemical changes create new substances with new properties, hard to reverse.
- Melting, dissolving, evaporating and squeezing are common physical changes.
文體小知識：記敘文記錄一次科學小探究的經過。跟著同學一起動手做實驗，比較物質的變化，找出物理變化與化學變化的分別。
§Text Passage
1In science lesson, the teacher set out a dish of ice, a cup of sugar water and a rusty iron nail, and asked what the three had in common. The answer: each had gone through a change. Ice is frozen water; the sugar water is sugar dissolved in water; the rust came from iron meeting air and water. Matter changes every day — but changes come in two kinds.
2The first kind is a physical change: only the shape, state or position changes, and no new substance appears. Melt ice and you get water — still water. Sugar hides in water, but one sip finds the sweetness still there, and evaporating the water brings the sugar back. Fold a crease into paper, unfold it, and it is the same paper. Most such changes can be undone by cooling, evaporating and so on.
3The second kind is a chemical change: matter turns into a new substance. In damp air an iron nail slowly grows brittle, reddish-brown rust, and rust is no longer iron. Burnt paper becomes ash and smoke. Food left out turns sour and bad, its taste and smell all changed. Chemical changes create new substances with entirely different properties, which usually cannot simply be turned back.
4The key test is only one: does a new substance appear? Cutting an apple changes only its shape — a physical change; the flesh slowly browning in the air is a chemical change. Water freezing to ice and melting back is water all along — a physical change.
5Changes are everywhere: rice becoming cooked rice, a candle burning with light — chemical changes; squeezing modelling clay into a puppy, tearing paper to fold a plane — physical changes. Learn to tell the two apart, and you will find signs of science hidden in the kitchen, the classroom and the street.
1Summary 重點摘要
Ice melting, sugar dissolving and paper folding make no new substance — physical changes. Iron rusting, paper burning and food rotting create new substances — chemical changes.
2Vocabulary cards Vocabulary
A change of shape or state that makes no new substance.
A change that creates a new substance, such as rusting and burning.
A solid turning to liquid when heated, such as ice melting into water.
A material spreading into a liquid until it cannot be seen, like sugar in water.
A liquid slowly turning to gas and drifting away, like wet clothes drying.
Iron reacting with oxygen and water in the air to grow reddish-brown rust — a chemical change.
A substance reacting quickly with oxygen, giving out light and heat — a chemical change.
Food broken down by microbes and going bad — a chemical change.
A material formed by a change, with properties different from the original.
Able to be undone by simple means, like ice melting to water and freezing back.
3Step by step Steps
- Step 1: Matter is always changingOpen your eyes and look: morning dew vanishes in the sun; bread left out grows mouldy; an old pair of scissors shows brown spots. Matter in this world changes all the time — as fast as fireworks bursting, as slow as rocks weathering. Scientists sort these countless changes into two great kinds: physical and chemical. Learning to tell them apart is like putting on new glasses: you see the rules behind the material world, and you learn how to keep food fresh and care for objects.
- Step 2: Physical changes — no new substanceA physical change alters only the shape, size, state or position of matter; the matter itself does not become something else. Water freezing to ice, ice melting to water, water evaporating to vapour — it is water all along, only its state switching. Sugar hidden in water cannot be seen, but the taste remains, proving the sugar never vanished. Cutting or folding paper, moulding clay into a figure, rolling dough into noodles — all are changes of shape alone. The rule of thumb: the look has changed, the essence has not.
- Step 3: Physical changes can usually be undoneMost physical changes can be reversed by simple means. Melted chocolate hardens again in the fridge; salt water left by the window gives back its salt once the water evaporates; a folded sheet, creased as it is, is still the same paper. Being reversible is a common mark of physical change — but not an absolute test: torn paper is hard to piece together, yet the pieces are still paper, no new substance, so it is still a physical change. The most reliable test remains whether a new substance appears.
- Step 4: Chemical changes — a new substanceIn a chemical change, the original matter turns into a new substance with different properties. An iron nail in damp air reacts with oxygen and water and grows brittle reddish-brown rust; rust is no longer iron and flakes off at a touch. Burning paper becomes ash and smoke, giving out light and heat. Food left unrefrigerated is broken down by microbes, turning sour, mouldy and smelly. In all these changes the original matter is transformed — colour, smell and properties all different — and generally cannot be turned back.
- Step 5: The test for telling changes apartFacing a change, ask two questions. First: has a new substance appeared? Sudden colour shifts, new smells, gas bubbles, light and heat are often clues of new substances. Second: can simple means such as cooling or evaporating undo it? Ice melting to water can be frozen back — physical. Bread burnt to charcoal will never become bread again however it cools — chemical. Both questions centre on the new substance, because that is the most fundamental difference between the two kinds of change.
- Step 6: Changes all around usApply the test to daily life. Egg white turning from clear to solid white as it cooks is a chemical change; beating the egg only changes its shape — physical. Apple flesh browning after cutting, a candle burning, an iron gate rusting, milk going sour — chemical. Blowing up a balloon, tearing open a wrapper, ice melting, boiling water turning to steam — physical. Knowing the types, you also see why food is refrigerated: low temperature slows down rotting, a chemical change.
5Worked example Example
Example: In science lesson a group does six things: puts an ice cube into warm water, stirs sugar into water, folds paper into a plane, leaves an iron nail on a wet tissue for a day, burns a sheet of paper, and leaves bread on the desk for three days. Which are physical changes? Which are chemical? Why?
- Ice melting and sugar dissolving create no new substance and can be undone by freezing and evaporating — physical changes.
- Folding a paper plane only changes the paper's shape; the paper is still paper — a physical change.
- The nail grows reddish-brown rust, a new substance different from iron — a chemical change.
- Paper burnt to ash and smoke, and bread going mouldy, both create new substances — chemical changes.
- Answer: Three are physical: ice melting, sugar dissolving and folding the paper plane, because no new substance appears. Three are chemical: iron rusting, paper burning and bread rotting, because each creates a new substance with different properties that cannot easily be undone.
6Common mistakes Mistakes
7Quick check Quick check
Q1What is the most basic difference between physical and chemical changes?
Q2Which kind of change is ice melting into water?
Q3What does rusting iron produce?
Q4Name one way to prevent iron from rusting.
Q5How can you prove the sugar is still there after it dissolves?
8Recap Recap
- Physical changes alter the look, create no new substance, and can usually be undone.
- Chemical changes create new substances with new properties, hard to reverse.
- Melting, dissolving, evaporating and squeezing are common physical changes.
- Rusting, burning and rotting are common chemical changes.
- One test decides all: does a new substance appear?