Because of various causes such as heat, light and pressure, change comes about in matter. In some changes only the state and the structure of the matter change, while in others there is a difference in their very properties. On this basis, the changes that happen in matter have been divided into two kinds: physical and chemical.
If no change ever came about in matter, what effect would there be in nature?
Physical change
A change in which the physical properties of matter, such as its state, colour, volume, shape and density, change but its chemical properties stay the same is called a physical change.
In daily life a great many examples of this kind of change can be seen. A lump of salt breaking up to become powdered salt; snow or ice melting to become water, and water freezing to become ice; cutting wood to make firewood; grinding spices to make powder; and mixing salt into water to make a solution are all examples of physical change.
Characteristics of physical change
| a | Even though the state of the object changes, its properties stay the same. |
| b | When a physical change happens, no new substances are formed. |
| c | The substance can be brought back to its earlier state. |
| d | In a physical change the change is temporary. |
| e | No change comes about in the weight of the substance. |
| f | A physical change happens through the use of energy such as heat, electricity and light on the substance. |
| g | No change comes about in the internal structure of the substance. |
Chemical change
The very process in which the property of one substance changes and a substance with a different property is formed is a chemical change. In this kind of change, a change comes about in the internal structure of the matter itself.
In the activity above, magnesium burning to form a white substance called magnesium oxide is a chemical change. When magnesium burns it reacts chemically with oxygen, and magnesium oxide is formed. Besides this, rust forming on iron; fruits and vegetables rotting when they are kept for a long time; the food we eat being changed into energy by reactions in the body; green plants making food; burning firewood; compost manure being formed from leaf litter; and hydrogen and oxygen combining to make water are all examples of chemical change.
Characteristics of chemical change
| a | When a chemical change happens, new substances with different properties are formed. |
| b | A chemical change is a permanent change, so the substance cannot be brought back to its earlier state. |
| c | A change comes about in the internal structure of the matter. |
| d | Chemical change happens in substances because of direct contact between them, and because of heat, light, pressure, electricity and catalysts. |
| e | In a chemical change there is a process of absorbing heat or of releasing heat. |
Comparing the two kinds of change
| Physical change | Chemical change |
|---|---|
| 1. A change comes about in the state of the object. | 1. A change comes about in both the state and the properties of the object. |
| 2. The substance can be brought back to its earlier state. | 2. The substance cannot be brought back to its earlier state. |
| 3. Substances with new properties are not formed. | 3. Substances with new properties are formed. |
| 4. The change is temporary. | 4. The change is permanent. |
Those two lists look like twelve separate facts to memorise, and they are not. They are all consequences of one idea, and you already have everything you need to see it, because you have just spent two topics learning about molecules and atoms. In a physical change, the molecules are moved about but never broken. In a chemical change, the molecules are broken and their atoms are rebuilt into different molecules. That is the whole difference. Take ice melting. Ice is made of water molecules held close together, and heating it lets them slide apart, but every molecule is still H₂O. Nothing was made and nothing was lost, so no new substance appears, the weight is unchanged, and cooling it simply lets the molecules settle back together again, which is why the change is reversible and temporary. Now take wood burning. The molecules of the wood are torn apart, their carbon and hydrogen atoms join with oxygen from the air, and out come new molecules of carbon dioxide and water vapour, along with ash. Those are genuinely different substances with genuinely different properties, and putting them back would mean taking apart the new molecules and rebuilding the old ones, which no amount of cooling will do. Every single line in both lists follows from that one sentence: physical change rearranges molecules, chemical change rearranges the atoms inside them.
The mass puzzle, and how to think about it honestly
Look again at characteristic (e) of a physical change: no change comes about in the weight of the substance. That is straightforward enough, since the molecules are the same molecules and none has gone anywhere. But it raises an obvious question about chemical changes, and it is worth answering carefully rather than guessing.
| What you see | What people conclude | What is really happening |
|---|---|---|
| Burn a heavy log and only a small heap of ash is left | That matter must have been destroyed | Nothing was destroyed. Most of the wood left as carbon dioxide and water vapour, which are invisible gases |
| Burn magnesium ribbon and the white powder weighs MORE than the ribbon did | That matter must have been created | Nothing was created. Oxygen from the air joined the magnesium, and that oxygen has mass |
Put those two rows side by side and the lesson is a good one. In a chemical change, atoms are never made and never destroyed; they are only rebuilt into different molecules. If you could weigh everything that took part, including the invisible gases going in and coming out, the total would be exactly the same before and after. Burning wood looks as though matter has vanished only because you can weigh the ash and cannot weigh the smoke. Burning magnesium looks as though matter has appeared only because you can weigh the powder and cannot weigh the oxygen that walked in from the air.
Why iron gates get painted
You know that after the grill, the windows and the gate of a house have been made, they are painted. Why might that be? And why is galvanising done on corrugated iron sheets? Both questions have the same answer, and it is about chemical change. Iron reacts chemically with the oxygen and moisture in the air to form rust, which is a new substance with new properties: it is reddish brown rather than grey, it is flaky rather than strong, and it cannot be turned back into iron by scraping. Since a chemical change is permanent, the only sensible approach is to stop it happening in the first place, and that means keeping the air and water away from the iron. A layer of paint does this, and so does the thin coat of zinc that galvanising puts on a sheet of iron. Notice why aluminium window frames are usually left unpainted: aluminium also reacts with oxygen, but the layer it forms is thin, tough and stays stuck, so it protects the metal underneath instead of flaking off as rust does.
Two items on the physical list surprise students every year, so it is worth meeting them now rather than in an examination. The first is dissolving salt in water. It looks final, because the salt disappears completely and you certainly cannot pick it out again with your fingers. But no new substance has been made: the salt is still salt, and boiling the water away leaves it sitting in the pan exactly as it was. So it is physical. The second is milk turning into curd, which looks similar to dissolving but is not. The milk has genuinely become a different substance with different properties, and no amount of heating or cooling will turn curd back into milk, so it is a chemical change. The two-question test sorts both of them correctly: is there a new substance, and can you get the old one back? Salt in water gives no and yes, which is physical. Milk to curd gives yes and no, which is chemical.
Sorting practice
| Change | New substance? | Can you get the old one back? | So it is |
|---|---|---|---|
| Water evaporating | No, still H₂O | Yes, cool the vapour | Physical |
| Steam condensing into water | No, still H₂O | Yes, heat it again | Physical |
| Sugar dissolving in water | No, still sugar | Yes, evaporate the water | Physical |
| Boiling an egg | Yes, the white has changed permanently | No, cooling does not undo it | Chemical |
| Mineral fuel burning | Yes, carbon dioxide and water vapour | No, not without another reaction | Chemical |
| Rust forming on iron | Yes, rust is not iron | No, scraping does not restore it | Chemical |
Notice how easily the third and fourth columns agree with each other. Whenever there is no new substance, you can get the old one back; whenever there is a new substance, you cannot. That is not a coincidence, and it is the same idea in two different words. If the molecules were only moved about, moving them back is easy. If the molecules were taken apart and rebuilt, putting them back needs another chemical reaction, and heating or cooling will not do it.
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