Lesson 17 of 19
The big picture
Show on a big screenTwo questions sort most changes.
| Idea | Remember | Example |
|---|---|---|
| Shape and size | Folding and cutting change the shape. The paper stays paper. | A cut scrap is still paper. |
| Melting and boiling | Ice, water and steam are one substance in three forms. | 200 g ice, 200 g steam. |
| Dissolving | Sugar in water is spread too thin to see. | Salt pans on the coast. |
| Cannot be undone | No new substance is made. Some changes still cannot be undone. | A broken clay pot. |
| A new substance | A chemical change makes a new substance. | Lime water turns milky. |
| Five signs | A sign is only a clue. Ask if the stuff at the end is new. | Steam is a gas, and it is water. |
| One candle | Wax melting is physical. Wax vapour burning is chemical. | A candle has both. |
| Two questions | Ask two questions. Is something new made? Can we undo it? | A broken pot: no, and no. |
| Slow changes | Rocks break in physical changes. Some rocks change into new substances. Most of these changes are slow. | Water freezes in a crack. |
| Burning | Combustion needs a combustible substance and oxygen from the air. It gives out heat, light or both. | Dry grass, charcoal, petrol |
| Fire triangle | A fire needs three things: fuel, oxygen and enough heat. Take one away and the fire goes out. | Water takes heat away |
| Word equations | In a word equation, the starting substances are on the left. The new substances are on the right. | magnesium + oxygen → magnesium oxide |
| Useful and harmful | A desirable change helps us. An undesirable change harms us. The same change can be both. | Rot: bad in a cupboard, good in a pit |
A chemical change makes a new substance. A physical change does not. Whether we can undo it is a different question.
Now open Chapter 5, Changes Around Us: Physical and Chemical. Read it with these pictures in mind.