Q1.
These students are describing some changes. What kinds of changes are they talking about?
Answer
They are talking about everyday changes that we notice with our senses. Two of them are physical changes (nothing new is formed) and two are chemical changes (a new substance is formed).
| What the student says | What is changing | Kind of change |
|---|---|---|
| “I placed a cube of ice here half an hour ago. It has now become water!” | Solid ice → liquid water (only the state changes) | Physical, and it can be reversed by freezing |
| “A bud that I saw yesterday on this rose plant has become a flower today.” | The bud grows and opens; new colour, new scent, new substances are made inside the plant | Chemical (a life process); it cannot be reversed |
| “The bottle containing cold water is not cold anymore.” | Only the temperature of the water has risen | Physical, and it can be reversed by cooling |
| “Yesterday, I saw some brown spots on a banana, but today it has more brown spots and a strong smell.” | Ripening and decay — colour and smell change because new substances form | Chemical; it cannot be reversed |
Why it happens: We spot a change with our senses — sight (ice becoming water, brown spots), touch (the bottle no longer cold), smell (the strong banana smell). But the sense only tells us that something changed. To decide the kind of change we must ask one question: has a new substance been formed? If yes, the change is chemical; if only shape, size, state or temperature changed, it is physical.
Tip: A strong new smell is one of the easiest clues to a chemical change. Milk turning sour, a banana over-ripening and food going bad all announce themselves by their smell.