The camphor experiment
The first part of the activity is about camphor — and it is the odd one out of the four.
Activity 16.1 (i)
Hold a metal spoon in a Bunsen flame, heat it strongly and set it aside. Put a camphor ball on it and observe. Cover it at once with another metal spoon. After a little while, observe the inner face of the covering spoon.
The observation
The camphor ball melts and turns into vapour. A white powder has formed on the inner face of the covering spoon.
What happened?
On the hot spoon the solid camphor melts and then becomes a vapour. On the cool surface of the covering spoon that vapour condenses again into a thin layer of solid camphor.
The substance did not change
Through all three states — solid, liquid and vapour — what was there was camphor. All that changed was its physical state, that is, the arrangement of its particles.
Physical change
Changes in which the substance itself does not change and only its physical state does are called physical changes.
The white powder on the covering spoon is not a new substance — it is camphor, returned to the solid state. The proof is that heating it again turns it straight back into vapour.