Page 38 - ICSE Chemistry 8

Curdling of Milk

When we add a tablespoon of curd to a container with milk in it and leave it at room temperature
overnight, we observe that the whole milk changes into curd which cannot be changed back to milk. This
shows that curdling of milk is a chemical change.

Baking a Chapatti

When we take some dough of fl our, roll a chapa out of it and bake it on a griddle, we will observe that
the dough changes into a baked chapa . Now, it is not possible to get back the dough of fl our. This shows
that baking a chapa is a chemical change.

Fermentation

Fermenta on is a process by which a substance is broken down by the ac on of microorganisms with the
release of heat and carbon dioxide. When dough is fermented, bread is made which is so . Fermenta on
of dough is a chemical change which is man-made, slow and desirable.

Decomposi on of water into hydrogen and oxygen gas, germina on of seed and burning of fuels are
some more chemical changes.

Activity 2.7

Aim: To show that change of colour occurs during a chemical change
Materials required: A burner, sugar, a pan
Procedure

• Take some sugar in a pan.
• Heat the sugar.

Observa on: The sugar fi rst melts and then changes its colour—fi rst to reddish-brown and then to black.
The black residue is charcoal.
Conclusion: This shows that change of colour takes place during a chemical change.

Table 2.1 shows some diff erences between physical and chemical changes.

Table 2.1: Diff erences between Physical Change and Chemical Change

Physical change Chemical change

A physical change is temporary. A chemical change is permanent.

In a physical change, no new substance is formed. In a chemical change, new substances are formed.

There is no change in the composition of the substance. The composition of the substance changes.

The change is generally reversible. The change is generally irreversible.
Very little heat energy may or may not be absorbed or Large amount of heat energy is absorbed or released.
released.

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