Chemical Reaction

What kind of change happens in following cases?

Milk is left at room temperature during summers.

Change: Milk gets curdled most of the time.

An iron girdle/pan/nail is left exposed to humid atmosphere.

Change: A brownish layer forms on iron girdle or pan or nail.

Grapes get fermented.

Change: Grapes develop a strange taste.

Food is cooked.

Change: Food becomes tastier and is easier to digest.

Food gets digested in our body.

Change: Food changes into simple nutrients.

We respire.

Change: Food is oxidized to produce energy for the body.

What is common in all these changes is that a new substance is formed after the change. You have read in earlier classes that when a new substance is formed after a change, the change is called a chemical change. Chemical Reaction is another name for chemical change.

Activity

Let us do an activity to understand chemical reaction or chemical change.

Things You Need

A magnesium ribbon, pair of tongs, Bunsen burner, watch glass

Process

• Hold the magnesium ribbon with a pair of tongs at one end.
• Burn another end of magnesium ribbon with the help of flame on Bunsen burner.

Observation

• Magnesium ribbon burns with white dazzling light.
• A powdery substance gets collected in watch glass.
• Smoke comes out of burning magnesium ribbon.

Conclusion

• There is a change in colour, which is shown by white dazzling light.
• Smoke shows a change in state from solid to gas.
• Combustion signifies change in temperature.
• Powdery substance shows a new substance is formed.

We can say that a chemical reaction involves all or any of the above changes. There can be a change in colour, change in state, change in temperature, etc. In all cases of chemical reaction, a new substance is formed.

Signs of Chemical Reaction

1. Change of state of substance
2. Change of colour of substance
3. Evolution of heat
4. Absorption of heat
5. Evolution of gas
6. Evolution of light
7. Evolution of sound, etc.