Chemistry
Browning of paper in old books is caused by

Frequent use
Collection of dust
Oxidation of cellulose
Lack of aeration

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Chemistry
Water is not effective in extinguishing a fire caused by petrol because

Water and petrol react chemically
Water and petrol are miscible with each other
The flame is too hot for water to cool it down
Water and petrol are immiscible with each other and petrol which forms the upper layer continues to burn

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