What is ‘gasohol'?

Hydrogen compounds are abundantly available on earth. Hydrogen can thus be available in plenty and it has high calorific value. But this is not used as a common fuel. Give any two reasons.

Which of the following phenomena may help in the making of more destructive weapon? Name the weapon.

(i) Nuclear fission (ii) Nuclear fusion 

The solar constant is $1.4 \mathrm{kW}$ per square metre. If all the heat is to be absorbed by a bucket containing $100 \mathrm{kg}$ of water, calculate the rise in the temperature of water in$1/2$ hour. Specific heat capacity of water is $4.2 \mathrm{kJ}/\mathrm{kg}°\mathrm{C}.$

What is the principle of working of solar cooker? Write two major drawbacks of all solar heating devices.

Give three reasons why the use of cow-dung in biogas plants is preferred to burning cow-dung cakes for cooking.