Oxygen and Hydrogen Qns

1. The first airships were filled with hydrogen gas. Now helium is used in airships.
   
   1. what property do hydrogen and helium have in common which makes them useful in airships?
   2. Explain why helium is now used and hydrogen is no longer used.
   3. write a balanced chemical equation to show what happens when hydrogen burns in oxygen gas?
2. (a) By giving one reason, explain the following facts:
   i. During laboratory preparation of oxygen gas, little manganese dioxide is added to hydrogen peroxide.
   ii. Fish can obtain oxygen for respiration although spend their life in water.
   iii. Oxygen gas can be used for welding activities although it does not burn.
   (b) Which property enables the use of hydrogen gas in:
   i. Fuelling.
   ii. Manufacturing of margarine.?