State some sources of hydrogen.

In free state, hydrogen is found in traces in earth's crust and atmosphere. Volcanic gases contain 0.025% of it, the earth's crust 0.98%, the earth's atmosphere 0.01% and the atmospheres of the Sun and the stars also contain hydrogen in large amounts.

In combined state :

1. Plants and animal tissues are made up of compounds of hydrogen with carbon, oxygen and nitrogen.
2. Hydrogen is the characteristic constituent of acids, alkalis, hydrocarbons and proteins. In addition to these, sugar, starch, petroleum products, proteins, carbohydrates and also fats contain hydrogen.
3. In water, it is 11.1% by weight.