New words：

hydrogen [ˈhaɪdrədʒən] n. 氢；氢气
hydrogen gas 氢气

Sample Sentences：

Water is composed of hydrogen and oxygen.
Hydrogen​​ is the lightest gas in the universe.
When ​​hydrogen​​ burns in air, it produces water.

compose [kəmˈpoʊz] v.组成；

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