Hydrogen directly combines with ______? 

The correct answer is Ca.

Key Points

• Calcium is more reactive than these metals and can directly combine with hydrogen to form calcium hydride (CaH₂).
• This reaction is commonly used for hydrogen storage and generation applications.
• Hydrogen does not directly combine with gold (Au), copper (Cu), nickel (Ni), or calcium (Ca) under normal conditions.
• These metals are relatively unreactive toward hydrogen, which means that they do not readily form chemical compounds with hydrogen by direct combination.

Important Points

• Under certain conditions, such as high temperature and pressure or in the presence of a catalyst, hydrogen can react with these metals to form compounds.
• For example, nickel can react with hydrogen at high temperatures and pressures to form nickel hydride (NiH₂), which is a useful hydrogen storage material.
• Similarly, copper and gold can form hydrides under extreme conditions.