Which of the following statements about hydrogen is incorrect ?

Answer: 1. Hydrogen has three isotopes of which tritium is the most common.

Key idea: check each statement against NCERT facts and pick the incorrect one.

• 1: Hydrogen has three isotopes, but protium (about 99.98%) is by far the most abundant; tritium occurs only in trace amounts. This statement is incorrect.
• 2: A bare \(\mathrm{H^+}\) does not exist as a cation in ionic salts. True.
• 3: \(\mathrm{H^+}\) is hydrated in water, so \(\mathrm{H_3O^+}\) exists in solution. True.
• 4: \(\mathrm{H_2}\) can reduce metal oxides on heating, so it does act as a reducing agent; this statement is also loosely wrong, but statement 1 is the plainly false one.

The answer is option 1. The copy of the key we had (option 2) disagrees with several independent solution pages and with the facts, so it was corrected. Option 4 makes the question slightly ambiguous.