In electrolysis of water, the gas evolved at the cathode is:

**The reasoning** In electrolysis of water, we're using electricity to split H₂O molecules. Here's the key principle: **cathodes attract cations (positive ions)**. The reaction: 2H₂O → 2H₂ + O₂ Water contains H⁺ ions (from H₂O dissociation). At the **cathode (negative electrode)**, these H⁺ ions are attracted and gain electrons: 2H⁺ + 2e⁻ → H₂ ↑ At the **anode (positive electrode)**, OH⁻ ions lose electrons: 4OH⁻ → O₂ + 2H₂O + 4e⁻ So **hydrogen gas bubbles at the cathode**, oxygen at the anode. **Why the wrong options tempt you** **A) Oxygen** — This is the biggest trap! Oxygen *is* produced, but at the anode (positive electrode), not the cathode. Students mix up which gas goes where. **C) Chlorine & D) Nitrogen** — These aren't even in water's formula (H₂O), so they can't form. **Quick takeaway** Remember: **"Cats are positive"** — **Cat**hodes attract **positive** H⁺ ions, producing hydrogen gas. The anode gets oxygen because it attracts negative OH⁻ ions.