When pure water boils vigorously, the bubbles that rise to the surface are composed primarily of

The correct answer is D, water vapour. At the boiling point, water turns into vapour inside the liquid itself, and these pockets of steam are the bubbles that rise and burst at the surface. Boiling is different from evaporation. Evaporation happens only at the surface and at any temperature, while boiling happens throughout the liquid once its vapour pressure equals the pressure of the air above it, which for pure water at sea level is 100 degrees Celsius. The heat supplied at this stage, the latent heat of vaporisation, goes into changing liquid into gas, so the temperature stays steady until all the water has boiled away. Option A is wrong because the small bubbles of dissolved air appear early, while the water is only warming, and are soon driven out. Option B is wrong because boiling is a physical change, so water does not break into hydrogen. Option C is wrong because splitting water into hydrogen and oxygen needs electrolysis or very high temperatures, not ordinary boiling. Exam tip: boiling is a physical change; the bubbles are steam, not separated gases.