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

When water is heated, the initial small bubbles that appear are often dissolved gases like nitrogen and oxygen escaping the liquid [3]. However, as the water reaches its boiling point and begins to boil vigorously (a 'rolling boil'), the phase change from liquid to gas occurs throughout the liquid [1]. At this stage, the movement of water particles becomes so energetic that they overcome interparticle forces, forming pockets of gaseous H2O, or water vapour, within the liquid [1]. These bubbles rise to the surface and release steam [2]. Unlike electrolysis, which chemically decomposes water into hydrogen and oxygen gases, boiling is a physical change where the chemical identity of the water molecules remains unchanged. Therefore, in vigorously boiling pure water, the bubbles are primarily composed of water vapour.

Sources

1. [1] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 7: Particulate Nature of Matter > 7.2.2 Liquid state > p. 105
2. [3] Science , class X (NCERT 2025 ed.) > Chapter 3: Metals and Non-metals > Activity 3.10 > p. 43
3. [2] Science ,Class VIII . NCERT(Revised ed 2025) > Chapter 8: Nature of Matter: Elements, Compounds, and Mixtures > 8.3 What Are the Types of Pure Substances? > p. 121