A glass of water does not turn into ice as it reaches 0°C. It is because

When water reaches 0°C, it does not automatically turn into ice because a phase change requires the removal of latent heat. Latent heat is the energy released or absorbed during a change in physical state that occurs without a change in temperature [2]. Specifically, for liquid water to solidify into ice at its freezing point, the 'latent heat of solidification' (or fusion) must be extracted from the water [1]. This energy removal is necessary to allow the molecules to form the rigid bonds characteristic of a solid [3]. Without removing this specific amount of energy—approximately 334 kJ/kg for water—the substance remains in a liquid state at 0°C. Additionally, factors like the absence of nucleation points can lead to supercooling, where water remains liquid even below 0°C until a seed crystal or disturbance triggers the release of latent heat [1].

Sources

1. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 22: Vertical Distribution of Temperature > Latent Heat > p. 294
2. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 22: Vertical Distribution of Temperature > Explanation > p. 295
3. [3] https://en.wikipedia.org/wiki/Supercooling