Introduction:

Water is an extraordinary substance that effortlessly demonstrates the three states of matter: solid (as ice), liquid (as familiar water), and gas (as water vapour). But what truly happens within matter when it shifts from one state to another? The answer lies primarily in the effect of changing temperature.

Let's trace the journey of ice as it transforms into water and then into vapor:

1. Solid to Liquid: Melting and the Melting Point

When you heat a solid, such as ice, you are supplying energy to its particles. This energy is absorbed as kinetic energy, causing the particles to vibrate with greater speed and intensity. As the heating continues, the supplied heat energy becomes sufficient to overcome the strong forces of attraction between the particles.

A critical point is reached when the solid melts and is converted to a liquid. The minimum temperature at which a solid melts to become a liquid at the atmospheric pressure is called its melting point. The melting point serves as an indicator of the strength of the force of attraction between its particles. For ice, its melting point is 273.15 K. This process of melting, that is, change of solid state into liquid state is also known as fusion.

The Mystery of Latent Heat of Fusion:

During melting, a peculiar phenomenon occurs: even as you continue to supply heat, the temperature of the system does not change after the melting point is reached, till all the ice melts. Where does this "hidden" heat energy go? This heat gets used up in changing the state by overcoming the forces of attraction between the particles. This "hidden" heat, absorbed without causing a rise in temperature, is called latent heat. Specifically, the amount of heat energy that is required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point is known as the latent heat of fusion. Consequently, water particles at 0?C (273 K) have more energy as compared to particles in ice at the same temperature.

2. Liquid to Gas: Boiling and the Boiling Point

When we supply heat energy to water, particles start moving even faster. At a certain temperature, a point is reached when the particles have enough energy to break free from the forces of attraction of each other. At this temperature the liquid starts changing into gas. The temperature at which a liquid starts boiling at the atmospheric pressure is known as its boiling point. Unlike evaporation (which occurs only at the surface), boiling is a bulk phenomenon; particles from the bulk of the liquid gain enough energy to change into the vapour state. For water, the boiling point is 373 K (100 °C).

The Power of Latent Heat of Vaporisation:

Similar to fusion, during boiling, the temperature remains constant even as heat is continuously supplied until all the liquid has turned into gas. This extra energy absorbed by the particles to overcome the intermolecular forces and transition into the gaseous state is the latent heat of vaporisation. This explains why particles in steam, that is, water vapour at 373 K (100 °C) have more energy than water at the same temperature, making steam capable of causing more severe burns than boiling water.

In summary, the state of matter can be changed into another state by changing the temperature. This transformation involves the absorption or release of energy, which impacts the kinetic energy of particles and their ability to overcome or succumb to the forces of attraction between them.

Fun Fact: The melting point of ice at 273.15 K is so precise that it's often used as a reference point in temperature calibration!

Question for You: If you want to cool a drink quickly, would you prefer to add ice at 0 °C or an equal mass of water at 0 °C? Explain your reasoning based on latent heat.