When a substance is in a solid state, its particles are very tightly packed together, resulting in a high density. As the substance is heated, the particles gain kinetic energy and begin to move more freely, causing the substance to expand and become less dense. This decrease in density continues until the substance reaches its melting point, at which point it becomes a liquid.
In a liquid state, the particles are still quite close together but have more freedom to move around, resulting in a lower density compared to the solid state. As the substance is heated further, the particles gain even more kinetic energy and move even more freely, causing the liquid to expand further and become even less dense.
