What causes a substance to change from solid to liquid?

A substance changes from solid to liquid due to the increase in temperature, which provides energy to break intermolecular bonds.

When a solid substance is heated, the temperature increases. This increase in temperature provides the energy needed to overcome the forces of attraction between the particles in the solid. These forces, known as intermolecular bonds, keep the particles in a fixed, rigid structure. As the temperature rises, the particles gain more kinetic energy and start to vibrate more vigorously.

Eventually, the energy supplied is sufficient to break these intermolecular bonds. This allows the particles to move more freely, transitioning the substance from a solid state to a liquid state. This process is known as melting. The specific temperature at which a solid turns into a liquid under standard pressure is called the melting point. Different substances have different melting points due to the varying strengths of their intermolecular bonds.

It's important to note that the substance doesn't change chemically during this process. It remains the same substance, just in a different state. This is a physical change, not a chemical one. The process is also reversible. If the liquid is cooled, it will lose energy, the movement of the particles will slow down, the intermolecular bonds will reform, and the substance will return to its solid state. This is known as freezing or solidification.

In summary, the transition of a substance from a solid to a liquid is caused by an increase in temperature, which provides the energy needed to break the intermolecular bonds that hold the particles together in the solid state. This allows the particles to move more freely, resulting in a change of state.