A chocolate manufacturer heats solid chocolate in a large vat until it melts completely. Once all the chocolate has melted, the manufacturer notices that the mass of the liquid chocolate is the same as the mass of the solid chocolate before heating. Explain why the mass is conserved during this change of state, and describe how the physical properties of the chocolate differ between the solid and liquid states in terms of particles.

Melting is a physical change, so no chemical reaction occurs and no particles are created or destroyed – the mass of the chocolate therefore remains the same.

During melting the chocolate is heated until the temperature reaches its melting point. The energy supplied increases the kinetic energy of the particles until the attractive forces that hold them in a fixed lattice are overcome.

In the solid state the particles are tightly packed in a regular arrangement and can only vibrate about fixed positions, held together by strong intermolecular forces.

In the liquid state the particles are still in contact but are free to move past one another; they can flow and take the shape of the container while the intermolecular forces still act between them.

Because the particles are merely rearranged and not destroyed, the mass is conserved.