What happens to an object's density as the object is heated?

As the particles in a substance are heated, the density of the substance decreases.

Heat is the total amount of energy possessed by the particles that make up a substance.  Oftentimes, the addition of heat energy results in an increase in the motion of the particles that make up the substance. In other words, the particles gain kinetic energy. Kinetic energy is the energy of movement. Because temperature is a measurement of the average kinetic energy of the particles of a substance, heating a substance is often accompanied by an increase in the temperature of the substance.  When particles gain kinetic energy, the particles begin to move more and faster. This causes the particles to move away from one another.

The formula for density is mass divided by volume. As a result of the increased distance between the heated particles, the volume of the substance increases but the substance's mass stays the same. Therefore, the density of the substance decreases.