When heating an object, its molecules refer to the individual particles that make up the material. Molecules are composed of atoms that are chemically bonded together. In most cases, the molecules in a solid, liquid, or gas are constantly in motion, vibrating or moving around within the material.
In a solid, the molecules are closely packed and have limited mobility. As the object is heated, the energy is transferred to the molecules, increasing their kinetic energy and causing them to vibrate more vigorously. This increased molecular motion is what we perceive as an increase in temperature.
In a liquid, the molecules have more freedom of movement compared to a solid. They can slide past one another but still remain relatively close together. When the object is heated, the molecules gain more energy, increasing their motion and causing the liquid to expand and potentially change into a gas if the temperature continues to rise.
In a gas, the molecules are far apart and move freely throughout the available space. When heat is applied, the molecules gain even more energy, increasing their speed and causing the gas to expand.
Overall, when an object is heated, the energy is transferred to its molecules, causing them to move more rapidly and increase the object's temperature.