Substance like iron expand on heating and it can be contracted by cooling, so it is a physical change or reversible change

**Introduction:**

When a substance expands on heating, it can be categorized as either a physical change or a chemical change. In order to determine which type of change it is, we need to understand the underlying processes that occur during heating.

**Physical Change:**

A physical change refers to a change in the physical properties of a substance without altering its chemical composition. Heating can often lead to the expansion of a substance due to the increased kinetic energy of its particles. This expansion is primarily a result of thermal expansion, where the average distance between the particles increases as they gain energy.

**Examples of Physical Change:**

1. **Thermal Expansion**: Many materials, such as solids and liquids, expand when heated. This is because the increased kinetic energy causes the particles to vibrate more vigorously, leading to a greater separation between them. Common examples include the expansion of metals when heated, the expansion of liquids in a thermometer, and the expansion of gases in a balloon.

2. **Phase Changes**: Heating can also cause substances to undergo phase changes, which are considered physical changes. For instance, when water is heated, it undergoes a phase change from a solid (ice) to a liquid (water) and eventually to a gas (steam). These changes occur due to the absorption of heat energy, which breaks the intermolecular forces holding the particles together.

**Chemical Change:**

A chemical change, also known as a chemical reaction, involves the formation of new substances with different chemical properties. Unlike physical changes, chemical changes result in a rearrangement of atoms and the breaking or formation of chemical bonds.

**Examples of Chemical Change:**

1. **Combustion**: When a substance undergoes combustion, it reacts with oxygen to produce new substances and release energy in the form of heat and light. For example, when wood is heated, it undergoes combustion and transforms into ash, smoke, and gases such as carbon dioxide and water vapor.

2. **Decomposition**: Some substances may decompose when heated, leading to the breakdown of complex molecules into simpler ones. An example is the thermal decomposition of calcium carbonate (CaCO3) into calcium oxide (CaO) and carbon dioxide (CO2) when heated strongly.

**Conclusion:**

In conclusion, when a substance expands on heating, it can be classified as either a physical change or a chemical change. If the expansion is solely due to thermal expansion or phase changes, it is considered a physical change. On the other hand, if the expansion is accompanied by the formation of new substances through chemical reactions, it is classified as a chemical change. It is important to analyze the specific processes occurring during heating to determine the nature of the change taking place.