which of the following is a physical change?

* sawing a piece of wood in half
* burning a piece of wood
* rust forming on an iron fence
* a copper roof changing color from red to green.

To determine which of the following is a physical change, we need to understand what physical changes are. Physical changes are those that only alter the physical properties of a substance, such as size, shape, or state, without changing its chemical composition.

Let's analyze each option:

1. Sawing a piece of wood in half: This is a physical change because it only changes the shape and size of the wood, but the chemical composition remains the same.

2. Burning a piece of wood: Burning wood is a chemical change because it involves a reaction with oxygen, resulting in the formation of new substances like ash and smoke. The chemical composition of the wood changes during the burning process.

3. Rust forming on an iron fence: Rust formation is a chemical change. When iron reacts with oxygen and moisture in the air, it forms iron(III) oxide, also known as rust. This process involves a change in the chemical composition of the iron.

4. A copper roof changing color from red to green: This is a chemical change as well. When copper is exposed to air and moisture, it forms copper carbonate, resulting in a greenish patina on the surface of the copper. The chemical composition of the copper changes during this process.

Based on the explanations above, sawing a piece of wood in half is the only physical change among the given options.