91Ó°ÊÓ

When water boils, the water molecules gain enough energy to overcome the attractive forces that hold them together in the liquid phase. These molecules escape into the air as water vapor. The bubbles observed when water boils are formed by this water vapor. Therefore, the correct answer is: iv. water vapor

To determine whether boiling water is a chemical or physical change, it is necessary to understand the differences between these two types of changes. A chemical change involves the formation of new substances with different properties than the original substances, usually due to the breaking and forming of chemical bonds. Some examples of chemical changes are rusting, combustion, and digestion. A physical change, on the other hand, does not involve the formation of new substances but merely changes the physical state or arrangement of the original substance. Examples of physical changes include melting, freezing, and breaking a solid object. In the case of boiling water, the substance is still water, but its state changes from liquid to gas. There are no new substances formed during this process, and the chemical bonds between the hydrogen and oxygen atoms in the water molecules remain unchanged. Therefore, boiling water is a: Physical Change As per the student's request, an explanation for boiling water being a physical change can be provided. When water boils, it undergoes a phase transition from liquid to gas. This phase change doesn't alter the fundamental structure of water molecules, and no new substances are formed. Since boiling water is only a change in the physical state and not a change in the chemical composition, it is considered a physical change.