1. Water freezing: When water freezes, it undergoes a physical change from a liquid to a solid state. This change is reversible, and the water can be melted back into a liquid by heating it.

2. Water boiling: When water boils, it undergoes a physical change from a liquid to a gas state. This change is also reversible, and the water vapor can be condensed back into a liquid by cooling it.

Chemical changes:

1. Electrolysis of water: When water is subjected to an electric current, it undergoes a chemical change and breaks down into hydrogen and oxygen gases. This change is not reversible, and the hydrogen and oxygen gases cannot be recombined to form water.

2. Hydrochloric acid dissolving limestone: When hydrochloric acid comes into contact with limestone, it undergoes a chemical change and reacts to form calcium chloride and carbon dioxide gas. This change is not reversible, and the calcium chloride and carbon dioxide gas cannot be recombined to form hydrochloric acid and limestone.