In order to observe a property of a substance, the substance must interact with another substance, such as when gasoline burns or when sodium metal reacts with water.

Observing a property of a substance can be done through experiments. Most of these experiments are done by a chemical reaction. A chemical reaction involves two substances interacting with each other. However, not all property of a substance can only be observed through a reaction. Like melting experiment, freezing point determination and boiling point determination, only heat is added or is removed to the substance in order to see that certain property.

Naoh + hbr -> nabr (salt) + h2o ca (oh)2 + 2hbr - - - > ca (br)2 + 2h2o