Properties can be observed only when the substance in a sample of matter are changing into different substance

The complete statement is "chemical properties can be observed only when the substance in a sample of matter are changing into different substance".

It states a key concept in chemistry.

A chemical property is the ability of a substance, element or compound, to transform into other substances either by decomposing or by combining with one or more substances.

This transformation is defined as chemical reaction.

During chemical reactions some chemical bonds are broken and others are formed leading to the formation of one or more different substances called products.

Some examples of chemical properties are: reactivity with oxygen, reactivity with water, acidity, basicity, oxidation, reduction. The only way to tell if a substance has certain chemical property is by letting it react and so observe the change of the original substance into one or more different substances.