Final answer:
It is true that observing changes in properties of matter can indicate a chemical reaction. These can include changes in color, temperature, light emission, gas production, and the formation of a precipitate, all of which are evidence of a substance undergoing a chemical change.
Step-by-step explanation:
True, a chemical reaction can often be detected by observing changes in the properties of matter. During a chemical change, one or more substances are converted into different substances with distinct chemical and physical properties. Some common observations that indicate a chemical change include unexpected color changes, temperature changes such as an increase or a decrease in temperature, light given off during a reaction, the formation of bubbles indicating gas production (especially when the substance isn't boiling), and the formation of a precipitate - a solid that forms from a solution during a chemical reaction.
Chemical reactions may also involve changes in chemical properties like flammability, or the ability to react with other substances. For example, when hydrogen reacts with oxygen it forms water, which is a different substance with different properties compared to the original hydrogen and oxygen.
When chemists describe matter and its changes, they refer to both physical and chemical properties. Physical properties like size, shape, color, and mass can be observed without changing the substance's identity. In contrast, observing chemical properties involves inducing a chemical change, which results in a new substance with different characteristics.