Chemistry is the study of matter and the changes it undergoes.
Matter is anything that occupies space and has mass. A substance is a form of matter that has a definite (constant) composition and distinct properties. A mixture is a combination of two or more substances in which the substances retain their distinct identities. In a homogenous mixture, the composition of the mixture is the same throughout. In a heterogeneous mixture, the composition is not uniform. Any mixture can be created and separated by physical means into pure components without changing the identities of the components. An element is a substance that cannot be separated further into simpler substances by chemical methods. A compound is a substance composed of atoms of two or more elements chemically united in fixed proportions. Compounds can be separated only by chemical means into pure components.
In principle, all substances can exist in three states – solid, liquid, and gas. The three states of matter can be interconverted without changing the composition of the substance.
A physical property of a substance can be measured and observed without changing the composition or identity of a substance. A chemical property of a substance can be observed only by carrying out a chemical change. Measurable properties of matter can be intensive or extensive. The measured value of an intensive property does not depend on how much matter is being considered; the measured value of an extensive property does. Intensive properties are additive; extensive properties are not. Measurements of macroscopic properties can be determined directly. Measurements of microscopic properties must be determined indirectly.
Mass is a measure of the amount of matter in an object. Weight is the force that gravity exerts on an object.
—July 2020
Lance Jones 