Matter that has a uniform and unchanging composition is a substance. A physical property is a characteristic that can be observed or measured without changing the sample's composition. Extensive properties are independent of the amount of substance present. Intensive properties are dependent of the amount of substance present. The ability of a substance to combine with or change into one or more other substances is a chemical property. All matter that exists on Earth can be classified as one of these physical forms called states of matter. A solid is a form of matter that has its own definite shape and volume. A liquid is a form of matter that flows, has constant volume, and takes the shape of its container. A gas is a form of matter that flows to conform to the shape of its container and fills the entire volume of its container. The word vapor refers to the gaseous state of a substance that is a solid or liquid at room temperature. A physical change is a type of change that alters the physical properties of a substance but does not change its composition. A chemical change is a process involving one or more substances changing into new substances; also called a chemical reaction.The law of conservation of mass states that mass is neither created nor destroyed during a chemical reaction it is conserved. A mixture is a combination of two or more substances in which each pure substance retains its individual chemical properties. A heterogeneous mixture is one that does not blend smoothly throughout and in which the individual substances remain distinct.