Composition of Matter I. Pure Substances i. A pure substance is either an element or a compound ii. Substances cannot be broken down into simpler components and still maintain the properties of the original substance a. Elements i. If all atoms in a substance are alike, that substance is an element. ii. There are 90 elements found in nature b. Compounds iii. A compound is a substance in which atoms of two or more elements are combined in a fixed proportion I. Mixtures II. A mixture is a material made up of two or more substances that can be easily separated by physical means a. Heterogeneous Mixtures i. A mixture in which different materials can be distinguished easily is called a heterogeneous mixture. ii. You might not be able to spot these mixtures with the naked eye. iii. Some components are easy to see, but others are not iv. The component you might see can be a mixture itself b. Homogeneous Mixtures v. A homogeneous mixture contains two or more gaseous, liquid, or solid substances blended evenly throughout. vi. Vinegar is a homogeneous mixture. It appears clear even though it is made up of particles of acetic acid mixed with water. vii. A solution is a homogeneous mixture of particles so small that they cannot be seen with a microscope and will never settle to the bottom of their container.