Chemistry is the science that studies the properties of matter and how matter interacts, combines and changes. There are several branches of chemistry including organic, inorganic, biochemistry, analytical, physical, agrochemistry, astrochemistry, electrochemistry, geochemistry and environmental chemistry. Matter exists in three phases - solid, liquid, and gas. The particulate theory of matter states that all matter is made of very small particles called atoms. Properties of matter can be physical, describing characteristics that don't change its identity, or chemical, describing how it reacts with other substances. Properties can also be either intensive, independent of amount, or extensive, dependent on amount.