Chemistry is the study of matter. It can be divided into two sections: inorganic and organic. Inorganic chemistry involves acids, bases, salts, elements, and the physical state of matter in which they are found. Organic chemistry involves compounds that contain carbon. The definition of matter is anything that occupies space and has mass. Matter can exist as a solid, liquid, or gas. Temperature and pressure can affect the physical state of a chemical, but not its properties. The hazards presented by a chemical may not be the same, depending on the physical state of the material. For example, only gases burn; solids and liquids do not burn, even though they may be listed as flammable. A solid or liquid must be heated until it produces enough vapors to burn. It is important to understand the states of matter in order to better understand the physical and chemical characteristics of hazardous materials. There can be some intermediate steps in the process of classifying solids, liquids, and gases. Some solids may have varying particle sizes, from large blocks to filings, chips, and dusts. Particle sizes of vapors may vary, from vapors that are small enough to be invisible to mists that are readily visible.