The substance of the universe—from a grain of sand to a star—is called matter. Scientists define matter as anything that occupies space and has mass. An object’s mass and its weight are related concepts, but not quite the same. An object’s mass is the amount of matter contained in the object and is the same whether that object is on Earth or in the zero-gravity environment of outer space. An object’s weight, on the other hand, is its mass as affected by the pull of gravity. Where gravity strongly pulls on an object’s mass, its weight is greater compared to its weight where gravity is less strong. An object of a certain mass weighs less on the moon, for example, than it does on Earth because the gravity of the moon is less than Earth's. In other words, weight is variable and is influenced by gravity. A piece of cheese that weighs a pound on Earth weighs only a few ounces on the moon.

All matter in the natural world is composed of one or more of the 92 fundamental substances called elements. An element is a pure substance distinguished from all other matter because it cannot be created or broken down by ordinary chemical means. The main elements that compose the human body are oxygen (O), carbon (C), hydrogen (H), and nitrogen (N). All the elements in our body are derived from the foods we eat and the air we breathe. An atom is the smallest quantity of an element that retains the unique properties of that element.

Most matter occurs in nature, either as pure substances or as mixtures. Pure substances have a constant composition. All specimens of a pure substance have the same makeup and properties. Pure substances may be divided into two classes: elements and compounds. Elements are fundamental substances, which cannot be broken into simpler substances. In nature, elements rarely occur alone. Instead, they combine to form compounds. A compound is composed of two or more elements joined by chemical bonds.

A mixture is composed of two or more types of matter that can be present in varying amounts and can be separated by physical changes, such as evaporation. A homogeneous mixture, also called a solution, exhibits a uniform composition and appears visually the same throughout. The air around us is a homogenous mixture of nitrogen, oxygen, carbon dioxide, water vapors, and other gases. A mixture with a composition that varies from point to point is called a heterogeneous mixture—for example, a mixture of sand in water.

This text is adapted from Openstax, Anatomy and Physiology 2e, Section 2.1: Elements and Atoms: The Building Blocks of Matter, and Openstax, Chemistry 2e, Section 1.2: Phases and Classification of Matter.