TLDR: Chemical substances are different forms of matter that have a specific composition and unique properties. They can be simple substances (made up of one element), compounds (made up of multiple elements), or alloys. Chemical substances can exist in different states (solid, liquid, gas, or plasma) and can change from one state to another through changes in temperature, pressure, and time. Chemical substances can also be combined or converted into other substances through chemical reactions.

A chemical substance is a type of matter that has a specific chemical composition and characteristic properties. It can be a simple substance, which means it is made up of only one type of element, like pure water or gold. It can also be a compound, which means it is made up of multiple elements combined together, like table salt (sodium chloride) or sugar (sucrose). Another type of chemical substance is an alloy, which is a mixture of two or more metals, like bronze or stainless steel.

Chemical substances that cannot be separated into simpler elements by physical means are considered "pure." For example, pure water has the same properties and the same ratio of hydrogen to oxygen, whether it is found in a river or made in a laboratory. However, in practice, no substance is entirely pure, and chemical purity is specified based on its intended use.

Chemical substances can exist in different states, such as solids, liquids, gases, or plasmas. They can change from one state to another through changes in temperature, pressure, and time. For example, water can exist as a solid (ice), a liquid (water), or a gas (steam), depending on its temperature. Chemical substances can also be combined or converted into other substances through chemical reactions, where atoms rearrange to form new molecules.

In the field of chemistry, chemical substances are classified and named according to their composition and properties. They can be identified by their systematic names, which follow specific rules set by organizations like the International Union of Pure and Applied Chemistry (IUPAC). Chemical substances can also be known by their common names, which are simpler and more commonly used, like glucose or ethanol.

Chemical substances play a crucial role in various fields, including medicine, industry, and everyday life. They are used to create medicines, develop new materials, produce energy, and much more. Understanding the properties and behavior of chemical substances is essential for scientists and engineers to design and create new products and technologies.