Everything is made of stuff.
Some stuff is just one thing. Pure water is one kind. It stays the same every time.
This helps us know the world. Can you find something pure?
Everything is made of stuff.
Some stuff is just one kind of thing. We call this a substance. It stays the same every time.
A substance can be a solid or a liquid. It can even be a gas. Changing the heat can change it. For example, water can turn into steam.
Sometimes, different things mix together. This makes a mixture. You can separate a mixture back into its parts. This is not a single substance.
Pure water is a great example. It is always made of the same parts. It is a special kind of substance.
Can you find a substance in your house?
A substance is a special kind of matter. It always has the same parts. These parts are called a constant composition.
Substances can be elements. An element is a substance made of only one kind of atom. You cannot break an element down into something else.
There are 118 known elements. Most elements are metals. Metals like gold or iron shine. They also move heat and electricity well. Other elements are non-metals, like oxygen.
Substances can also be compounds. A compound is made when two or more elements join together. They do this through a chemical reaction.
Pure water is a compound. It always has two hydrogen atoms and one oxygen atom. This ratio is always 2:1.
Sometimes, substances mix together. This makes a mixture. A mixture does not have the same parts every time. You can separate a mixture using simple ways. For example, you can use a magnet to pull iron out of sulfur.
This shows it is a mixture, not a single substance.
A chemical substance is a unique form of matter. It always has a constant composition, which means its parts are always the same. This substance will also show characteristic properties. These are special traits that help us identify it.
Substances can exist in different physical states. These states are called phases. A substance might be a solid, a liquid, or a gas. It can even be a plasma. A substance changes between these phases when the temperature or pressure changes. For example, water can be liquid or steam. Even though it looks different, the chemical parts stay the same.
There are two main ways substances are built. One way is through elements. An element is a substance made of only one kind of atom. You cannot break an element down using a chemical reaction.
The other way is through chemical compounds. A compound is formed when two or more elements join together. They do this through a chemical reaction. This creates a new substance with its own set of rules. All compounds are substances, but not all substances are compounds.
Some compounds are organic, meaning they use carbon and hydrogen. Others are inorganic. Some are even called organometallic if they use a bond between carbon and a metal.
Scientists have worked for a long time to understand these rules. In the late eighteenth century, a chemist named Joseph Proust did important work. He studied pure chemical compounds like basic copper carbonate. He found that all samples of a compound have the same proportions of elements. This idea is called the law of constant composition.
Later, researchers faced a hard job with something called isomerism. Isomers are substances that have the same parts but a different arrangement. Friedrich August Kekulé helped explain the structure of benzene. This helped people understand how atoms are put together in different ways.
We can find many different substances in our world. There are 118 known elements in total. About 80 of these are stable, so they do not change on their own.
Many elements are metals like gold, iron, or copper. Metals often shine and carry heat or electricity well. Other elements, like oxygen or nitrogen, are non-metals. Some substances are very common, like table salt or refined sugar. Pure water is a great example of a substance. It always has a ratio of two hydrogen atoms to one oxygen atom. This is written as H2O.
It is easy to confuse a substance with a mixture. A mixture contains two or more substances that are just mixed together. They do not have a fixed composition.
You can often separate a mixture using mechanical ways. For instance, you can use a magnet to pull iron away from sulfur. A mixture like butter or soil is very different from a pure substance. Even things that look smooth can be mixtures. Cranberry glass looks like one color, but it contains tiny gold particles. Understanding the difference helps us see how the world is built.
A chemical substance is a unique form of matter. It possesses a constant chemical composition and specific characteristic properties. These properties allow scientists to identify and distinguish one substance from another.
Substances can exist in several different physical states, also known as phases. These phases include solids, liquids, gases, and plasma. A substance transitions between these phases when temperature or pressure changes. Even when the phase changes, the chemical composition remains exactly the same. For example, liquid water and steam are both the same substance.
There are two primary categories of substances: elements and chemical compounds. An element is a substance made of only one kind of atom. It cannot be broken down into something else by a chemical reaction.
Elements can be classified into different groups based on their traits. Metals, such as gold or iron, often have a characteristic lustre. They also conduct heat and electricity well and are malleable. Non-metals, like oxygen or nitrogen, lack these metallic properties. Some elements, called metalloids, show characteristics of both groups.
A chemical compound is a substance formed when two or more elements combine through a chemical reaction. All compounds are substances, but not all substances are compounds. Compounds can consist of atoms bonded in molecules or in a crystalline lattice.
Chemical compounds are further organized by their internal structures and compositions. Organic compounds are those based primarily on carbon and hydrogen atoms. Inorganic compounds include all other types of chemical compounds. Some specialized substances, called organometallic compounds, contain bonds between carbon and a metal. Compounds can also be classified by how their atoms are held together. Covalent compounds involve atoms that share electrons. Ionic compounds, often called salts, consist of oppositely charged ions. Some complex substances, known as coordination complexes, use a dative bond to hold a metal center and a ligand together.
Understanding the history of these definitions is essential to modern chemistry. In the late eighteenth century, Joseph Proust conducted important research on pure compounds. He studied substances like basic copper carbonate to understand their makeup. Proust deduced that all samples of a compound have the same proportions of elements by mass. This discovery is known as the law of constant composition.
Later, researchers encountered the complex problem of isomerism. Isomers are substances that have the same chemical composition but different atomic arrangements. Friedrich August Kekulé provided clarity by describing the structure of benzene. The discovery of stereoisomerism further explained how three-dimensional arrangements create distinct substances.
Chemistry uses many specific numbers to describe the world of substances. As of 2025, there are 118 known elements in the periodic table. Approximately 80 of these elements are considered stable. Stable elements do not change through radioactive decay. Pure water is a perfect example of a substance with a strict ratio. Every molecule of water contains exactly two hydrogen atoms and one oxygen atom. This 2:1 atomic ratio is a characteristic property of H2O. Other substances like refined sugar have much larger and more complex molecular formulas.
It is important to distinguish a pure substance from a chemical mixture. A mixture contains two or more substances that are combined without reacting. Mixtures do not have a fixed composition like pure substances do.
You can often separate mixtures using mechanical processes like distillation or evaporation. For example, you could use a magnet to separate iron from sulfur. In a mixture, the components retain their own identities. In contrast, a chemical reaction creates a new substance with entirely new properties. Even things that look uniform can be mixtures. Cranberry glass appears to be one color, but it is actually a mixture containing gold particles.
Different scientific fields apply the term "substance" in unique ways. In geology, inorganic solid substances with uniform composition are called minerals. When minerals combine into aggregates, they are defined as rocks. In the field of law, the term can include mixtures with a defined manufacturing process. For example, charcoal is a complex mixture, but it can be identified by how it is made. Polymer chemistry also uses unique definitions for long chains of molecules. Even though polymers like polyethylene are mixtures of different molecular masses, they are defined by their specific chemical precursors.
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