What Is Matter in Chemistry?

In chemistry, matter is anything that has mass and takes up space, meaning it has volume. Both halves of that test have to be true. A desk passes, a glass of water passes, and so does the air around you, even though you cannot see it. Light, heat, sound, and ideas all fail, because none of them has mass or occupies space. Nearly everything chemistry studies is matter, so knowing where that boundary falls is worth a few minutes.

Key takeaways

• Matter is anything that has mass and takes up space. Fail either half of that test and it is not matter.
• Light, heat, sound, and energy in general are not matter. They have no rest mass and no volume.
• Mass is the amount of matter in a sample and never changes with location. Weight is the pull of gravity on that mass, so weight does change.
• All matter is built from atoms, most of which measure about 0.1 to 0.5 nanometers across.
• Matter divides into pure substances (elements and compounds) and mixtures (homogeneous and heterogeneous).
• Physical changes keep a substance's identity. Chemical changes make new substances. Mass is conserved either way.

What is matter, and how do you test for it?

Matter is anything that has mass and takes up space. Treat that as a two-part test. Mass measures how much material is present, usually in grams or kilograms. Volume measures the space that material occupies.

Air is the example that trips people up, since you cannot see it. One liter of dry air has a mass of roughly 1.2 grams at room temperature and ordinary pressure. A bedroom measuring 3 by 4 by 2.5 meters holds 30 cubic meters of air, which comes to about 36 kilograms, close to the mass of a large dog.

Air takes up space too, which is why an inflated basketball is firm rather than floppy. Matter also appears in different physical forms called states: solid, liquid, gas, and plasma. Ice, liquid water, and steam are all matter.

What is not matter?

If something has no mass and occupies no space, it is not matter. That rules out a large slice of the physical world, and almost every exclusion is a form of energy rather than a substance.

• Light. Photons have no rest mass and no volume. Switching on a lamp does not make a room heavier.
• Heat. Heat is energy moving from a hotter object to a cooler one, not a fluid that pours into things.
• Sound. Sound is a pressure wave traveling through matter. The air is matter; the wave crossing it is not.
• Fields and ideas. Magnetic fields, gravity, thoughts, time, and temperature all fail the two-part test.

Electricity is the useful edge case. A current is a flow of electrons, and electrons have mass, so the particles moving through a wire really are matter. The energy they deliver is not. A flame splits the same way: the glowing gas and soot are matter, the light and heat pouring out are not.

What is the difference between mass and weight?

Mass and weight are different measurements, even though everyday speech treats them as one. Mass is the quantity of matter in an object, measured in kilograms or grams. Weight is the force gravity exerts on that mass, measured in newtons.

Weight equals mass times gravitational field strength. On Earth that strength is about 9.8 newtons per kilogram, so a 60 kg student weighs 60 × 9.8 = 588 N. On the Moon it is about 1.6 N/kg, so the same student weighs roughly 96 N, one-sixth as much. The mass is still 60 kg, because none of the matter went anywhere.

Pair mass with volume and you get density, the mass packed into each unit of volume. A one-cubic-centimeter cube of aluminum has a mass of about 2.7 g; the same cube in lead has a mass of about 11.3 g.

What is matter made of?

All matter is made of particles, and the basic particle is the atom, the smallest unit of an element that is still that element. Most atoms measure about 0.1 to 0.5 nanometers across, so ten million of the smallest would fit in a row inside one millimeter.

Two or more atoms bonded together form a molecule, such as the O₂ in air or the H₂O in a glass. A single drop of water, roughly 0.05 mL, holds about 1.7 × 10²¹ of those molecules. Not all matter is molecular, though: copper metal is a lattice of atoms, and table salt is a lattice of alternating ions.

Almost all of an atom's mass sits in a tiny central nucleus, so matter that feels perfectly solid is mostly empty space. The particles are never still either, which is why food coloring spreads through motionless water on its own.

How is matter classified?

Every sample of matter belongs to one of two families, pure substances or mixtures. The question that separates them is whether the composition is fixed.

A pure substance has the same composition throughout and cannot be split up by physical means. An element contains only one kind of atom and cannot be broken down chemically; 118 are known. A compound contains two or more elements bonded in a fixed ratio, so water is always two hydrogen atoms per oxygen atom, close to 11% hydrogen and 89% oxygen by mass in every sample.

A mixture holds two or more substances that are physically combined rather than bonded, so the proportions can vary and each ingredient keeps its own properties. Homogeneous mixtures look the same throughout and are also called solutions: salt water, air, and brass. Heterogeneous mixtures have visibly different regions, such as sand in water or granite.

That boundary depends on scale. Milk and fog look uniform to the eye but hold droplets large enough to scatter light, so chemists class them as heterogeneous colloids.

Iron and sulfur show the difference cleanly. Stir iron filings into powdered sulfur and you have a mixture: any ratio works, and a magnet lifts the iron back out. Heat the blend and the two react into iron(II) sulfide, a compound with a fixed 1:1 ratio that ignores a magnet.

Physical or chemical: properties and changes

Properties come in two types, split by whether observing them destroys the substance. A physical property can be measured without changing what the substance is: color, hardness, state, melting point, boiling point, conductivity, and density.

A chemical property describes how a substance turns into a different substance. Flammability, reactivity with acids, and the tendency to rust are chemical properties, and confirming one means letting the change happen.

Changes split the same way. In a physical change the substance is the same afterward: melting ice is still H₂O, and sugar dissolved in tea comes back when the water evaporates. In a chemical change, atoms are rearranged into new substances, as when iron and oxygen make rust or methane burns to carbon dioxide and water.

Bubbles, a new solid in a clear liquid, an unexpected color change, and released heat or light are clues that a reaction happened. Treat them as clues, not proof, since boiling water bubbles hard and is purely physical. The real question is whether the substances at the end differ from the ones you started with.

Can matter be created or destroyed?

Not in a chemical reaction. The mass of the products always equals the mass of the reactants, a rule called the law of conservation of mass. Antoine Lavoisier established it by weighing sealed reaction vessels before and after.

Burn 12.0 g of carbon in 32.0 g of oxygen and you collect 44.0 g of carbon dioxide. The atoms were rearranged, not removed, which is exactly why chemical equations have to balance atom for atom.

Apparent exceptions usually involve a gas nobody weighed. A burning log seems to vanish because carbon dioxide and water vapor drift away. A rusting nail really does gain mass, because oxygen from the air becomes part of the iron oxide.

One genuine exception exists. Nuclear reactions convert a measurable amount of mass into energy through E = mc². Chemical reactions do the same in principle, but burning methane shifts the total mass by roughly one part in ten billion, far below what any balance can read.

Frequently asked questions

Is air matter?

Yes. Air has mass, about 1.2 grams per liter at room temperature and ordinary pressure, and it takes up space, which is why a filled balloon holds its shape. A sealed flask of air weighs measurably more than the same flask pumped empty.

Is light matter?

No. Photons have no rest mass and occupy no volume, so light fails both parts of the test. Light does carry energy, which is why sunlight warms whatever it lands on, but energy is not the same thing as matter. Heat, sound, and magnetic fields are excluded for the same reason.

What is the difference between a compound and a mixture?

A compound's elements are chemically bonded in a fixed ratio, and only a chemical reaction can separate them: water is always two hydrogen atoms to one oxygen atom, whatever its source. A mixture's ingredients are only physically combined, so the proportions vary and physical methods such as filtering or evaporating will pull them apart.

Does an object's mass change on the Moon?

No. Mass measures the amount of matter present, and none of that matter is lost on the trip. Weight does change, because the Moon's gravitational field is roughly one-sixth as strong as Earth's. A 60 kg astronaut weighs about 96 N there instead of 588 N.

Want precise definitions of element, compound, and mixture? Search any word in the A to Z Chemistry Dictionary.

By the Chemistry Dictionary Editorial Team · Published September 15, 2026