Short answer
- The 3 states of matter are solid, liquid, and gas. They are the three classical states in which ordinary matter is usually described.
- A solid keeps its shape and its volume. Its particles are packed closely together and mainly vibrate in place.
- A liquid keeps its volume but not its shape. It flows and takes the shape of its container, while its particles stay close together and slide past one another.
- A gas keeps neither its shape nor its volume. It expands to fill its container, and its particles are far apart and move freely and rapidly.
- Heating and cooling, along with changes in pressure, move matter between states. Melting, freezing, evaporation, condensation, sublimation, and deposition are the names for those changes.
- Plasma is commonly listed as a fourth state, an ionized gas found in stars and lightning, but it does not replace the three state account used in this article.
Introduction
Look around the room and you will see matter in more than one form. The chair holds its shape. The water in a glass takes the shape of the glass. The air around you has no shape of its own at all. Those three everyday forms are the three states of matter most people learn first: solid, liquid, and gas.
This guide explains what each state is, how particles behave inside it, and what makes matter change from one state to another. It uses water as the main example because water is familiar and because it moves through all three states at temperatures people meet in daily life. It also clears up common mixups, such as the difference between steam and water vapor, and explains why plasma is often named as a fourth state without changing the three state picture used here.
What Is a State of Matter?
Matter is anything that has mass and takes up space. A state of matter is a physical form that matter can take. The state depends on how much energy the particles have and how strongly they hold on to one another. When particles have little energy and strong attractions, they stay in a fixed arrangement. When they gain energy, those attractions weaken, and the particles gain freedom to move.
That is why temperature matters so much. Heating a substance usually adds energy to its particles. Cooling removes energy from them. Pressure matters too, because squeezing particles together or letting them spread out can change how freely they move. For most everyday purposes, though, temperature is the control people notice first: ice melts in a warm room, water boils on a stove, and vapor condenses on a cold window.
It is also important to say what a change of state is not. A change of state is a physical change. The substance itself does not turn into a different chemical. Ice, liquid water, and water vapor are all water. The molecules are the same kind of molecules in each case. What changes is how they are arranged and how they move, not what they are made of. That point explains why changes of state are usually reversible. Cool the vapor and it can become liquid again. Freeze the liquid and it becomes solid again.
Scientists sometimes use the words state and phase for the same idea, and both words appear in textbooks. In this article, state is used throughout for consistency. A single sample of matter at one temperature and pressure normally sits in one state, although two states can exist side by side during a change. A glass holding ice and water at the melting point is a good example: solid and liquid are present together while the change is under way.
Solids: Fixed Shape, Fixed Volume
A solid has a definite shape and a definite volume. A brick keeps the shape of a brick whether it sits on a table, in a box, or in your hand. Cut it in half and you get two smaller solids, each with its own shape. The solid does not flow to fill a container, and it does not need a container to hold its form.
Inside a solid, particles are packed closely together in a regular pattern in most cases. The particles are not still. They vibrate in place, and heating makes that vibration stronger, but they do not wander away from their neighbors in the way liquid particles do. Strong attractions between particles hold the arrangement together, which is why a solid resists being pulled apart or squashed into a new shape without breaking or bending.
Not every solid has a tidy crystal pattern. In a crystalline solid, such as table salt or quartz, particles sit in a repeating ordered arrangement. In an amorphous solid, such as glass, the particles lack that long range order. Glass is still a solid for everyday purposes: it holds its shape, it does not pour, and it does not need a container. The distinction matters mainly because it shows that solid is a broad category, not a single perfect pattern.
Solids also vary widely in hardness, strength, and flexibility. A rubber eraser, a steel nail, a wax candle, and a block of ice are all solids, yet they respond very differently to force. What they share is the basic test: left alone, each one keeps its own shape and volume. If a material flows slowly to take a new shape on its own, it is behaving as a liquid, even if the flow is slow. Honey is a liquid, for example, even though it pours slowly. A solid may bend or break under force, but it does not flow on its own.
Everyday solids include wood, metal, stone, plastic, cloth, and ice. Each grain of sand is also a solid, although a pile of sand can be poured because the grains slide past one another.
Liquids: Fixed Volume, Flowing Shape
A liquid has a definite volume but no definite shape of its own. Pour water from a tall glass into a wide bowl and the volume stays the same, but the shape changes completely. The liquid takes the shape of the lower part of whatever container holds it, and its top surface stays level when the container is still. That combination, fixed volume with a flowing shape, is the signature of the liquid state.
Inside a liquid, particles are still close together, much closer than in a gas, but they are not locked into a fixed pattern. They can slide and roll past one another, which is why a liquid flows. The attractions between particles are strong enough to keep the liquid together as a body, so it does not drift apart like a gas, but they are not strong enough to fix each particle in place. A liquid therefore has a surface that a gas does not have in the same way, and a spill spreads out rather than vanishing into the air at once.
Liquids also vary in viscosity, which is how easily they flow. Water flows readily, while honey and syrup flow slowly. Warm honey pours more easily than cold honey, because its particles have more energy and move past one another with less resistance.
Another everyday liquid property is that liquids are hard to compress. Push on a sealed container of water and the water barely shrinks, because its particles are already close together and have little empty space to give up. Gases behave very differently, as the next section explains. That difference is useful in machines: hydraulic systems use liquids to transmit force precisely because a liquid keeps its volume under pressure.
Common liquids include water, milk, cooking oil, juice, and alcohol based cleaners. Mercury is a well known exception to the usual pattern, because it is a metal that is liquid at room temperature, while most metals are solids at room temperature. Liquid examples also include molten materials, such as melted wax or molten rock, which remind us that state depends on conditions. Wax is solid on a cool shelf and liquid in a burning candle. The substance has not changed its identity. Its temperature has.
Gases: No Fixed Shape, No Fixed Volume
A gas has neither a definite shape nor a definite volume. Release a gas into a container and it spreads out to fill the whole container, whatever its size or shape. Let it out of the container and it spreads into the room, and then beyond. That is why a gas always needs a sealed container to be held in one place, and why a balloon must be tied shut if the gas inside is to stay put.
Inside a gas, particles are far apart compared with a solid or a liquid, and they move rapidly in all directions. They collide with one another and with the walls of their container, and those collisions produce pressure. A gas is also easy to compress, because so much of its volume is empty space between particles. Squeeze a gas into half its volume and the particles simply have less room to move, so they hit the walls more often and the pressure rises.
The air around you is a mixture of gases, mostly nitrogen and oxygen, with smaller amounts of other gases. Each gas in the mixture spreads through the whole volume available to it. That is why the smell of cooking can travel from a kitchen to another room: gas particles move and mix freely, even without wind. Heating a gas makes its particles move faster, and cooling slows them down, just as in other states, but in a gas the effect on pressure and volume is especially easy to see. Warm air in a sealed container pushes harder on the walls than cool air does.
Gases are sometimes confused with vapors. A vapor is the gaseous form of a substance that is normally a liquid or a solid at room temperature, so water vapor is the gaseous form of water. Steam is often used loosely for the white cloud above a kettle, but that cloud is actually tiny liquid droplets formed when water vapor cools and condenses. The water vapor itself is invisible.
The Three States Compared
A short comparison makes the pattern easy to hold in mind. The table below sets out shape, volume, particle spacing, particle motion, and compressibility for each state.
| Property | Solid | Liquid | Gas |
|---|---|---|---|
| Shape | Fixed | Takes the shape of its container | Expands to fill its container |
| Volume | Fixed | Fixed | Expands to fill its container |
| Particle spacing | Very close, in a pattern | Close, with no fixed pattern | Far apart |
| Particle motion | Mainly vibration in place | Sliding past neighbors | Rapid, free motion in all directions |
| Compressibility | Very hard to compress | Hard to compress | Easy to compress |
| Energy of particles | Lowest of the three | Between solid and gas | Highest of the three |
The energy row needs a small qualification. It describes the same substance at increasing temperature, moving from solid to liquid to gas. It does not mean that every gas is hotter than every liquid in the world. A cold gas can be colder than a warm liquid of a different substance. The comparison is about the energy needed to move one substance through its own states, not a contest between unrelated materials.
Density usually falls as a substance moves from solid to liquid to gas, because particles are packed less closely. Water is the famous exception: ice is less dense than liquid water, which is why ice floats and can insulate the water below it in winter.
Particles, Energy, and Why States Change
The particle picture ties the whole topic together. Imagine a substance as a crowd of tiny particles held together by attractions. In a solid, the crowd stands in rows, shuffling in place. In a liquid, the crowd is still close, but people can move through it, changing neighbors as they go. In a gas, the crowd has scattered across a large hall, with people moving quickly and bumping into the walls.
Adding energy, usually as heat, makes the particles move more vigorously. In a solid, stronger vibration strains the attractions that hold the pattern together. At the melting point, the particles gain enough energy to break free from their fixed positions, and the solid becomes a liquid. Add more energy and the liquid particles move faster still. At the boiling point, enough particles gain the energy to escape the liquid entirely, and the substance becomes a gas.
Cooling reverses the process. Remove energy from a gas and its particles slow down, attractions take hold again, and the gas condenses into a liquid. Remove more energy and the liquid particles settle into a fixed pattern, freezing into a solid. Pressure can shift the exact temperatures at which these changes happen, and in extreme cases it can force a change on its own. For example, squeezing a gas can help turn it into a liquid, which is how many gases are stored and transported.
One instructive detail is what happens to temperature during a change of state. While ice is melting, the temperature of the ice and water mixture stays at the melting point until all the ice has melted, even though heat is still being added. The added energy is used to break attractions between particles, not to raise temperature. The same happens during boiling, which is why a pot of boiling water does not get hotter if you turn up the heat. It simply boils away faster.
The Six Changes of State
Every move between two of the three states has a name. Learning the six names, and the direction of each one, makes the topic much easier to discuss and to recognize in daily life.
| Change | From | To | What is happening |
|---|---|---|---|
| Melting, also called fusion | Solid | Liquid | Particles gain enough energy to leave fixed positions |
| Freezing, also called solidification | Liquid | Solid | Particles lose energy and settle into a fixed pattern |
| Evaporation | Liquid | Gas | Particles at the surface gain enough energy to escape |
| Boiling, a form of vaporization | Liquid | Gas | Particles throughout the liquid gain enough energy to form vapor |
| Condensation | Gas | Liquid | Particles lose energy and gather into a liquid |
| Sublimation | Solid | Gas | Particles pass directly to gas without becoming liquid |
| Deposition | Gas | Solid | Particles pass directly to solid without becoming liquid |
Evaporation and boiling are both forms of vaporization, but they are not identical. Evaporation happens only at the surface of a liquid and can happen below the boiling point, which is why a puddle dries on a cool day and a wet towel dries on a line. Boiling happens throughout the liquid, when its vapor pressure matches the surrounding pressure, and it happens at a specific temperature for a given pressure. Condensation is the reverse: when vapor cools or is compressed, its particles lose energy and form liquid droplets, as on a cold drink on a humid day or on a bathroom mirror after a hot shower.
Sublimation and deposition skip the liquid state entirely. Dry ice, the solid form of carbon dioxide, sublimates at ordinary pressure, turning straight into invisible gas; the fog effect used on stages comes from water vapor in the air condensing in the cold gas. Frost forms by deposition, when water vapor in cold air turns directly into ice crystals on a surface, without first becoming liquid water. Mothballs also shrink over time by sublimation, which is why they slowly disappear from a closed drawer even though they never melt into a puddle.
Freezing and melting are reverse processes, and for a pure substance they happen at the same temperature. That temperature is called the melting point when the change runs from solid to liquid and the freezing point when it runs the other way. The names differ, but the temperature does not. Mixtures behave less tidily, because different components may freeze or melt at different temperatures, which is why salt water and other mixtures freeze over a range rather than at a single sharp point.
Water Through All Three States
Water is the best worked example because its changes happen at temperatures people can check with an ordinary thermometer. At standard atmospheric pressure, water freezes and melts at 0 degrees Celsius, which is 32 degrees Fahrenheit, and it boils at about 100 degrees Celsius, which is 212 degrees Fahrenheit. Between those two points it is a liquid. Below the freezing point it is ice. Above the boiling point it is water vapor.
Start with an ice cube in a pan on a stove. At first the ice warms but stays solid. Once it reaches 0 degrees Celsius, it begins to melt, and the temperature holds steady while solid and liquid coexist. Only when the last ice has melted does the temperature rise again. The liquid then warms until it reaches about 100 degrees Celsius and begins to boil, again holding steady while liquid and vapor coexist.
Cooling runs the story backward. Vapor condenses into droplets, forming clouds and dew. Liquid water cooled to 0 degrees Celsius freezes into ice, again holding steady during the change. Ice can also sublimate slowly below its melting point, which is why old ice cubes in a frost-free freezer shrink over time.
Water also has a special point worth knowing by name. At a specific low pressure and a temperature of 0.01 degrees Celsius, which is 273.16 kelvin, solid ice, liquid water, and water vapor can all exist together in equilibrium. Scientists call this the triple point of water. It is a precise reference point used in temperature measurement, and it shows neatly that state is not fixed by temperature alone. Pressure is part of the story too.
States of Matter in Everyday Life
The three states shape daily life far more than a textbook list suggests. Cooking is a sequence of state changes. Butter melts in a pan, water boils for pasta, steam condenses on a pot lid, and a sauce thickens as water evaporates from it. Freezing preserves food because cold slows the motion of particles and the chemical changes that depend on that motion, and because water locked up as ice is not available in liquid form for the processes that spoil food.
Heating and cooling systems also depend on state changes. A refrigerator moves heat by evaporating and condensing a working fluid in a closed loop: evaporation inside the cabinet absorbs heat, and condensation outside releases it. An air conditioner works on the same principle on a larger scale. Even a simple fan helps evaporation, by carrying away the vapor above sweating skin so that more liquid can evaporate and take heat with it.
Industry uses the same ideas at larger scale. Metals are melted, poured into molds as liquids, and cooled into solid shapes. Gases are compressed into cylinders, and distillation separates liquids by their different boiling points. In each case, the engineer is choosing the state that makes the material easiest to move, shape, or separate.
Weather is the largest state change system most people ever see. Water evaporates from oceans, lakes, and land, rises as invisible vapor, condenses into cloud droplets, and falls again as rain or snow. Frost, fog, dew, hail, and sleet are variations on that same cycle of water moving between states.
Common Mixups and the Fourth State
Several mixups come up again and again, and clearing them up makes the topic much firmer.
First, melting is not dissolving. An ice cube melting in a drink is a change of state. Sugar dissolving in tea is not. The sugar spreads through the liquid as individual particles, but it does not melt, and the tea does not change state. Evaporate the water away and the sugar remains as a solid.
Second, a gas is not the same as air. Air is a mixture of gases. Oxygen, nitrogen, carbon dioxide, and water vapor are each gases in their own right, and air is what you get when they mix together. Saying that something turns into air is vague. It is clearer to name the gas it becomes.
Third, the white cloud above a kettle is not steam in the strict sense. As noted earlier, water vapor itself is invisible. The visible cloud is liquid droplets that formed when the vapor met cooler air and condensed. The same reasoning explains why breath is visible on a cold day: warm vapor from the lungs condenses in cold air into tiny droplets.
Fourth, not all solids melt cleanly. Some substances, such as wood, break down chemically when heated instead of melting into a liquid. That is a chemical change, not a simple change of state, and it does not contradict the three state picture.
Finally, there is plasma, the state most often listed fourth. Plasma is an ionized gas, a gas in which some electrons have been separated from their atoms, leaving a mixture of charged particles. NASA describes plasma as the fourth phase of matter after solid, liquid, and gas, and notes that stars are the most common example, along with lightning and the aurora. Plasma is far less common in daily life on Earth than the other three states, because it needs very high temperatures or strong electric fields to form and persist. Its existence does not make the three state account wrong. It extends it. The three classical states remain the right framework for the matter people handle every day, and plasma is the additional state that appears under more extreme conditions.
FAQ
What are the 3 states of matter? The 3 states of matter are solid, liquid, and gas. A solid has a fixed shape and volume, a liquid has a fixed volume but takes the shape of its container, and a gas has neither a fixed shape nor a fixed volume and expands to fill its container.
What is the difference between a solid, a liquid, and a gas? The difference lies in particle arrangement, motion, and energy. Solid particles are closely packed and mainly vibrate in place. Liquid particles are close together but can slide past one another, so the liquid flows. Gas particles are far apart and move freely and rapidly, so the gas spreads out and is easy to compress.
What causes matter to change state? Heating, cooling, and changes in pressure cause matter to change state. Heating adds energy and weakens the attractions holding particles together, moving matter from solid toward liquid and then gas. Cooling removes energy and lets those attractions take hold again, moving matter back toward liquid and then solid.
What are the six changes of state called? The six changes are melting (solid to liquid), freezing (liquid to solid), evaporation and boiling (liquid to gas), condensation (gas to liquid), sublimation (solid to gas), and deposition (gas to solid). Evaporation and boiling are both forms of vaporization, with evaporation at the surface and boiling throughout the liquid.
Is water the only substance with three states? No. Many substances can exist as solids, liquids, and gases, but water is the most familiar example because its changes happen near everyday temperatures. Most metals are solids at room temperature and become liquids when heated enough, and gases such as oxygen and carbon dioxide become liquids when cooled and compressed enough.
Why does ice float on water? Ice floats because it is less dense than liquid water. In most substances the solid is denser than the liquid, but water is an exception: its solid structure takes up slightly more space than the same amount of liquid water, so ice rises to the surface instead of sinking.
What is the difference between steam and water vapor? Water vapor is the invisible gaseous form of water. The white cloud often called steam above a kettle is actually tiny liquid droplets formed when water vapor cools and condenses in the air. The vapor itself cannot be seen.
Is plasma a fourth state of matter? Yes, plasma is commonly listed as a fourth state. It is an ionized gas found in stars, lightning, and auroras. It is not part of the three state picture used in most everyday explanations because it needs extreme temperatures or strong electric fields, but it is a real and important state in astrophysics and related fields.
Does temperature stay the same during a change of state? Yes, for a pure substance at a fixed pressure, the temperature stays constant while the change is under way. Added heat during melting or boiling is used to overcome attractions between particles rather than to raise the temperature. Once the change is complete, further heating raises the temperature of the new state.
Can a substance skip a state? Yes. Sublimation moves a solid directly to a gas, as with dry ice, and deposition moves a gas directly to a solid, as with frost forming from water vapor. Both skip the liquid state entirely.
Keep reading: What Causes Climate Change? Causes and Evidence Explained
Keep reading: The Layers of Earth’s Atmosphere, From the Ground Up
Keep reading: The First Law of Thermodynamics, Explained
Keep reading: How Many Planets Are There? The Full Count, Verified
Sources & references
- National Aeronautics and Space Administration, Goddard Space Flight Center, Imagine the Universe: Matter in Extreme Conditions, describing plasma as the fourth phase of matter after solid, liquid, and gas, with stars as the most common example.
- National Aeronautics and Space Administration, Goddard Space Flight Center, International Solar Terrestrial Physics historical pages: Plasma, describing plasma as a gas in which atoms have been broken up into free electrons and positive ions.
- National Institute of Standards and Technology, SI Units: Temperature, stating that on the Celsius scale water freezes at 0 degrees Celsius and boils at about 100 degrees Celsius, and giving the triple point of water as 273.16 kelvin.
- National Institute of Standards and Technology, Kelvin: Present Realization, describing the triple point of water as the point at which water exists simultaneously as a solid, liquid, and gas in thermal equilibrium.
- Chemistry LibreTexts, university introductory chemistry courses: Classifying Matter According to Its State, Solid, Liquid, and Gas, describing the shape, volume, and particle arrangement of each state and plasma as a sometimes added fourth state.
- Chemistry LibreTexts, university introductory chemistry courses: Phase Changes, describing freezing, melting, evaporation, condensation, deposition, and sublimation, and the constant temperature during a phase change.
