How matter changes state, and what stays the same
TEAS 7 outline: Physical properties of matter and changes of matter
Also covers HESI A2 Chemistry (Matter)
- Solids keep shape and volume; liquids keep volume; gases fill their container.
- Melting, vaporization and sublimation absorb energy; the reverse changes release it.
- Temperature stays constant while a substance is changing phase.
- Density = mass ÷ volume and does not depend on sample size.
- Physical changes keep identity; chemical changes make new substances; mass is conserved.
Key facts
- Solid particles are tightly packed and vibrate in place; liquid particles are close but slide past each other; gas particles are far apart and move independently.
- Melting, vaporization, and sublimation absorb energy; freezing, condensation, and deposition release energy.
- Sublimation is solid to gas (dry ice); deposition is gas to solid (frost).
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- Temperature stays constant while a substance is changing phase.
- Density = mass ÷ volume. It does not depend on sample size, so cutting an object in half does not change its density.
- A physical change keeps the substance's chemical identity; a chemical change produces one or more new substances.
- The law of conservation of matter: total mass stays the same in both physical and chemical changes.
Quick check1 question · 2 more at the end
A substance keeps the same volume no matter what container it is poured into, but it always takes the shape of that container. Which description fits the particles of this substance?
Correct answer: C.
A definite volume but no definite shape describes a liquid. Liquid particles stay close together, which keeps the volume nearly fixed, but they have no regular arrangement and can move past each other, which lets the liquid flow into any shape. Solids keep both shape and volume, and gases take both the shape and the volume of their container.
A Tightly packed particles in a regular pattern that vibrate in place describe a solid, which keeps its own shape instead of taking the container's shape.
B Widely spaced particles that move independently describe a gas, which would expand to fill the container instead of keeping a fixed volume.
C Correct: liquid particles are close together (fixed volume) but have no regular arrangement and slide past one another (no fixed shape).
D This mixes features of a gas (far apart) and a solid (regular pattern, fixed positions) and does not describe any normal state of matter.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: Solids have a fixed shape and volume, liquids have a fixed volume but take the container's shape, and gases fill their whole container. A physical change, like melting or dissolving, keeps the substance's identity; a chemical change, like rusting or burning, makes new substances. In both, total mass is conserved.
1 How are solids, liquids, and gases different?Everything around you is made of tiny particles that attract each other and are always moving. 1 min
Everything around you is made of tiny particles that attract each other and are always moving. Which state of matter a substance is in depends on the tug-of-war between those attractions and how much energy the particles have.
| State | Shape | Volume | What the particles do |
|---|---|---|---|
| Solid | Definite | Definite | Tightly packed, often in a regular pattern; vibrate in place |
| Liquid | Takes the container's shape | Definite | Close together, no regular pattern; slide past each other |
| Gas | Takes the container's shape | Fills the container | Far apart; move independently until they collide |
Because gas particles are so far apart, a gas's volume changes a lot with pressure and temperature. Squeeze a gas and it takes up less space. The volumes of solids and liquids barely change with pressure. A fourth state, plasma, is a gas that contains many electrically charged particles.
2 What are the names of the phase changes?When particles gain enough energy, they break free of their attractions and move to a less ordered state. 1 min
When particles gain enough energy, they break free of their attractions and move to a less ordered state. When they lose energy, attractions pull them back together.
| Change | From → to | Energy |
|---|---|---|
| Melting (fusion) | Solid → liquid | Absorbed |
| Freezing | Liquid → solid | Released |
| Vaporization (evaporation, boiling) | Liquid → gas | Absorbed |
| Condensation | Gas → liquid | Released |
| Sublimation | Solid → gas | Absorbed |
| Deposition | Gas → solid | Released |
Real examples help these stick. Dry ice (solid carbon dioxide) shrinks without ever forming a puddle: that is sublimation. Frost forming on a cold window is deposition. Sweat cools you because evaporation pulls heat out of your skin.
One more rule: while a substance is changing phase, its temperature stays constant. Heat a pot of ice and the thermometer sits at 0 °C until all the ice has melted. The added energy goes into pulling particles apart, not into raising the temperature.
3 What is density, and why does ice float?Density is mass divided by volume: D = m ÷ V. 1 min
Density is mass divided by volume: D = m ÷ V. Solids and liquids are usually given in grams per cubic centimeter (g/cm³, the same as g/mL). Gases are given in grams per liter, because gases are so much less dense.
An object floats in a liquid if its density is lower than the liquid's and sinks if it is higher. Liquid water is about 1.0 g/cm³, while ice is about 0.92 g/cm³, so ice floats. A block of wood at 0.65 g/mL floats; a block of iron at 7.9 g/mL sinks.
Density is an intensive property: it does not depend on how much you have. Cut a block in half and both the mass and the volume are halved, so the density is unchanged. Mass and volume themselves are extensive properties, because they do depend on the amount.
4 What is the difference between a physical change and a chemical change?A physical property can be observed without changing what the substance is made of: color, hardness, density, melting point. 1 min
A physical property can be observed without changing what the substance is made of: color, hardness, density, melting point. A physical change alters state or appearance but keeps the substance's identity. Melting wax, dissolving sugar, grinding a solid, and condensing steam are all physical changes.
A chemical property describes a substance's ability (or inability) to turn into a different substance: flammability, toxicity, how readily iron rusts. A chemical change always produces at least one new substance. Rusting, burning, cooking, digestion, and rotting are chemical changes.
Clues that a chemical change happened include a new color (meat browning on the grill) or a gas forming from a reaction. Other clues are heat and light (a flame) or a solid appearing when two solutions are mixed. Treat these as clues, not proof: boiling water also makes a gas, yet it is still water.
5 Does mass change when matter changes?The law of conservation of matter says there is no detectable change in total mass when matter undergoes a chemical change or moves between solid, liquid, and gas. 1 min
No. The law of conservation of matter says there is no detectable change in total mass when matter undergoes a chemical change or moves between solid, liquid, and gas. Melt 50 g of ice in a sealed bag and you have 50 g of water.
If a reaction seems to "lose" mass, check whether it happened in an open container. A gas product can drift away, so the balance reads less, but the atoms still exist. In a closed container, the total mass stays the same.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: Sublimation goes solid to gas, not gas to solid; gas to solid is deposition.
Watch out: Dissolving sugar in water is a physical change, not a chemical one; the sugar is still sugar.
Watch out: Condensation and freezing release heat, not absorb it; only the changes that pull particles apart absorb energy.
Watch out: A smaller piece of the same material has the same density, not a lower one; mass and volume shrink together.
Watch out: Mass that seems to disappear in an open container usually escaped as a gas; it was not destroyed.
How to remember it
Memory hook: "Up the ladder takes energy." Picture solid at the bottom, liquid in the middle, gas at the top. Any change moving up (melting, vaporization, sublimation) absorbs energy; any change moving down (freezing, condensation, deposition) releases it.
Self-check:
- Name all six phase changes and say whether each absorbs or releases energy.
- Explain why half of a brick has the same density as the whole brick.
- Sort these as physical or chemical: dissolving salt, toasting bread, frost forming, a nail rusting.