States of Matter and Phase Changes

This topic sits between chemistry and physics and gets asked from both sides. The whole of it follows from one idea: adding energy makes particles move more, and moving more is what separates a solid from a liquid from a gas.

The three states

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A liquid has a fixed volume and no fixed shape. That middle row is the one questions target, because it is the state that is definite in one respect and not the other.

Plasma is sometimes given as a fourth state: a gas heated until its atoms lose electrons, which is what stars and lightning are made of. It appears occasionally and is worth recognizing.

The six phase changes

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Going right on that diagram absorbs energy; going left releases it.

Sublimation skips the liquid entirely. Dry ice, which is solid carbon dioxide, does this at room temperature - it produces fog and no puddle, which is the giveaway in a question. Deposition is the reverse, and frost forming directly on a cold window is the example.

Sublimation and deposition are the two the test asks, because the other four are familiar from daily life.

Temperature during a phase change

The idea worth understanding rather than memorizing.

While a substance is changing state, its temperature stays constant.

Heat a pot of ice at 0 degrees Celsius and it will sit at 0 until every piece has melted, and only then will the water start warming. Boil water at 100 degrees and it stays at 100 until the last of it has turned to steam.

The energy going in is being used to break the bonds holding the particles together, not to speed them up. Temperature measures how fast the particles move, so while the energy is going elsewhere, temperature does not rise.

That energy is called latent heat, and it explains several everyday things: why steam burns worse than boiling water at the same temperature, and why sweating cools you - evaporation takes its latent heat from your skin.

Density

Density = mass / volume, and the usual units are grams per cubic centimeter or kilograms per cubic meter.

Density decides floating: an object floats if it is less dense than the fluid it is placed in. The comparison is always object against fluid, in that order.

Water is 1 gram per cubic centimeter, which makes it the reference point. Ice is about 0.92, which is why ice floats.

Water is unusual in expanding when it freezes. Nearly everything else contracts and becomes denser as a solid, so most solids sink in their own liquid. Ice floating is the exception, and it is asked because of that.

Density does not depend on the amount. A drop of water and a bathtub of water have the same density. Cutting an object in half halves the mass and the volume and leaves the density unchanged.

Gas behavior

Lightly asked, and two relationships cover it.

At constant temperature, pressure and volume are inversely related. Squeeze a gas into half the volume and the pressure doubles. That is Boyle's law.

At constant pressure, heating a gas expands it. That is why a balloon grows in the sun and a sealed container can burst when heated. That is Charles's law.

At constant volume, heating a gas raises its pressure - Gay-Lussac's law - which is why an aerosol can says not to heat it. The combined gas law puts all three together.

Gas law temperatures must be in kelvins. Double the Kelvin temperature and the particles' average kinetic energy doubles; double the Celsius figure and nothing so tidy happens. Absolute zero, 0 kelvins or -273 degrees Celsius, is where particle motion would stop.

Both follow from the particle picture: pressure is particles striking the walls, so more collisions in less space, or faster collisions when hotter, means more pressure.

Why evaporation cools

The molecules in a liquid move at a range of speeds, and the fastest ones - those with the highest kinetic energy - are the ones that escape from the surface. Losing its fastest molecules lowers the liquid's average energy, so it cools. That is the mechanism behind sweating.

The markers on the Celsius scale are water itself: it freezes at 0 degrees and boils at 100 degrees at sea level. Steam at 100 degrees burns worse than water at 100 degrees, because as it condenses on your skin it also releases its latent heat.

What you can skip

Across all 2,104 General Science questions in our bank, the ideal gas law (PV = nRT) appears once and no question asks you to calculate with a gas law. Know which quantities move together and which move oppositely; that is what is asked. Plasma, the fourth state, does appear (15 mentions across the subtest, mostly in the stars lesson) - know that it is an ionized gas, the state of the Sun and of lightning.

Where people lose points

Saying temperature rises during melting or boiling. It holds steady.

Confusing sublimation and deposition. Solid to gas is sublimation.

Saying a liquid has no fixed volume. It does; it has no fixed shape.

Getting the floating rule backward. Less dense floats.

Thinking density changes with the amount of substance.

Saying all substances contract when freezing. Water expands.

Work one in under a minute

A block has a mass of 240 grams and a volume of 300 cubic centimeters. Will it float in water?

Density is 240 divided by 300, which is 0.8 grams per cubic centimeter.

Water is 1.0, and 0.8 is less, so it floats.

The whole question is one division and one comparison against 1.0, which is why knowing water's density as a round number is worth so much here.

Where this leads

The latent heat idea reappears in the heat lesson, and density is what buoyancy and hydraulics are built on in Mechanical Comprehension.