IB Physics SLTopic B.1 — Heat & Thermal TransferPaper 1 & 2States of Matter~7 min read
States of Matter
Solid, liquid or gas — every substance is really just the same particles, arranged and moving differently. The kinetic theory of matter is the model that explains why.
📘 What you need to know
Matter exists in three familiar states: solid, liquid and gas
The kinetic theory of matter models particles as small spheres in constant motion
What separates the three states is the arrangement, spacing and energy of these particles — not what the particles actually are
Intermolecular forces of attraction hold particles together to different degrees in each state
Properties like shape, volume, density and compressibility all follow directly from how tightly and how energetically the particles are packed
The Kinetic Theory of Matter
The kinetic theory of matter is a model used to explain the observed properties of solids, liquids and gases. In this model, particles are treated as small, rigid spheres. What changes between the three states isn’t the particles themselves, but how closely they’re packed, how they’re arranged, and how much kinetic energy they carry.
Solids
In a solid, particles are:
closely packed together with no space between them
arranged in a fixed, repeating pattern known as a lattice structure
only able to vibrate around fixed positions, not move past one another
at low energy compared with particles in liquids or gases — too low to overcome the intermolecular forces holding them in place
Particles in a solid sit in a fixed lattice, with no space between them, and can only vibrate about their fixed positions
These features give solids their familiar behaviour: a fixed shape (though some solids can be deformed under enough force), a fixed volume, strong resistance to compression, and the highest densities of the three states.
Liquids
In a liquid, particles are:
closely packed, but no longer locked into a fixed pattern
randomly arranged, with small gaps opening up between them
free to flow past one another
at a medium energy — enough to partially overcome the intermolecular forces holding them together, but not enough to escape entirely
Particles in a liquid stay close together but are randomly arranged, allowing them to flow past one another
As a result, liquids take the shape of whatever container holds them, but still have a fixed volume. They’re difficult to compress, and sit at a medium density — lower than solids, but higher than gases.
Gases
In a gas, particles are:
far apart — roughly ten times further apart than particles in a solid or liquid
randomly arranged, with no meaningful structure at all
moving in all directions at a range of speeds, occasionally colliding with each other and the walls of their container
negligible in size compared with the volume they occupy
at high energy — enough to fully overcome the intermolecular forces of attraction holding them together
Particles in a gas are widely spaced and move freely in all directions, occasionally colliding with each other and the container walls
This gives gases no fixed shape and no fixed volume — they expand to fill whatever space is available. Because there’s so much empty space between particles, gases can be compressed easily, and have by far the lowest densities — roughly a thousand times smaller than solids or liquids.
Quick recap: Solids — fixed shape, fixed volume, high density. Liquids — no fixed shape, fixed volume, medium density. Gases — no fixed shape, no fixed volume, low density. The difference always comes down to particle spacing, arrangement and energy.
WE 1
A sample of gas is compressed into a much smaller container without changing its temperature. Explain, using the kinetic theory of matter, why this is possible for a gas but would not be possible for a solid.
Reasoning
In a gas, particles are far apart with large amounts of empty space between them, so pushing them closer together is mostly a matter of reducing that empty space.
Contrast with a solid
In a solid, particles are already closely packed with no space between them, so there’s essentially nothing left to compress.
Gases compress easily; solids effectively don’t
WE 2
Explain, in terms of particle arrangement, why a liquid takes the shape of its container but a solid does not.
Reasoning
Particles in a liquid are randomly arranged and free to flow past one another, so the liquid has no rigid internal structure to hold a shape of its own.
Contrast with a solid
Particles in a solid are locked into a fixed lattice and can only vibrate about set positions, giving the solid a shape that doesn’t change on its own.
Liquids flow into their container’s shape; solids hold their own
💡 Top tips
Whenever you’re asked to explain a property of a state, think in terms of spacing, arrangement and energy — almost every answer traces back to these three
“Fixed volume” and “fixed shape” are not the same thing — liquids have one but not the other
Remember gas particles aren’t negligible because they’re empty — they’re negligible only in size compared to the space they occupy
Density differences between states are a direct consequence of how tightly particles are packed, not what the particles are made of
⚠ Common mistakes
Describing gas particles as having no volume at all, rather than a volume that’s simply negligible compared to the container
Assuming liquids can be compressed as easily as gases, when their particles are actually already closely packed
Forgetting that particles in a solid still move — they vibrate, they just can’t travel from place to place
Mixing up “arrangement” (fixed pattern vs. random) with “energy” (low, medium, high) — these are two separate ideas that both matter
Up next: Density — where we turn today’s ideas about particle packing into an actual number you can calculate.
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