Recognise states and particle-model explanations

Shape and volume tell us different things

Matter has mass and takes up space. Volume is the space a sample occupies; shape is its form. An invisible gas still takes up space. Compare a sample without heating it, adding material or changing its state

1. 1Solid An intact solid keeps its own shape and volume when moved to another container, unless it is squeezed, bent or broken
2. 2Liquid A liquid flows and takes the shape of the part of its container it occupies. Its volume stays nearly the same when it is poured without loss
3. 3Gas A gas flows and spreads through the available space in a container. It has no fixed shape or volume

Use more than one observation

Suppose a continuous sample changes shape when poured, has a level surface at rest and keeps the same measured volume. The observations together fit a liquid. A gas would spread into the space above that surface

1. 1Do not decide from flow alone Liquids and gases can both flow. A flowing sample needs another observation before we can distinguish those states
2. 2Check the conditions Losing some sample or changing its temperature would make this comparison less useful. Compare the same amount under the stated conditions

A sealed sample spreads through all the available space when its container is enlarged. Nothing enters or leaves, and its state does not change. Which state fits this observation?

Ask whether the sample keeps its own volume or takes the available volume

Read a particle model, not a photograph

The next diagrams use twelve circles to represent particles of one substance. Real samples contain vastly more particles. A circle is not a visible grain, drop or bubble. A and B are labels for following two particles, not different substances

1. 1What the positions mean The diagrams compare particle arrangement and spacing. The circles are drawn at a useful size, not to scale
2. 2What the snapshots do not measure A snapshot is one selected arrangement. The gaps between snapshots are not measured times, and no paths or speeds are shown. Temperature, pressure and changes of state are not simulated
3. 3What the boundaries mean A dashed outline marks a small region of solid or liquid being modelled, not a container wall. The gas model uses a solid outline for a sealed container

A solid can keep its shape while particles move

In a solid, particles are close together and vibrate around fixed positions. Vibrate means move back and forth around a position. They do not need to stop moving for the whole sample to keep its shape

1. 1One solid example The next model has an ordered arrangement. Not every solid has this regular structure
2. 2Make a prediction When you change the snapshot, predict whether A and B will exchange places or move only a little around their positions

Compare snapshots 1 and 2, then try 3, 4 and 5. Follow A and B. The dashed line marks the model region, not a container. These are separate snapshots, not a timed animation

Which description fits the changes you can see?

A liquid changes shape without spreading out like a gas

Liquid particles stay close together but can move past one another. Staying close accounts for a nearly fixed volume. Changing neighbours allows the sample to flow and take a new shape

1. 1Look at the particles, not a surface The next diagram shows part of the liquid at particle level. It is not a cup with drops or bubbles inside
2. 2Make a prediction Predict whether A and B must stay beside one another as the snapshot changes. Keep track of their labels, not the colour

Compare snapshots 1 and 5, then inspect the snapshots in between. Follow A and B as their neighbours change. All twelve circles still represent particles of the same substance

Which combination explains liquid flow with nearly unchanged volume?

Gas particles have space between them

Gas particles are far apart compared with their size and move through the available space. A sealed gas can occupy less volume because the spaces between particles can become smaller. The particles do not need to shrink or disappear

1. 1Keep the comparison fair First keep the snapshot fixed and change only the container width. Nothing enters or leaves this model
2. 2Make a prediction Before moving the compression control, predict what will change: the spaces, the number of circles, or the size of each circle
3. 3A bounded comparison Compression settings are not pressure measurements. This model does not calculate temperature, speed or changes of state

Keep snapshot 1 selected. Compare compression settings 0 and 3, then try the settings in between. Count the circles and compare their size separately from the spaces. Finally change the snapshot while keeping the width fixed

What accounts for the smaller volume of this sealed gas model?

A pile is not one particle

A pile of small solid beads can be poured. The pile changes shape because separate beads move past one another, while each bead keeps its own shape. Pouring the pile does not make the bead material a liquid

1. 1Keep the levels separate A visible bead contains very many particles. It is not one of the particles represented by a circle in the earlier model
2. 2Ask what is being classified Use observations of each bead when deciding the state of its material. Do not use only the changing shape of the whole pile

A collection of small pieces is poured into a tray. Each piece keeps its own shape and volume. What does this evidence support about each piece?

The question is about each piece, not the shape of the whole collection

Optional check: a record says only that a sample flows. Can that fact alone distinguish a liquid from a gas?

Think of both states before making a claim. This check is optional and unscored

The circles in our model are coloured so they are easy to see. No observation of the sample's colour is given. What can we conclude about its actual colour?

A drawing convention is not a measurement of the sample

Connect observations to the model

Use shape and volume together, then ask how particle arrangement and movement can explain them. Reviewing this lesson records learning activity on this device, not mastery or an exam result

1. 1Compare the states Solid: close particles vibrate around fixed positions. Liquid: close particles change neighbours. Gas: separated particles move through available space, and smaller gaps allow compression
2. 2Use the model carefully A circle is not a visible grain. Labels and colours help us read the model; they are not measurements of a sample. The snapshots do not measure speed or time
3. 3What still needs other evidence Choosing supplied explanations does not assess writing your own explanation, drawing a justified model or carrying out an investigation. Optional practice has nine cases; repeated cases and shuffled choices are not new evidence of mastery