Classify grains droplets and trapped gas
Use the material's properties rather than a misleading bulk shape or mixture.
Cross-referenced from the related Cambridge IGCSE Chemistry 0620 or Physics 0625 teaching to the 0652 syllabus. Exact 0652 Core/Supplement objective coverage and this final-series course’s examiner fit still need specialist review. Written practice is self-checked and does not predict a Cambridge grade or demonstrate supervised laboratory competence.
Before you begin
- Recall each state's shape and volume properties.
- Distinguish a material from a mixture of different components.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What distinguishes a pile from one grain in that pile?
What you will learn
- Explain why pouring solid grains does not make them a liquid.
- Identify separate states within a described mixture.
Individual grains can remain solid
Dry sand pours and takes a container's bulk shape, but each grain can retain its own shape and volume. The moving grains rearrange as whole solid objects. Pouring alone therefore does not establish that the grain material is a liquid. The gaps between grains can contain air, making the bulk sample a mixture of solid grains and gas.
Packing grains more closely may reduce the volume occupied by the bulk pile by reducing gaps. That is not evidence that each grain is readily compressible like a gas. If measurements are supplied, distinguish total pile volume from the volumes of the individual grains. Do not claim that every powder is safe to handle or that all powders have identical properties.
Several states can occur in one sample
A mist contains tiny liquid droplets suspended in a gas. The droplets are liquid water; the surrounding air is gas. A visible mist is not the same as invisible water vapour, which is water in the gaseous state. Calling the whole sample 'gas' can hide the separate liquid component that makes the mist visible.
Foam may contain gas bubbles surrounded by liquid films. Its bulk compressibility can come largely from the gas, so one bulk test does not prove that the liquid itself is readily compressed. Interpret supplied observations component by component. This lesson uses described observations; actual investigations require appropriate teacher supervision and suitable apparatus.
Sources for this lesson
Pause and explain
A visible water mist consists of small droplets in air. Which description is accurate?
Worked example
Dry sand pours and its pile becomes smaller when shaken. Does this prove that each grain is liquid or readily compressible?
Show the worked solution
- Each described grain retains its own shape and volume.
- Shaking can reduce gaps between those solid grains.
- Bulk pouring and packing do not establish liquid grains or gas-like grain compression.
Answer No; the grains remain solid and the gaps can shrink.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Explain why pouring dry sand does not establish that its individual grains are liquid.
Give me a hint
Check the properties of each grain rather than only the pile's outline.
Compare my reasoning
- The grains move as separate objects.
- Each grain retains its own shape and volume.
- Their rearrangement lets the pile pour without a liquid state change.
Pouring is bulk rearrangement of solid grains.
Look for these in your work
- Distinguish grain and pile.
- Use the solid properties.
A pile occupies 120 cm³ before shaking and 100 cm³ after. No material is lost and the grains retain their volume. Explain the 20 cm³ reduction, then identify the states in a water mist.
Give me a hint
Account for changing gaps and classify separate mist components.
Compare my reasoning
- The pile reduction is 120 − 100 = 20 cm³.
- Reduced spaces between unchanged grains account for the bulk change.
- Mist contains liquid water droplets and a surrounding gas.
Gaps decrease by 20 cm³; mist contains liquid droplets in gas.
Look for these in your work
- Preserve the stated grain volumes.
- Name both mist components.
A foam sample is readily compressed. A student concludes that its liquid films must be gas. Evaluate the conclusion.
Give me a hint
The supplied sample contains more than one state.
Compare my reasoning
- Foam includes gas bubbles and liquid films.
- Compression of the bubbles can reduce the bulk volume.
- The observation does not prove a state change of the liquid films.
The conclusion is not established; gas bubbles can explain the bulk compression.
Look for these in your work
- Consider the mixture.
- Avoid assigning every component the bulk behaviour.
Common mistakes
- Bulk pile shape is not each grain's state.
- Visible mist includes liquid droplets.
- A mixture's compressibility need not match every component.
What can you explain now?
What two states are present in liquid droplets suspended in air?
Compare with the explanation
Liquid and gas.
The droplets are liquid and the surrounding air is gas.
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Make it stick
Explain grain rearrangement and identify liquid and gas components without confusing visible droplets with gaseous molecules.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.