Particle motion and material properties

Mixing from random motion

Particles in gases and liquids keep moving in different directions. Diffusion is mixing that results from their random motion

Concentration describes how much of a substance is present in a given volume. With equal volumes, the region containing more of that substance has the higher concentration

When concentrations differ, random motion produces an overall transfer from higher to lower concentration. Individual particles can cross in both directions; they do not choose a destination

A dissolved substance is still present even when separate pieces are no longer visible. Dissolving puts it into a solution; diffusion can then spread the dissolved particles through that solution

A supplied model record shows only what it states. Counts at different snapshots do not measure a diffusion rate unless measured times and an appropriate scale are provided

Predict what happens to the left/right difference, then inspect all three supplied records. The regions have equal volume, the boundary is open between them, and no selected particles enter, leave or react. These are three illustrative records of twelve selected dissolved particles, not a simulation, a measured timescale or the whole real sample

What changes across these records?

Explain the net direction

Uniform does not mean motionless

Once a closed mixture is uniform, particles still move and cross between regions. There is no persistent concentration difference driving an overall transfer one way

Equal concentrations do not require exactly equal crossings in every short interval. Random fluctuations are possible even when the average distribution is uniform

Stirring, wind and currents move whole regions of fluid. They can carry a substance as well as diffusion, so a faster visible spread does not automatically measure diffusion alone

Track motion without a net count change

Distinguish diffusion from a moving fluid

Changes in size without particle growth

Thermal expansion means an increase in a sample's dimensions as its temperature changes. Contraction is a decrease. Most materials expand when warmed and contract when cooled over everyday temperature ranges, if free to change size

For an expanding solid, particles vibrate more and their average spacing increases. They remain near positions in the solid; the particles themselves do not become larger

Cooling an ordinary contracting sample reduces average motion and spacing without shrinking each particle. Expansion and contraction can occur without a change of state

Conditions and exceptions matter. A rigid gas container keeps volume fixed when warmed, while liquid water contracts when warmed from 0 °C to 4 °C at ordinary pressure. Use the actual record rather than saying every material always expands

Explain an expanding solid

A metal rod is free to expand. A record shows that gentle warming increases its length while it remains solid. No material is added, lost or chemically changed

Explain the rod's increased length using the particle model, while preserving the stated substance and state

The rod retains the same solid substance and particles while its length increases

In this expanding material, increased vibration is associated with greater average spacing, not enlarged particles or a change to liquid

Respect the rigid boundary

Use a limited exception

Resistance to flow

Viscosity is resistance to flow. Under matched driving conditions, a more viscous liquid flows less easily. It is not the same property as density, which compares mass with volume

Attractions between neighbouring particles and their size and shape affect how easily they move past one another. Different liquids can therefore resist flow by different amounts

Many liquids become less viscous when warmed while remaining liquid. Greater average motion energy helps particles move past their neighbours and attractions; the individual particles do not melt or shrink

A comparison needs a suitable method with controlled apparatus, sample amount, temperature and driving conditions. A stopwatch alone does not make two flow times comparable

Interpret matched flow times

Explain a warmer liquid flowing more easily

Find the comparison limits

Construct three distinct explanations

Written task, not automatically marked

Record A: dissolved dye becomes more evenly distributed in a closed still liquid with no bulk flow. Record B: a metal strip warms, becomes longer and stays solid without losing material. Record C: a liquid stays liquid when warmed and takes less time to pass through the same suitable apparatus under a controlled driving pressure and equal sample volume

Explain each record using particle motion and state one unchanged feature. Distinguish diffusion, expansion and reduced viscosity rather than treating all three as the same process. For Record C, identify what must be controlled before attributing the result to temperature

Match each explanation to its record

Record A: dissolved dye becomes more evenly distributed in a closed still liquid with no bulk flow. Record B: a metal strip warms, becomes longer and stays solid without losing material. Record C: a liquid stays liquid when warmed and takes less time to pass through the same suitable apparatus under a controlled driving pressure and equal sample volume

A describes diffusion: random motion redistributes dissolved particles, giving an overall transfer from higher to lower concentration. The dye particles remain present with unchanged identity

B describes thermal expansion: greater vibration is associated with increased average spacing in the solid. The particles retain their size and the strip remains solid

C describes lower viscosity: greater average motion energy helps liquid particles move past neighbours and attractions. The liquid retains its substance and state while flowing more easily

A temperature comparison for C keeps the liquid identity, sample volume, apparatus and driving conditions matched. The result is about this tested liquid and interval, not a universal claim about every fluid

Review motion, spacing and flow

Diffusion changes the distribution through random motion; even a uniform mixture contains moving particles

Thermal expansion or contraction changes average spacing and sample dimensions without growing or shrinking the particles. State the boundary conditions and use observed exceptions

Viscosity describes resistance to flow, not mass per volume. Use controlled comparisons and distinguish temperature effects from apparatus or sample differences