GRADE 7 · STRUCTURE OF MATTER

Particles and diffusion

Molecules, particle motion, Brownian motion and diffusion.

18 minutes4 theory cards

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Understanding step by step

A drop of dye gradually spreads even without a spoon. What mixes the water when no one stirs it? Let’s test an idea: an apparently still liquid may contain particles that never stop moving.

A drop of dye was placed in still water and nothing was stirred. After a while, the dye appears all over the glass: the substance did not disappear, but dispersed between the water particles.

A model of a substance

Imagine repeatedly dividing water into smaller portions. Details become invisible to the unaided eye long before we reach individual particles. We therefore use a model: a substance consists of atoms, molecules or ions, with gaps between them; these particles keep moving and interacting. A water molecule consists of atoms, while table salt contains ions. The circles in a particle diagram are symbols: neither their size nor the gaps need be drawn to scale.

An enlarged particle model: the particles are separated by gaps and are moving. The circles are symbols, not a drawing to scale.

Why the boundary spreads out

Imagine a boundary around a drop of dye. Randomly moving particles cross it in both directions. Initially there are far more dye particles inside, so on average more leave than return. The boundary spreads out. This intermixing of particles of substances in contact, caused by thermal motion, is diffusion. Even when the colour becomes uniform, the particles keep moving; the two opposing transfers simply balance on average.

On the left, the two substances have just touched; on the right, their particles are mixed due to thermal motion. Empty and filled circles represent different substances; the number of particles of each is conserved. The arrow signifies the passage of time.

What else carries a smell?

Open a scent bottle at one end of a room and switch on a fan. The smell reaches the other end sooner, but has the fan created the molecules’ thermal motion? Its main effect is to create an air flow that carries the added substance along. Diffusion continues alongside this bulk transport. These are different mechanisms, so a smell arriving quickly does not by itself measure the diffusion rate. To investigate diffusion, we must reduce air currents.

Without air movement, the smell spreads in all directions on its own due to the thermal movement of particles. The fan creates a flow that transports the impurity much faster, and this is a different mechanism.

The visible speck is not a molecule

Under a microscope, a suspended speck changes direction unpredictably even when its container is still. Imagine tiny impacts from all sides. They almost balance, but at any instant the impacts on one side may be slightly greater. The speck receives one random push, then another. This is Brownian motion of small particles suspended in a liquid or gas. We see a speck much larger than the surrounding molecules and infer the invisible impacts from its motion.

Liquid molecules hit a small suspended speck from different sides. Unequal impacts produce its zigzag track. The speck itself is not a molecule.

Temperature and movement

Why does heated water usually mix faster? At higher temperatures, the thermal motion of particles is on average more intense. Temperature describes the state of the entire medium, and not the speed of one selected molecule: the speeds of particles differ and change. Absolute temperature is denoted by T and in SI is measured in kelvins, K; in everyday life they use degrees Celsius, °C. Heating a vessel and stirring it with a spoon are different ways to change the observed picture.

In the same substance, higher temperature means more intense particle motion on average. Arrow lengths represent speed schematically.

Compare under controlled conditions

Compare the same dye in warm and cold water, keeping the container shape and the way the drop is added the same. Diffusion is usually faster in warmer water. But predict a possible complication: uneven heating can create water currents that carry the dye. To investigate diffusion itself, use conditions with no noticeable flow. Gas particles move more freely; mixing in liquids, and especially in solids, is usually much slower.

In a gas, a particle moves freely between collisions; in a liquid, it is surrounded by close neighbors. In a solid body, vibrations around the equilibrium position predominate; diffusive movements are usually much slower.

Diffusion without stirring

Dye is carefully added to two identical containers of water, one warm and one cold. Neither is stirred. Why does the boundary usually spread out faster in warm water?

The same dye was carefully introduced into two identical vessels: the water in them differs only in temperature, and it is not stirred. It is necessary to explain where the boundary will blur faster.

Let's look at an example · 1

At higher temperatures, the thermal movement of particles is more intense, so they mix faster.

The same sections of the border are enlarged after the same time. Filled circles indicate dye, empty circles indicate water. Particles of both substances move across the previous boundary; Mixing is usually faster when heated.

Let's look at an example · 2

But heating can create streams of water. To ensure that the conclusion relates specifically to diffusion, conditions without noticeable flow are compared.

If you heat a vessel from below, the water begins to rise in the middle and fall at the walls. The dye is then carried away by the current, and such transfer can no longer be attributed to diffusion.

Let's check the result

Heating usually speeds up diffusion; distinguish it from transport by a flow.

Key idea · 1

The apparent rest of a substance is compatible with the continuous movement of its particles.

Key idea · 2

Diffusion reduces differences in composition. Particles cross in both directions, but initially more leave the region where they are more concentrated.

Key idea · 3

Brownian motion reveals the effect of invisible molecules on a visible speck. A flow and a temperature change affect the experiment through different mechanisms.

Where do we go next?

Particles move in both a stone and water. Next, we will find out why these materials have such different properties.

Basic Concepts

3 concepts

Molecular structure

A substance consists of tiny particles with gaps between them; the particles are always moving and interacting.

The sizes of molecules are much smaller than usual bodies, so their properties are determined by observed effects and special instruments.

Brownian motion

Random movement of small particles in a liquid or gas due to uneven impacts of the molecules of the medium.

Pollen seen under a microscope changes direction without an external nudge, providing evidence of random thermal molecular motion.

Diffusion

Spontaneous mutual penetration of particles of contacting substances due to their thermal movement.

Thermal movement carries particles across the boundary of substances and gradually mixes them. Heating usually speeds up diffusion. The rapid spread of odor throughout a room is often also associated with air flows, which must be distinguished from diffusion.

Physical quantities

1 magnitude

Temperature of the medium

A quantity associated with the intensity of thermal motion of particles.

Unit: kelvin · \mathrm{К}

Problems on this topic

Statements, answer options and the walkthrough.

Structure of matterGrade 7 · 18 problems