States and Changes of State of Matter
Senior 1 · Chemistry
States and Changes of State of Matter
Lesson 5 of 5
22 min
When brochettes are grilled at a roadside stall in the evening, you can smell them from far down the street, although there is no wind and nobody has carried the smell to you. When a tea bag is dropped into a cup of hot water, brown colour creeps out into the water without any stirring. Something is moving on its own. These everyday observations are some of the best evidence that matter is made of tiny particles that never stop moving.
Fill a beaker with water and leave it to stand until the water is completely still. Using tweezers, drop one small crystal of potassium manganate(VII) into the centre of the beaker. Do not stir or move the beaker. Observe it at the start, after 10 minutes and after an hour (Figure 5.1).
Figure 5.1: The purple colour spreads through still water by itself.
Warning
Safety. Potassium manganate(VII) is an oxidiser and stains skin and clothes brown: handle the crystal with tweezers, not fingers, and wear goggles. Dispose of the purple solution as your teacher directs, not down the sink, because it harms water life.
The crystal dissolved, and its purple particles spread out through the water until the colour was even everywhere. Nobody stirred the water: the particles moved by themselves, colliding with the moving water particles and wandering further and further from the crystal. This spreading is called diffusion. Overall, particles move from where there are many of them (a high concentration) to where there are few (a low concentration), until they are evenly spread.
Definition
Diffusion: the spreading out of particles from a region where they are at a higher concentration to a region where they are at a lower concentration, caused by the random movement of the particles. Example: the smell of perfume spreading across a room.
Definition
Concentration: how much of a substance is present in a given volume. A strong-coloured part of a solution has a high concentration of the coloured particles; a pale part has a low concentration.
In a fume cupboard, your teacher pushes a plug of cotton wool soaked in concentrated ammonia solution into one end of a long, dry glass tube, and a plug soaked in concentrated hydrochloric acid into the other end, closing both ends with rubber bungs at the same moment. After a few minutes a white ring appears inside the tube (Figure 5.2). The ring is solid ammonium chloride, formed where the two gases meet:
Figure 5.2: Ammonia and hydrogen chloride diffuse towards each other; the white ring forms where they meet.
Warning
Teacher only, in a fume cupboard. Concentrated ammonia and concentrated hydrochloric acid are corrosive and give off choking fumes. The plugs are handled with tongs, and the bungs are fitted at once so no gas escapes into the room.
Worked example
Problem. In Figure 5.2 the tube is 100 cm long and the ring forms 59 cm from the ammonia end. (a) Which gas diffused faster? (b) About how many times as far did it travel? (c) An ammonia particle has a mass of 17 units and a hydrogen chloride particle 36.5 units. What do the results suggest?
Solution.
Table 5.1: Factors that affect the rate of diffusion
| Factor | Effect | Reason |
|---|---|---|
| Temperature | higher temperature, faster diffusion | particles have more kinetic energy and move faster |
| Mass of the particles | lighter particles diffuse faster | at the same temperature lighter particles move faster |
| State | fastest in gases, slower in liquids, hardly at all in solids | gas particles are far apart and move freely |
| Difference in concentration | a bigger difference gives faster spreading at first | more particles move from the crowded region each second |
Worked example
Problem. Mutesi repeats Experiment 1 with two beakers, one of cold water and one of warm water, at the same time. In which beaker will the colour spread faster? Explain.
Solution. In the warm water. The water particles and the dissolved purple particles have more kinetic energy, so they move faster and collide more often, and the purple particles spread through the water more quickly. This is also why a tea bag colours hot water much faster than cold water.
In the 1820s the botanist Robert Brown looked through a microscope at tiny grains from pollen floating in water and saw them jiggling about in a restless, zigzag way, although the water was still. The same thing is seen when smoke is trapped in a small glass cell and lit from the side: under a microscope, each smoke particle shows up as a bright speck that darts about at random (Figure 5.3 A).
The explanation came from the kinetic theory. Air (or water) particles are far too small to see, but they are moving very fast in all directions. They bombard each smoke particle from every side. At any instant more hit one side than the other, so the smoke particle is pushed first one way, then another (Figure 5.3 B). In 1905 Albert Einstein used this idea to show that atoms and molecules are real.
Definition
Brownian motion: the random, zigzag movement of small particles (such as smoke or pollen grains) suspended in a gas or a liquid, caused by uneven collisions with the fast-moving particles of the gas or liquid.
Figure 5.3: Brownian motion of a smoke particle, and its cause.
Warning
Common mistake: saying that a smell spreads "because the air carries it". In still air there is no wind; the smell particles spread because they, and the air particles, are always moving at random. Wind only speeds it up.
Tip
Diffusion always goes from crowded to empty: high concentration to low concentration, until everything is evenly mixed.
Royal Society of Chemistry: Diffusion in action, the reaction of ammonia and hydrogen chloride (4 min). Note where the white ring forms and compare it with Figure 5.2.
Practice
Further reading
Sources and credits
