Particles move and attract each other

The particles never stop

Open a bottle of perfume in one corner of a room. After a short time, the smell reaches the other corner. Put a drop of blue ink gently into a glass of water. Slowly, the blue spreads through all the water, even if you do not stir.

These facts tell us that the particles of matter are continuously moving. They have energy of motion (kinetic energy).

You cannot see this motion, because the particles are too small. But in 1827, a botanist, Robert Brown, looked at tiny grains of pollen in water through a microscope, and saw them jump about in a zigzag way. The grains were pushed by the invisible particles of water that hit them from all sides. This Brownian motion is a direct sign of the motion of the particles of matter.

Diffusion

When the particles of one substance spread into another substance by themselves, this is called diffusion. It happens because the particles are moving, and there are spaces between them.

• A gas diffuses into a gas. The smell of food spreads in the air. Diffusion in gases is fast.
• A liquid diffuses into a liquid. Ink spreads in water. Diffusion in liquids is slower, because the particles are closer together.
• A gas can diffuse into a liquid. Oxygen from the air dissolves in water, and fish and other water animals use it to breathe.
• Even solids can diffuse into each other, very slowly. If a block of copper sulphate is placed in contact with water, the colour slowly spreads upwards through the water.

Diffusion always goes from where there are many particles of a substance to where there are few, until they are spread evenly. That is why a smell is strongest near the source, and weaker far away.

The effect of temperature

Put a drop of ink in a glass of cold water, and a drop of ink in a glass of hot water. The ink spreads faster in the hot water.

When we heat a substance, the particles take in energy and move faster. So diffusion becomes faster. The picture shows this.

This also explains daily facts. Sugar dissolves faster in hot tea than in cold water. The smell of cooking food reaches you faster from a hot pan than from a cold one. A wet cloth dries faster in the Sun.

You should find that the ink spreads about twice as far in ten minutes in water at $55\ ^\circ\text{C}$ as in water at $20\ ^\circ\text{C}$.

A race in a glass tube

Lighter particles move faster, so they spread faster. A neat experiment shows it. Put cotton soaked in ammonia solution into one end of a long glass tube, and cotton soaked in hydrochloric acid into the other end. Close both ends. Ammonia gas and hydrogen chloride gas spread toward each other. Where they meet, they make a white ring of ammonium chloride.

The ring forms nearer the acid end. So the ammonia particles, which are lighter, travelled farther in the same time. They spread about $1.47$ times as fast.

The particles attract each other

Is it easy to break a piece of chalk? Yes. Is it easy to break an iron nail? No. Can you cut a stream of water with your hand? No, because the water closes again.

These facts show that there is a force of attraction between the particles of matter. It keeps the particles together. The force is strong in an iron nail, and weaker in chalk or in water. The strength of this force is different in different kinds of matter.

Another test: you can easily pull a thread of cotton apart, but a thin steel wire of the same size will hold a large weight before it breaks. The steel particles hold each other more strongly.

A drop of water on a leaf makes a small round bead, because the particles of water attract each other and pull the drop together. This pull between the particles of a liquid is what makes small drops round.

Two ideas together

The particles are always moving, which tends to spread them out. The particles also attract each other, which tends to hold them together. The balance of these two decides whether a substance is a solid, a liquid or a gas. In a solid, the attraction is strong, and the particles only vibrate in their places. In a gas, the particles move so fast and so far that the attraction hardly matters. The next course, on the states of matter, builds on this.

Practise with new numbers

Try it in Python

The program finds where the white ring forms in tubes of three lengths.

Common mistakes

• Thinking that particles move only when the substance is heated. They always move. Heating makes them move faster.
• Thinking that diffusion needs stirring or wind. It happens by itself, because of the motion of the particles. Stirring only makes it faster.
• Thinking that solids have particles that do not move at all. They vibrate about their places.
• Mixing up the speed of the particles and the size of the particles. Heating changes the speed, not the size.

Remember these

• The particles of matter are continuously moving. They have kinetic energy.
• Diffusion is the spreading of the particles of one substance into another. It is faster in gases than in liquids, and it is fastest at a higher temperature.
• There is a force of attraction between the particles of matter. It is different for different matter.
• Brownian motion is a sign of the motion of invisible particles.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Why can you smell a cake baking in the kitchen from your bedroom?

2. A sugar cube dissolves faster in hot milk than in cold milk. Why?

3. In a tube $60\ \text{cm}$ long, the white ring forms $36\ \text{cm}$ from the ammonia end. Which gas spreads faster, and about how many times as fast?

4. Why does a steel wire hold a much heavier load than a cotton thread of the same thickness?