Particle Theory

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Matter & Particles

What is 'stuff' made of?

Our world and everything in it is made up of thousands of substances that are generally called 'matter'.

Matter is anything that takes up space, and includes all things you can see and touch (e.g. a rock or water), as well as all invisible things, such as the air we breathe. Some pieces of matter like a lump of ice can be held in your hands, while other sorts of matter like water are more difficult to hold on to.

Particle Theory

Scientists believe that all matter is made of particles. This is referred to as the particle theory of matter.

The particles of matter can be arranged together in several ways:

Independent atoms that remain alone, e.g. the gas helium which is used in 'lighter than air' balloons.
Small groups of atoms called molecules (e.g. carbon dioxide molecules and water molecules). The atoms in each molecule are bonded together.
Large, rigid networks of atoms called crystal lattices. The atoms in the lattice are all held together by bonds. Because the atoms are arranged in a regular pattern, they can form crystals that have distinctive shapes and colours (e.g. copper sulfate crystals and diamonds).

Particles in Motion

Particles of matter are in constant motion - they are moving all the time. The energy associated with the motion of particles of matter is called thermal energy.

Particles of matter can have three forms of motion - free motion, rotation and vibration.

In solids, the particles can only vibrate in thier positions. In liquids and gases however, the particle movement is random.

The movement of particles of matter in a solid, liquid or gas is affected by three factors:

The thermal energy gained by the particles
The more thermal energy the particles receive, the faster they move. The temperature of a substance is a measure of the thermal energy of its particles.
The mass of the particles
The greater the mass of the particles of matter involved, the more slowly they tend to move.
The forces of attraction between the particles
The stronger the forces of attraction between the particles, the less freely the particles are able to move.

For particles of matter to stop moving altogether, they have to be cooled to -273°C. This temperature is called absolute zero (as cold as you can get!).

Force and Motion

Particles of matter in solids have less energy and mostly just vibrate. The strong forces of attraction btween the particles keeping them in fixed places.

Particles of matter in liquids have moer energy and move freely past each other, as well as vibrating and rotating. Moderate forces of attraction keep the particles close together.

Particles in gases have high energ and can move freely in all directions. There are only weak forces between the gas particles, so they spread out.

Evidence

Brownian Motion

The effect of moving particles can be seen when you look down a microscope at a pollen grain in water. The grain appears to jiggle slightly because it is continually pushed by the water molecules around it. In a similar way, even in still air, specks of dust 'jump around' in different directions, because the specks are colliding with moving air particles. Scientists call this movement Brownian motion.

Diffusion

Diffusion occurs when particles spread out and mix evenly with other particles as they bump into each other. The following observations show that matter contains particles.

In a beaker of water, a purple crystal of potassium permanganate will dissolve and then diffuse and spread throughout the beaker, making all the liquid purple. Water particles bump into the purple crystal particles causing the purple particles to separate and mix around so all the contents of the beaker are coloured.

In a glass of water, two drops of red food colouring diffuse to make all the water red. The food colouring particles spread out and mix evenly with the water particles.

In a gas jar, red-brown bromine gas particles and colourless air particles mix evenly. The red-brown colour gets paler as the gas spreads out and mixes with the colourless air.

Using Scientific Models

Scientists use the particle theory of matter to explain how matter behaves. But particles (either atoms or molecules are extremely small and their behaviour cannot be observed. So scientists use models to show what they believe is happening at the particle leve.

Consider what happens when you dissolve one liquid into another. When 50 ml of water is dissolved into 50 mL of meths, the solution has a volume of 95 mL only. Why is there less volume than expected?

Scientists know that water particles are smaller than meths particles, so an explanation could be that water molecules fit into gaps ebtween the meths molecules and so take up less space.

We can use a model to demonstrate this. Small green plastic beads are used to represent the water particles and larger red beads to represent the meths particles. Equal volumes (50 mL) of green and red beads are measured out and then mixed as shown above.

In the model, mixing smaller and larger beads results in teh final volume being less than the total of the separate volumes. This is because the green beads pack into spaces between the red beads.

So, by using the particle theory of matter and our model, we can show what happens at the particle level when water dissolves into meths.

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