What Is the Particle Model: A Guide to Solids, Liquids and Gases
The Particle Model
One of the first topics I teach my pupils when they arrive in Year 7 (11 years old) is the particle model. This model states what solids, liquids and gases are, and explains the differences in their behaviour.
Everything around you is made of particles so tiny that we cannot see them even with the most powerful microscope. These particles are arranged and move differently in each state of matter. Take water as an example; the size, shape and chemical composition of the water particles remains the same whether it is solid water (ice) or gaseous water (steam) - but how those particles move and are arranged is different for each state.
The particle model has four main tenets:
- All substances are made of particles.
- The particles are attracted to each other (some strongly, others weakly).
- The particles move around (have kinetic energy).
- As temperature increases, the particles move more (their kinetic energy increases).
How to Draw a Particle Diagram - Solid
- Particles are packed closely together
- Particles arranged in rows
- No overlapping particles
- All particles are the same size
Solids are easy to describe - they are solid! You cannot pass through a table because both you and the table are solid. Solids have the least energy of the three traditional states of matter. The particles are arranged into a regular pattern (think army ranks) with very little space between the particles. Their particles are held together by strong forces and can only vibrate around a fixed position. Due to this, solids have high density and a fixed shape and volume. If you leave a table for a few days it doesn't spread into a thin layer of wood across the floor or expand to fill your living room!
How to Draw a Particle Diagram - Liquids
- Particles fill up from the bottom of the container
- Each particle is touching at least one other particle
- Spaces between particles too small to fit another particle
- Particles form 'bridges' over gaps by leaning on other particles
- Arranged so particles cannot be squashed into a tighter space
- All particles same size
- No overlapping
Just like solids, the particles in liquids are packed closely together albeit in a more jumbled-up, random way. Unlike solids, however, we can pass through liquids; this is due to there being fewer and slightly weaker forces between the particles. The particles can move around each other and solids can be forced between liquid particles.
Liquids also have a fixed volume (like solids) but not a fixed shape. Liquids will flow to take up the shape of the bottom of container. All liquids can flow, but some liquids are runnier than others. The thicker (more viscous) a liquid is, the stronger the forces between its particles; syrup has stronger forces between its particles than lemon juice, for example.
Liquids have much more kinetic energy (movement energy) than solids but much less than gases.
How to Draw a Particle Diagram - Gases
- Particles are spread out randomly
- Particles do not look like a 'dice pattern'
- Particles are shown to move in straight lines (arrows)
- Particles same size
- No overlapping
The particles in gases are far apart and randomly arranged. This state of matter has the highest kinetic energy of the three traditional states; there are almost no forces between the particles. The particles are constantly moving in all directions (but only in straight lines), bashing into each other and the sides of their container - this causes pressure. Gases also flow to completely fill their container, regardless of its size or shape - gases have no fixed shape or volume.
How to Draw Particle DiagramsClick thumbnail to view full-size
Properties of Solids, Liquids and Gases
High density - particles very close together
Fairly High density - particles are close together
Low Density - particles are far apart
Cannot be compressed - no space for the particles to be pushed together
Cannot be compressed - no space for the particles to be pushed together
Can be compressed - there is plenty of space for the particles to be pushed together
Fixed shape as particles are held in place by strong forces
Takes shape of its container
No fixed shape as particles move randomly in all directions
Can diffuse as particles can change places
Can diffuse as particles can move in all directions
Cannot cause pressure
Can cause some pressure
Can cause a lot of pressure
What state of matter are these substances?
- Sponge Cake
- Ice Cream
Many substances can exist as all three states of matter. Water is usually a liquid, but heat it up and you get water vapour, cool it down and you get ice. These changes are called state changes.
As you increase temperature, the kinetic energy of the particles increases - the particles move around more. This causes the particles in a solid to vibrate more. If the particles vibrate enough, they can break some of the bonds holding them in regular rows and start to move over each other. The substance has now melted: turned from a solid into a liquid
The melting point of a substance is the temperture at which it changes from a solid to a liquid. The stronger the forces holding the particles together, the higher the melting point.
As you cool a substance, the kinetic energy of the particles goes down. This means that the particles move less and less. If a liquid gets cold enough the particles move slowly enough for forces to attract them together again, pulling them into rigid rows and preventing movement. At this point, the liquid has frozen - turned from liquid to a solid.
The freezing point and melting point of a substance are the same.
Condensing works on the same principle as freezing. If a gas gets cold enough its particles move slowly enough for forces to attract them together again. The gas will change into a liquid. The particles still have enough energy to keep moving and rolling over each other and so are not pulled into rigid rows.
As with melting, evaporating is down to raising temperature increasing kinetic energy. When you heat a liquid, the particles roll around more quickly. Some particles will move around so much that they overcome all of the forces holding them close to other particles and escape from the surface of the liquid. Evaporating is the process of a liquidchanging to a gas.
The more the liquid is heated, the faster it evaporates. Boiling occurs when evaporating takes place throughout the liquid. The bubbles in boiling water are pockets of water vapour (a gas) escaping.
The temperature at which something boils is known as boiling point. This depends on the strength of forces between particles and the ambient air pressure. This higher the pressure, the higher the boiling point as the pressure forces the particles to stay together for longer.
On Everest, water boils at 72°C due to the low air pressure.
Dry Ice Sublimation
What is Sublimation?
Sublimation is when a substance goes from a solid to a gas without become a liquid (the opposite is called deposition). The classic example of this is dry ice: solid carbon dioxide. When you heat dry ice with a hair dryer, you don't leave a patch of liquid carbon dioxide, it turns straight into gaseous carbon dioxide. This occurs when heating of a substance in the solid phase causes all forces between the particles to be completely broken. This usually requires some interesting pressures or conditions to achieve.
(Note - Gaseous Carbon Dioxide is invisible - the misty smoke you are seeing is water vapour in the air rapidly condensing into a liquid because the dry ice has cooled the air so much)
What is Plasma?
Plasma is the most abundant state of matter in the universe - and yet I barely teach it to my pupils. Plasma is almost always badly defined - often as a high energy gas. This would be like defining a solid as a super-low energy gas!
Plasma is a state of matter with extremely high kinetic energy, containing a high proportion of particles that are ionized. When given sufficient heat energy, the particles of a gas release a number of electrons, causing the particle to become a charged ion. When enough particles have become ionised to significantly affect the electrical properties of the gas, it has changed into a plasma.
Stars are mainly plasma and it has been estimated that 99% of the visible universe is made of plasma.
A Superfluid Fountain - Liquid Helium
What Happens to Particles at Absolute Zero?
Heat is a measure of how much the particles in a substance move - how much kinetic energy they possess. Temperature is merely a scaled measure of this. If you cool particles enough you can get to a theoretical temperature at which the particles stop moving - this is Absolute Zero: 0 Kelvin or -273.15°C - the coldest possible temperature.
At this temperature, weird things start happening...Particles can overlap each other allowing solids to pass through other solids. Liquids can flow uphill or even climb out of its container as in the video.
Bose-Einstein Condensates are another state of matter where all of the individual particles behave as one 'super-atom.' This means that BECs have no viscosity - you can set it spinning and it will never stop! Spinning bodies are usually stopped by losing energy to friction - as BECs are in the lowest energy state possible, they just keep spinning! These BECs also have zero electrical resistance for the same reason - the substance simply cannot lose any more energy