MYP Integrated Sciences States and Properties of Matter Notes
MYP Integrated Sciences Topic States and Properties… | RevisionDojo
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The Particle Model Explains the Behavior of All States of Matter
The particle model of matter is a simplified but powerful framework for understanding the behavior of solids, liquids and gases on a microscopic level.
At its core, the model assumes that:
All substances are made of tiny particles
These particles may be atoms, molecules or ions.
Particles are in constant motion
The higher the temperature, the faster the particles move (they have more kinetic energy).
Forces of attraction exist between particles
These forces can be stronger or weaker, holding particles closely together (as in many solids) or allowing them to move more freely (as in liquids and gases).
There are spaces between particles
The amount of space between particles helps explain properties such as density and compressibility.
Note
You might expect that the forces of attraction between particles in a liquid are always much weaker than in a solid, because liquids can flow while many solids cannot.
This is often true, but water is unusual:
In ice (solid water), each molecule forms up to four hydrogen bonds in a fixed, open lattice structure.
In liquid water, hydrogen bonds are still present but are continually breaking and reforming, and the molecules can move closer together on average.
As a result, liquid water is actually more dense than ice, even though both involve hydrogen bonding.
This is why ice floats on water, which is a very important feature for life on Earth.
The Relationship Between Particle Arrangements and Properties
Understanding how particles are arranged and how they move helps explain three key properties of solids, liquids and gases:
Density
Compressibility
Diffusion
You can think about the three states of matter like this:
Property
Solids
Liquids
Gases
Density
Particles packed close together
Particles close together but with more space
Particles far apart
Compressibility
Particles in fixed, close positions
Particles can slide past one another
Particles fill entire container, move freely
Diffusion
Little space between particles, making solids incompressible and dense
Moderate diffusion due to spaces between particles
Rapid diffusion due to high kinetic energy
Density
Definition
Density
It explains how much matter is packed into a certain volume.
$$ \text { Density }=\frac{\text { mass }}{\text { volume }} $$
In solids, particles are packed closely together, so solids often have high density.
Liquids are usually slightly less dense than solids because there is a little more space between particles.
Gases have very low density because their particles are far apart.
Analogy
Think of a sponge and a brick of the same size.
They have the same volume, but the brick is much heavier.
In the brick, particles are tightly packed, making it denser.
The sponge contains lots of air spaces, so its overall density is lower.
Compressibility
Definition
Compressibility
It describes how much a substance can decrease in volume when external pressure is applied.
In solids, particles are locked in fixed, close positions with very little empty space between them. Solids are therefore almost incompressible.
Liquids also have particles close together, with only slightly more space than in solids, so they are nearly incompressible.
In gases, particles are widely spaced, so they can be pushed closer together. Gases are therefore highly compressible.
Analogy
Squeeze a plastic bottle filled completely with water –> its volume hardly changes because the liquid is almost incompressible.
Squeeze a bottle that contains mostly air –> the gas can be compressed, and the volume of the gas region decreases significantly.
This shows the difference in compressibility between liquids and gases.
Diffusion
Definition
Diffusion
The process in which particles spread from regions of high concentration to regions of low concentration.
In solids, diffusion is extremely slow because particles can only vibrate in fixed positions.
In liquids, diffusion occurs slowly as particles can slide past each other.
In gases, diffusion is rapid because particles have high kinetic energy and large spaces between them, allowing them to spread out quickly.
Example
Add a drop of food coloring to still water.
The color slowly spreads through the water, showing diffusion from an area of high concentration (the drop) to low concentration (the rest of the water).
When you are cooking, the smell of food travels from the kitchen to other rooms.
This is diffusion of gas molecules through the air, moving from where they are crowded (near the pan) to where they are less concentrated (farther away).
The Kinetic Theory Explains Particle Motion
The kinetic theory of gases extends the particle model to explain the behaviour of gases in terms of the motion of their particles.
It states that:
Gas particles are in random, constant motion.
They move in straight lines until they collide with each other or with the walls of the container.
Collisions between gas particles are (ideally) perfectly elastic.
In a perfectly elastic collision, no kinetic energy is lost overall.
Energy can be transferred between particles, but the total kinetic energy remains constant (as long as temperature stays constant).
The average kinetic energy of gas particles is directly proportional to the absolute temperature (in Kelvin).
If the temperature in Kelvin doubles, the average kinetic energy of the particles also doubles.
Note
Kinetic energy is the energy an object has because it is moving.
Faster-moving particles → higher kinetic energy.
Example
When a gas in a balloon is heated, the particles move faster (their kinetic energy increases).
They collide more frequently and with greater force with the walls.
At constant external pressure, the balloon expands until the inside pressure matches the outside pressure again.
Changes of State: From Solid to Gas and Back Again
Solids, Liquids, and Gases: The Three Classical States of Matter
Solids, liquids and gases have distinct properties because their particles are arranged and move differently.
Solids
Particles are tightly packed in a fixed, regular arrangement (a lattice in many solids).
Forces of attraction between particles are very strong.
Particles can only vibrate about fixed positions.
Solids have a fixed shape and a fixed volume.
Liquids
Particles are close together, but not in fixed positions.
Forces of attraction are strong enough to keep particles close, but weak enough to allow them to move past each other.
Liquids have a fixed volume, but they take the shape of their container and can flow.
Gases
Particles are far apart, with very weak (often negligible) forces of attraction between them.
Particles move randomly and rapidly in all directions.
Gases have no fixed shape and no fixed volume – they fill the entire container.
From Melting to Sublimation: The Processes of State Changes
When matter changes state, it undergoes physical changes such as melting, boiling, condensation, freezing, sublimation and deposition.
These changes can usually be reversed by heating or cooling.
They are physical changes because the chemical composition of the substance remains the same.
Latent Heat – Energy During State Changes
During a change of state, the temperature of the substance stays constant even though energy is being transferred.
This extra energy is called latent heat.
At the plateaus, average kinetic energy (temperature) stays the same, but potential energy of the particles changes as the arrangement becomes more or less ordered.
Definition
Latent heat
Latent heat is the energy absorbed or released without a change in temperature while a substance changes state.
Definition
Latent heat of fusion
Energy required to change 1 kg (or 1 mole) of a substance from solid to liquid at constant temperature.
Definition
Latent heat of vaporisation
Energy required to change 1 kg (or 1 mole) of a substance from liquid to gas at constant temperature.
Tip
Melting and boiling are endothermic (energy is absorbed by the system).
Freezing and condensation are exothermic (energy is released to the surroundings).
Melting and Freezing
Melting and freezing are opposite processes between the solid and liquid states.
Melting (solid → liquid)
When a solid is heated, particles gain kinetic energy and vibrate more strongly.
At the melting point, particles have enough energy to overcome some of the forces holding them in fixed positions.
The regular structure breaks down and the solid becomes a liquid.
During the process:
Energy is absorbed from the surroundings (endothermic).
Temperature stays constant while the solid melts – the energy goes into breaking intermolecular forces (latent heat), not raising temperature.
Freezing (liquid → solid)
When a liquid is cooled, particles lose kinetic energy and move more slowly.
At the freezing point, particles no longer have enough energy to move freely and settle into fixed positions.
The liquid becomes a solid with a more ordered arrangement.
During the process:
Energy is released to the surroundings (exothermic).
Temperature stays constant while the liquid solidifies – energy is released as intermolecular forces form and particles become more ordered.
Boiling and Condensation
Boiling (liquid → gas)
When a liquid is heated, particles gain kinetic energy.
At the boiling point, particles have enough energy to break the forces of attraction and form a gas.
Bubbles of gas form throughout the liquid and rise to the surface.
During the process:
Energy is absorbed from the surroundings (endothermic).
Temperature remains constant while all the liquid changes to gas.
The extra energy increases the potential energy of the particles as they move much further apart.
Condensation (gas → liquid)
When a gas cools, particles lose kinetic energy and move more slowly.
They come closer together and the forces of attraction become more important.
Particles form a liquid with a more ordered arrangement.
During the process:
Energy is released to the surroundings (exothermic).
Temperature stays constant while gas turns into liquid – energy is released as particles form stronger interactions in the liquid.
Evaporation
Evaporation is a slower process than boiling and occurs at the surface of a liquid at any temperature below the boiling point.
Some surface particles have enough kinetic energy to escape into the gas phase.
These high-energy particles leave the liquid, so the average kinetic energy of the remaining particles decreases.
As a result, evaporation causes cooling.
Example
When you sweat, water on your skin evaporates.
The escaping molecules take energy away from your skin, so you feel cooler.
This is why evaporation is called a cooling process.
Sublimation and Deposition
Sublimation: A change from solid directly to gas without passing through the liquid state.
Deposition: The reverse process, where a gas changes directly to a solid without becoming a liquid.
Example
Dry ice (solid CO₂) sublimes directly to carbon dioxide gas at room temperature.
This is why it is used to create fog or smoke effects in theatres and concerts.
The Role of Gases in Diffusion, Compressibility, and Density
Gases show some extreme forms of the properties we introduced with the particle model.
Diffusion – spreading out evenly
In gases, particles move rapidly and randomly, so they spread out to fill the entire available space.
Example
If you spray perfume in one corner of a room, the scent molecules diffuse through the air and eventually reach all parts of the room.
Compressibility – gases can be squashed
Gas particles are far apart, with a lot of empty space between them.
When a gas is compressed, particles are pushed closer together, reducing the volume.
This is why gases are much more compressible than liquids and solids.
Density – less mass in more space
Gases have low density because their particles are spread far apart.
There are fewer particles per unit volume compared with liquids and solids.
Active recall
State the four main assumptions of the particle model of matter.
State three key ideas of the kinetic theory of gases.
Describe the arrangement and movement of particles in a solid, liquid and gas.
Describe what happens to particle energy and arrangement when a gas condenses into a liquid.