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Syllabus
Syllabus
The three states of matter are solid, liquid and gas.
Melting and freezing take place at the melting point, boiling and condensing take place at the boiling point.
The three states of matter can be represented by a simple model. In this model, particles are represented by small solid spheres.
Particle theory can help to explain melting, boiling, freezing and condensing.
The amount of energy needed to change state from solid to liquid and from liquid to gas depends on the strength of the forces between the particles of the substance.
The nature of the particles involved depends on the type of bonding and the structure of the substance. The stronger the forces between the particles the higher the melting point and boiling point of the substance.
(HT only) Limitations of the simple model above include that in the model there are no forces, that all particles are represented as spheres and that the spheres are solid.
In chemical equations, the three states of matter are shown as (s), (l) and (g), with (aq) for aqueous solutions.
Students should be able to:
include appropriate state symbols in chemical equations for the reactions in this specification.
predict the states of substances at different temperatures given appropriate data
explain the different temperatures at which changes of state occur in terms of energy transfers and types of bonding
recognise that atoms themselves do not have the bulk properties of materials
(HT only) explain the limitations of the particle theory in relation to changes of state when particles are represented by solid inelastic spheres which have no forces between them.
What does this mean?
What does this mean?
Melting and Freezing Points.
Melting and Freezing Points.
As we can see from the above, different substances have different melting and boiling points.
It is the forces of attraction between particles that keep them together as solids and liquids.
So Alcohol must have fairly weak forces; Aluminium and Iron must have much stronger forces.
At -200oC all of the substances are below their melting point, so they will all be solids.
If we start heating, nothing will change state until -130oC when alcohol will all become a liquid.
At zero degrees the water will also become a liquid.
At 78oC the alcohol boils into a gas, the water is still liquid and the metals are still solids.
You'll see that metals usually have very high melting points, non metals generally have very low ones.
Heating until 100 oC causes the water to turn to steam, the metals won't melt until much hotter temperatures.
Notice that the Melting and Freezing points are the same, and the that the boiling and condensing points are the same.
So if we cool everything to 100oC, the steam will condense back into a liquid.
The alcohol will condense at 78 oC.
Questions
Questions
1. What state will Copper be in at 0oC?
2. What state will Sulphur be in at 60oC?
3. Give a temperature at which Magnesium is a liquid but Carbon is still solid.
4. What is the lowest temperature at which all the elements are gases?
5. What is the highest temperature at which all the elements are solids?
6. Which metal element melts at the lowest temperature?
7. One non-metal element has very strong forces between atoms, which one? How do we know?
8. Give a temperature that Helium is liquid.
9. What can we say about the strength of forces within Helium? How do we know?
10. Which metal has the weakest forces? How do we know?
State Symbols
State Symbols
Chemists are lazy - they like to write equations rather than words.
You probably know that the chemical formula for water is H2O.
This means that each molecule contains two H atoms and one O atom chemically bonded together.
When water freezes to ice this doesn't change its formula, just the way the particles (molecules) of water are arranged.
So water, ice and steam are all H2O.
Ice is H2O as a solid so we give it a symbol to show this H2O(s).
Water is H2O as a liquid so we give it a symbol to show this H2O(l).
Steam is H2O as a gas so we give it a symbol to show this H2O(g).
How can we write an equation for Ice melting?
How can we write an equation for Ice melting?
An equation needs two halves with an arrow to show that one side is changing to another.
H2O(s) --> H2O(l)
THe formula of the substance hasn't changed because Ice and water are the same substance in a different state.
So, all that changed was the state symbol (s) to0 (l)
Questions - Equations for state changes
Questions - Equations for state changes
Write an equation for water boiling?
Write an equation for Water freezing?
When we heat solid Iodine it sublimes - it turns straight to a gas without melting. (Iodine has the formula I2.) Write an equation for the sublimation of Iodine.
When we cool Carbon Dioxide gas it is deposited - it turns straight from a gas to a solid without condensing. (Carbon Dioxide has the formula CO2.) Write an equation for the deposition of CO2.
H2O(l) --> H2O(g) Only the state symbol changed!
H2O(l) --> H2O(s) Only the state symbol changed!
I2(s) --> I2(g) Only the state symbol changed!
CO2(g) --> I2(s) Only the state symbol changed!
Problems with the particle model
Problems with the particle model
We tend to think of particles as little snooker balls.
All the particles in a solid are touching in nice, neat rows and columns.
But we said that when we heat a solid the particles move further apart.
But if they stop touching they would be a gas!
So, the model we use is a bit misleading.
In reality, particles aren't solid, they can overlap.
Which means that they can move apart without losing contact with each other.
Video
Video
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PAST PAPER QUESTIONS
2019
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