Lesson video

Hi, I'm Mrs. Hudson, and today I'm going to be teaching you a lesson called predicting states of matter and limitations of the particle model.

This is a chemistry lesson and it comes under the unit titled states of matter.

So let's get going.

The outcome of today's lesson is I can use the particle model to predict the state of matter of a substance at different temperatures and discuss the limitations of this model for explaining how particles behave.

There'll be some keywords that are used frequently during today's lesson, and they are physical change, chemical change, particle model, and limitation of a model.

So let's have a look in more detail at what each of those words mean.

A physical change is a change in which no new substances are formed, such as a change in state.

For example, melting.

A chemical change occurs when a reaction takes place and atoms or ions in the reactants are rearranged to make new products or substances.

The particle model is a model that helps us to understand physical properties of substances.

It uses circles or spheres to represent particles, i.

e.

atoms or compounds.

And finally, the limitation of a model is a point at which we cannot use the model to help us explain a scientific phenomena.

Today's lesson is going to be split up into three different parts.

In the first part of the lesson, we're going to be looking at physical and chemical changes, and then we're going to move on to look at predicting states.

And then in the final part of the lesson, we're going to look at limitations of the particle model.

But let's get going first of all with physical and chemical changes.

During a physical change, there are changes in particle, arrangement and energy.

So here we can see an image of how the particles are arranged in the solid state.

They have a regular arrangement, so they look like they're in neat rows and they are vibrating on the spot.

Here we can see what particles would look like in the liquid state and they no longer have a regular arrangement.

The particles are touching each other, but they are able to slide over each other in the liquid state.

So particles have more energy in the liquid state.

And then in the gas state, we can see that the particles are randomly moving in all directions.

There is more space between the particles and they are at their highest energy in the gas state.

So they are moving around very quickly.

Now, if you go from a solid state to a liquid state, this is the change of state known as melting.

And then the liquid state to the gas state, this is the change of state known as boiling.

And for those to occur, you would need an input of energy.

And as you go from solid state to liquid state to gas state, there is an increasing movement of the particles and also an increasing energy.

So gas state particles exist at the highest energy in comparison to solid and liquid state.

The energy of particles can increase or decrease depending if energy is being added or removed from the system.

So here again, we've got our solid and liquid and gas states.

And this time, if we are removing energy from the system, then going from gas to liquid is the change of state known as condensing.

And then again, if you remove energy from the system when particles are in the liquid state, this would be known as freezing.

And when we remove energy from the system, going from gas to liquid to solid state, there's a decreasing movement of the particles and also a decreasing energy of the particles.

A chemical change is different to a physical change.

So looking first at a chemical change, during a chemical change, chemical bonds are broken and formed, rearranging the atoms or ions, making new substances.

So this is an example here.

We could have some hydrogen and some oxygen molecules, and if they react together in a chemical reaction, there is a chemical change and you would form a new substance, which is water in this example.

And you can see there's the balance symbol equation there.

So two H2 plus O2 makes two H2O, which is the water.

The main thing to take away from here is that the hydrogen and the oxygen atoms on the left-hand diagram are connected within the molecule by bonds, and those bonds will be broken during a chemical reaction and new bonds will be formed to make the new substance, water.

If we have a look more closely at the water bill, this can then change in a physical change.

But during a physical change, the arrangement of the particles changes, but the chemical bonds between them remain the same.

So water, if it undergoes a physical change, it could go from a gas to a liquid.

And the only thing that has changed there is the arrangement and the energy that the particles have.

So in the gas state, the molecules are further apart, but in the liquid phase, they are touching each other and can move around each other.

But there's still water.

It's still the same substance.

Let's check our understanding so far.

True or false, a chemical change takes place when particles are rearranged and become bonded to different particles.

True or false? This is true.

So well done if you got that right.

Now, to justify your answer, A, an example of a chemical change is when a substance in the solid state melts to a liquid state, or B, an example of a chemical change is when magnesium atoms bond to oxygen to become magnesium oxide.

This is B.

Well done if you got that right.

Next question.

When nail varnish remover, propanone, evaporates, it is A, a physical change, B, a chemical change, or C, neither a physical nor a chemical change.

This is A, a physical change.

If propanone is evaporating, it's changing state from a liquid to a gas and changes of state are physical changes.

And then the next question, state what change is occurring between carbon, C, and oxygen, O, to make carbon dioxide, CO2.

A, a chemical change, or B, a physical change.

And then can you say how you know that.

This is a chemical change.

And the reason we know that this is an example of a chemical change is the atoms are rearranged and bonded to different atoms to make a new substance.

So the bonds have been broken and reformed to make new substance, and that new substance is carbon dioxide.

Well done if you got that right.

And then the final question, state what change is occurring to carbon dioxide, A, a chemical change, or B, a physical change.

And can you say why? So this is B, a physical change.

And the reason we know that is because there's a change of arrangement of particles.

So no new substances are formed.

Fantastic job if you got those right.

Well done.

Forces of attraction between particles determine the amount of energy needed for state changes.

For example, melting and boiling.

So here we've got substance A, and you can see the particles of substance A are represented by the spheres.

And then those double-headed arrows are showing you the forces of attraction that exist between the particles in substance A.

Now, this diagram is only showing you the forces of attraction acting on one particle within substance A, but in reality, there will be forces of attraction existing between all the particles in substance A.

So we can see there another particle with the forces of attraction on.

The stronger the forces of attraction, the more energy is needed to overcome them.

The higher the melting point, the higher the boiling point.

Here we've got two different substances, substance A and substance B.

Substance A has a lower melting point than substance B because it has weaker forces of attraction holding the particles together.

So we can see of substance B, the forces of attraction are on that diagram already, and the width of the line represents how strong the forces of attraction are between the particles.

So if substance A has a lower melting point, the forces of attraction between the particles is going to be weaker.

So you can see when we put onto the diagram the forces of attraction, there's thinner lines showing weaker forces of attraction.

Let's check our understanding.

True or false: all substances have the same strength forces of attraction between their particles.

This is false.

Well done if you got that right.

Let's justify our answer now.

So A, different substances have different melting points due to different strength forces.

B, a substance with weak forces between the particles will have a high melting and boiling point.

This is A.

Different substances have different melting points due to different strength forces.

B is incorrect because substances with weak forces between the particles will have a low melting and boiling point.

So well done if you recognised that.

We're going to look now at different examples of substances and the forces of attraction between the particles or the molecules and how that affects their melting and boiling point.

So we're going to start by looking at substances that are made of small molecules.

And while we're looking at this diagram, we're going to use a key.

So forces between the molecules are going to be represented by the pink arrows and covalent bonds between the atoms are going to be represented by the blue arrows.

And we can see on the diagram here, we have got three small molecules represented within that square.

Small molecules, like the example on this slide, have only weak forces between the molecules and forces between molecules are called intermolecular forces.

So small molecules have weak intermolecular forces and they're usually gases or liquids, and they have low melting and boiling points.

Now, if we look at one of the molecules on its own, it's made up of five atoms, and those atoms are joined by covalent bonds, which we can see with the blue arrow here.

And small molecules have strong covalent bonds between the atoms that are not broken during a change of state, for example, in melting.

So when you melt a small molecule, you're not trying to break the covalent bonds within the molecule, you're trying to overcome the weak forces of attraction between the molecules.

And because the forces that exist between the molecules are very weak, small molecules have low melting and boiling points.

Intermolecular forces between small molecules increase with the size of the molecules.

So larger molecules will have higher melting and boiling points.

So we're going to start with a smaller molecule now, which in our example is methane, and it's got a boiling point of minus 162 degrees Celsius.

And if we look now at propane, we can see that those individual molecules are slightly larger than methane and therefore, there will be stronger forces of attraction that exist between those molecules because propane is a larger molecule than methane.

And then finally, if we look at poly propane, which is the largest example of the three here, there will be stronger forces of attraction between the longer molecules, and we can see that because the melting point of poly propane is 160 degrees, which is the highest of all of those molecules.

Let's check our understanding of that.

Which of the substances below would have the lowest melting point? A, ethanol B, sucrose, or C, glucose? If it's the lowest melting point, this will be the smallest molecule, which is ethanol.

So well done if you got that right.

Now we're going to have a look at ionic compounds and metals.

Ionic compounds and metals have giant lattices with bonds formed by strong electrostatic forces of attraction in all directions, which exist between oppositely charged ions in an ionic compound and positive metallic ions and electrons in metals.

Therefore, large amounts of energy are needed to break the many strong bonds.

So they have high melting points and high boiling points.

So ionic compounds and metals have high melting and boiling points due to the strong electrostatic forces of attraction.

And we can see here there's an example which is lithium iodide, which has a melting point of 446 degrees Celsius.

Now we're going to look at giant covalent structures.

An example here is buckminsterfullerene, which sublines at 800 degrees Celsius under a vacuum.

So giant covalent structures are made from atoms that are held together by strong covalent bonds.

So each one of those bonds holding the black spheres together in buckminsterfullerene is a very strong covalent bond and it requires large amounts of energy to overcome these strong bonds to change the state, but therefore, they're solids at room temperature and have very high melting points.

Let's check our understanding of that.

So select the correct statement or statements.

A, a large amount of energy is needed to overcome strong bonds and change state.

B, a large amount of energy is needed to overcome weak bonds and change state.

C, very high melting and boiling points occur in giant structures and there are lots of strong bonds to break.

D, very high melting and boiling points occur in giant structures as there are lots of weak bonds to break.

So the correct statements here are A and C, so well done if you managed to get those right.

Next question, select all types of substance that have high melting and high boiling points.

A, substances with metallic bonding.

B, substances that consist of small molecules, C, substances that have giant covalent compounds and D, substances that have a giant ionic lattice.

The correct answers here are A, C and D.

So small molecules were the only example that had low boiling points, and that's because there are weak forces of attraction between molecules.

So great job if you got that right.

The next question, which melting point below indicates the substance that has the strongest forces between the particles? A, oxygen melting point at minus 218 degrees Celsius.

B, water melting point at zero degrees Celsius.

C, sulphur melting point at 115 degrees Celsius.

And D, poly vinyl chloride melting point at 260 degrees Celsius.

This is going to be D, poly vinyl chloride with a melting point of 260 degrees.

And this is because the higher the melting point, the stronger the forces between the particles.

So well done if you got that right.

We're ready now to move on to the first task of the lesson, task A.

And in part one, Lucas has been discussing chemical and physical changes.

Use the choices in the brackets to make his sentences correct.

So we can see the different choices there.

And you need to select the choice that is correct for the statement that Lucas has made.

Pause the video and then press play, ready for me to go through the answers.

Let's see how we did.

So in the first statement, when a substance changes from a liquid state to a gas state, it is a physical change.

So well done if you got that right.

A physical change often involves a change of state, e.

g.

boiling.

And then the third one, in a chemical reaction, particles are rearranged and become chemically bonded to different particles.

So fantastic job if you managed to get all three of those right? Moving on to the second part of task A.

So in part A, is this a physical change or a chemical change? And give a reason for your answer.

You've got sulphur plus oxygen making sulphur dioxide, and then in part B, it's the same question, but we've got substance A turning into substance A.

So again, give this your best go and then press play ready for me to go through the answers.

So in the first part of 2a, this is showing a chemical change.

And we know this because atoms are rearranged and now bonded to different atoms. Sulphur is now bonded to oxygen in sulphur dioxide and a new substance has been produced.

Then in part B, this is a physical change.

It's only a state change from the liquid state to the solid state.

Substance A freezes, i.

e.

it changes to different arrangement of particles, but not chemically bonded to different types of particles.

There is no new substance formed.

Fantastic job if you managed to get those right.

Well done.

And now moving on to part three of task A.

Laura has been writing about forces of attraction between particles.

Complete the missing words in those sentences.

So you have a go at that now.

Let's see how we did.

So the missing word in the first sentence is stronger and the missing word in the second sentence is higher.

So fantastic job if you got that right.

Well done.

Now moving on to part four of task A, match up the boxes about bonding and melting and boiling points.

So you need to draw lines between the first column to the second column, the second column to the third column, and the third column to the fourth column to show how the bonding and the forces and therefore, the melting and burning point link to that substance.

Let's see how we did.

So ionic substances have strong electrostatic forces of attraction, which need lots of energy needed to overcome or break them, and therefore, they have high melting and boiling temperatures.

Metallic substances have strong forces of attraction, lots of energy needed to overcome or break, and high melting and boiling temperatures.

Giant covalent have many strong covalent bonds, which need lots of energy to overcome and therefore, high melting and boiling temperatures.

And small covalent substances have weak forces of attraction between molecules, low amount of energy needed to overcome, therefore, low melting and boiling temperatures.

Really great job if you managed to get those right.

If you need to go back to check your answers, please do, but if not, we're going to carry on with the rest of the lesson.

Great job so far.

Let's look now at predicting states.

If you know the melting and boiling point of a substance, you can predict which state of matter it will exist as at a specific temperature.

It can be helpful to use a number line to help you visualise this.

So if I give you an example, that's a number line here and that's showing the temperature in degrees Celsius, and we're going to mark zero onto that line and 100 degrees onto that line.

And then in the middle of those two numbers, you've got 50.

Now, zero degrees is the melting point of water, and 100 degrees is the boiling point of water.

So therefore, using that number line, we know below zero degrees is going to be in a solid state if you've got water, and therefore, zero degrees up to 99 degrees will be the liquid state.

And then 100 degrees and above will be the gas state.

But that number line has helped us to visualise the changes of state.

If I do another example of this, so ethanol has a melting point of minus 114 degrees Celsius and a boiling point of 78 degrees Celsius, in what state of matter is ethanol minus 120 degrees Celsius? So we're going to start with our number line, which is showing the temperature.

We're going to, on our number line, do the zero degrees and then minus 114, which is the melting point, and 78, which is showing you the boiling point.

So anything below 114 is a solid state.

Then we've got the liquid state.

Above 78 is the gas state.

In what state of matter is ethanol at minus 120 degrees? 120 is below minus 140.

So ethanol is in the solid state at minus 120 degrees.

Your turn to have a go now.

Methanol has a melting point of minus 98 degrees and a boiling point of 65 degrees.

What state of matter is methanol at 25 degrees? But you draw your number line on like that and then try and work out the state of matter of methanol at 25 degrees.

So we should have our zero degrees on our line here minus 98 is the melting point, and 65 is the boiling point.

So it's going to be solid state, liquid state and gas state at these temperatures.

So at 25 degrees, which sits below the boiling point, methanol is going to be liquid state at 25 degrees Celsius.

Fantastic job if you got that right.

Well done.

We're ready now to move on to task B of this lesson.

So in part one, Sofia has been discussing state changes.

Correct her sentences.

So these are the sentences that Sofia has said and you need to correct them.

So pause the video and then press play when you're ready for me to go through the answers.

Let's see how we did.

So minus 113 degrees Celsius is a higher temperature than minus 117 degrees Celsius.

A substance is in the liquid state when it is above the melting point, but below the boiling point.

And a substance that boils at minus 100 degrees Celsius will be in the gas state at room temperature of 25 degrees.

And then finally, a substance is in the solid state is below the melting point.

A really great job if you managed to get those right.

Well done.

Now looking at part two, predict the state of matter of the substances at the different temperatures.

So you've got the different substances, water, gold, oxygen, nitrogen, silicon dioxide, and potassium bromide.

You need to say what the state of matter is at the temperatures minus 200, zero degrees, 25 degrees, 100 degrees, and 1,500 degrees.

So give this your best go and then press play when you're ready for me to give you the answers.

Let's see how we did.

So water is a solid, solid, a liquid, a gas, and then a gas.

Gold is solid, solid, solid, solid and liquid.

Oxygen is liquid, and then gas in all the other temperatures.

Nitrogen is liquid and then gas at all the other temperatures.

Silicon dioxide is solid at minus 200 degrees and zero degrees and also all of the other temperatures.

And then potassium bromide is solid at all the temperatures apart from 1,500 degrees where it is a gas.

So fantastic job if you managed to get those right.

Well done.

Moving on to the third part of task B.

Water is a small molecule.

Using information from your table, suggest other substances that might also be small molecules.

Explain why you chose these substances.

So pause the video, have a go, and then press play when you're ready for me to feed back.

Let's see how we did.

You should have had oxygen and nitrogen as small molecules because they have low melting and boiling points, not so much energy is needed to overcome the intermolecular forces.

And gold, silicon dioxide and potassium bromide are not small molecules because they have high melting and boiling points.

They cannot be small molecules because too much energy is needed to break the bonds in these substances to change state.

Really great job if you managed to get those right.

Well done.

We're ready now to move on to the final part of today's lesson, limitations of the particle model.

This is how the particle model is used to represent the changes of state.

And you can see that we're looking at the boxes, which have the particles within them that exist within each state.

Particles are usually represented by spheres or circles and can be drawn in several different ways.

So description, it could just be a circular outline and that could be interpreted as a having an empty shell, a bit like an electron diagram.

You could have shading in of spheres, and that could be interpreted as solid sphere with no elasticity.

And then you could just have a circle.

That could be interpreted as being in 2D, not 3D, having only a circular shape and then outlining a coloured circle or sphere.

And that could be interpreted as having an outer shell filled with a substance.

So you can see that using spheres or circles actually can be interpreted in the wrong way that actually gives an idea about particles that are not actually true.

Particles are usually represented by spheres or circles that are the same size and shape.

However, in reality, particles vary in both size and shape, and atoms are mainly empty space.

Let's check our understanding of that so far.

How are particles represented in the states of matter diagrams? Select all statements that are correct.

So in this example, A, B, and C are all correct answers.

D is incorrect because particles are not represented using squares.

A limitation of the particle model is that it does not show how the particles are attracted together or the strength of these attractions.

Arrows have been added below to suggest what happens, and we can see that on this diagram here.

Arrows have been added to try and show you the forces of attraction that exist between particles that the particle model doesn't actually show us.

In reality, the particles are either bonded together, metals, ionic, and giant covalent compounds, or attracted by forces.

So the particle model needs to be adapted to show this.

The metallic bonds are formed by the attraction between the metal ions and the delocalized electrons.

Ionic bonding involves the attraction between oppositely charged ions.

For example, in sodium chloride, you've got the negatively charged ion, which is the Cl, and the positively charged ion, which is sodium.

Simple molecular substances are attracted together by weak intermolecular forces.

For example, in water here you can see the molecules of water and labelled there are the weak intermolecular forces between the molecules, but atoms within the water molecule are covalently bonded together.

In summary, the limitations of the particle model are all particles are represented as the same size circles or spheres.

The particles are shown as inelastic solid spheres, often in 2D when they are mainly empty space.

And thirdly, there's no attraction between the particles shown, for example, bonds or forces.

Let's check our understanding.

True or false.

All particles are inelastic solid spheres.

This is false.

And then justify your answer.

Particles are often different shapes and mostly empty space, not solid and inelastic or B, particles are always spherical but are not solid and inelastic.

This is A, particles are often different shapes and mostly empty space, not solid and inelastic.

Well done if you got that right.

Next question, what does this image of the particle model not attempt to show? Select all that apply.

A, movement of particles, B, arrangement of particles, C, forces between particles, and D, elasticity of particles.

So the correct answers are A, the movement of the particles, C, the forces between the particles and D, elasticity of particles.

B is incorrect because that diagram does show you the arrangement of the particles.

So well done if you got that right.

We're now ready to do the final task of the lesson.

This diagram shows a particle model for argon gas.

Give three or more limitations for this particle model diagram.

Pause the video, give this your best go, and then press play when you're ready for me to feed back the answers.

Let's see how we did.

So particles are shown as inelastic solid spheres.

It does not show the forces of attraction between the particles and it is only two dimensional.

So well done if you managed to get that right.

We're ready now to move on to the summary of everything we've learned this lesson.

So in today's lesson, we said there are changes in arrangement, movement, and energy of particles during state changes.

We also said there are two types of change, physical and chemical.

And the physical state of a substance can be predicted at specific temperatures.

We said the attraction between particles has a role in determining the amount of energy needed for state changes, and the particle model has limitations, for example, by showing particles as the same sized, inelastic spheres.

The model does not show the attraction between the particles, which affects the amount of energy needed to change the state.

You've done a really fantastic job this lesson.

Well done.

I really enjoyed it.

I hope you have too, and I look forward to seeing you next time.