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I can explain the shape of a heating/cooling curve by describing the energy changes through the heating/cooling of a substance.

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Licence

This content is © Oak National Academy Limited (2023), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

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Key learning points

An experiment can be planned to measure the temperature change through continuous heating of a substance
Energy is needed to overcome forces of attraction between particles so temperature does not change when changing state
The change in state of pure substances can be seen on a heating/cooling curve as a horizontal line
There is a change of energy, arrangement, and organisation of particles of a pure substance at each stage

Common misconception

Pupils struggle to understand what is taking place at the horizontal parts of the graph.

Be concise with your language and ask pupils to use the same language. 'have enough energy to overcome the forces of attraction between particles', 'to slide past each other', 'to move past each other'.

Keywords

Heating curve - shows temperature changes of a substance over time as it is heated.
Cooling curve - shows temperature changes of a substance over time as it is cooled.
Intermolecular forces - are forces between particles.

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Hi, I'm Mrs. Hudson, and today we're going to be doing a lesson called Heating and Cooling Curves.

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 explain the shape of a heating cooling curve by describing the energy changes through the heating of a substance.

During today's lesson, there will be three key words that are used frequently throughout the lesson, and they are heating curve, cooling curve, and forces of attraction.

So, let's look in more detail at what each of those phrases mean.

A heating curve shows temperature changes of a substance over time as it is heated.

A cooling curve shows temperature changes of a substance over time as it is cooled.

Forces of attraction are forces between particles.

If you want to pause the video to write those keywords down, then please do and then press play ready for me to carry on.

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 talking about heating curves, then we're going to look in more detail at cooling curves, and then in the final part of the lesson, we're going to be planning a heating curve experiment.

But let's get going with the first part, heating curves.

Substances are not fixed in one physical state.

A substance in the liquid state at room temperature, for example water, can be cooled and changed into the solid state, for example ice.

Changing a substance's state requires an energy transfer either from the surroundings to the substance or from the substance to the surroundings.

So, if we had two substances, which are here represented by the red and the blue block, if energy is transferred from the surroundings to the substance, then this would be heating the substance up, whereas if energy was transferred from the substance to the surroundings, this would be cooling.

You can see that represented by the red block, which is being heated up, and the blue block, which is cooling down.

When a substance, for example, water, is heated, energy is transferred from the surroundings to the substance.

So here we can see that we've got a Bunsen burner with a tripod and gauze and resting upon that is a beaker filled with water with a thermometer inside, and if heat is transferred from the Bunsen burner to the beaker, the water is going to heat up.

The transfer can be measured using a thermometer to read the temperature change in degrees celsius.

A substance in the solid state is placed in a beaker with a thermometer.

The beaker is heated, and the temperature measured every 10 seconds.

We can see a beaker with some water and a thermometer inside, and if we heat that water, we're going to measure the temperature every 10 seconds.

A graph of the results can then be plotted, and it is called a heating curve, and on that graph, on the x-axis, you have the time, which is in seconds, and the temperature, in degrees Celsius is on the y-axis.

Data can be plotted on a graph using x-y coordinates.

A cross is drawn where the two coordinates meet.

So, we can see here that we've got a table which has got the time in seconds, and then you have recorded the temperature at regular set intervals.

And you can plot the data onto a graph, so for example, the time in 30 seconds has a temperature of 16 degrees, which we can see is plotted on the graph where the arrows are pointing towards there, and you can do the same for each point on the graph.

So, the x-axis is the time, and the y-axis is the temperature.

The temperature of a pure substance increases as the energy is transferred to the substance.

The particles remain in a fixed position, but the vibrations increase.

So, if you had a solid, for example, and you transferred energy from the surroundings to the solid so you heat the solid, then the particles will remain in fixed positions, but the vibrations increase as the heat is transferred to the particles.

When the substance starts to melt, the energy is used to overcome the forces of attraction between the particles enough so that they can move past each other, and once the particles are able to move past each other, this is when the substance will have turned into a liquid.

And what this looks like on a graph on your heating curve you initially have a sloped line, which is showing you the temperature increase as you are heating the solid, and then when the substance melts, there's no increase in temperature anymore, and you have a horizontal line going across, and this shows you the melting point of that substance.

The reason why there's a horizontal line is because the energy is being used to overcome the forces of attraction in the particles and it's not being used to increase the temperature, and the horizontal section after the solid state is going to be melting.

The temperature does not increase until the substance has completely melted.

Once the substance has melted, the temperature of the substance will start to increase again as long as energy is still being transferred to the particles, and as energy is transferred to the substance, its particles start to move more.

When the substance starts to boil, the energy is used to further overcome the forces of attraction between the particles until they are free to move away from each other, and at this point we will get another horizontal section on the graph, and that is the boiling point.

And again, the reason why the temperature doesn't increase during the change of state boiling is because the energy is being used to overcome the forces of attraction between the particles.

Once the substance has boiled and is in the gas state, the temperature of the gas will start to increase again, which you can see is represented on the graph by the sloped line increasing and the temperature increasing further.

Determining the melting and boiling points for a pure substance from a heating curve.

So, we can see here we've plotted some points to show a heating curve, and there we would have the solid state, the liquid state, and the gas state as the time increases and the temperature increases.

The melting and boiling points can be determined by reading the temperatures of the horizontal sections.

So, for example, we know that the first horizontal section between solid and liquid state is melting, and the second horizontal section is the boiling point, and if we extrapolate across from the horizontal line to the y-axes of the temperature, we can read off the melting point, and we can do the same thing for the boiling point by extrapolating the horizontal boiling point line across to the temperature.

Pure substances have specific melting and boiling points, so we can see on this graph of a pure substance there is a horizontal line representing the change of state, and if we extrapolate that across to the temperature, it is one specific value for the melting and the boiling point.

Let's check our understanding.

Which section of the graph below represents when a substance is in the gas state? This is D.

Well done if you got that right.

Which section of the graph below represents when a substance is boiling? This is C, and we know that because it's a horizontal part, so that must be a change of state, and B is melting because it's occurring at a lower temperature, whereas C is boiling because it's occurring at a higher temperature.

Well done if you got that right.

Which of the graphs below has a ring around the section that represents when a substance is melting? A, B, or C.

This is C, and this is the only graph that's actually representing a state change here because it's horizontal.

Which section or sections of the graph below represents when the forces of attraction between the particles are being overcome? This is B and C.

Well done for recognising that.

Remember, the temperature doesn't rise during a state change because the energy is being used to overcome the forces of attraction, and this is when the actual state change is taking place.

So, on the graph, we can see this when the horizontal.

Really great job with those checks for understandings.

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

Andeep has labelled his diagram incorrectly.

Correct the mistakes he has made.

So, the title of his graph is warming curve of water.

He's put on the label so that graph, and he has identified melting points.

So, look at that graph and identify the incorrect things and correct them.

Sure, you're going to do a fabulous job.

Press play when you're ready for me to go through the answers.

Let's see how we did.

So, the first mistake Andeep made was that he said it was a warming curve of water rather than a heating curve.

So, great job if you recognise that.

He also hasn't got the correct label on his y-axis, which is temperature in degrees celsius.

He put melting point rather than boiling point, and zero degrees is the melting point of water.

And he's also solid state in the first part of the graph and gas state at the top part of the graph at a later time.

So, well done if you managed to get those right.

In the second part of task A, you need to complete the gaps with the following words or phrases, and the words you need to use are increases, decreases, stays the same, attraction, and repulsion.

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

Let's see how we did.

So initially the temperature of the substance increases until it reaches its melting point.

Once it reaches its melting point, the energy needed to overcome the forces of attraction, for example, intermolecular and electrostatic forces.

This means that the temperature of the substance stays the same.

Once all the substance is in the liquid state, the temperature starts to increase again until it reaches its boiling point.

The energy is then needed to further overcome the forces of attraction between particles, so the temperature stays the same.

Once all the substance is in the gas state, its temperature starts to increase again.

Really great job if you managed to get those right.

If you need to pause the video to just check what you've got and add any detail in, then please do, but we're going to carry on with the rest of the lesson.

Great job! We know about heating curves, so let's now look at cooling curves.

A pure substance in the gas state can be cooled.

Scientists can measure the temperature at regular time intervals and plot their results on a graph.

The graph is called a cooling curve, and this should look familiar because it's similar to a heating curve.

It's just that the substance starts in the gas state, and it starts at a higher temperature, and energy is transferred from the substance to the surroundings, so the substance is cooling.

So therefore, we start in the gas state there's a horizontal section, and then as the temperature decreases, it turns into the liquid state.

There's another horizontal section, and then as the temperature decreases further, we enter the solid state.

When a substance cools down, its particles transfer some of their energy to the surroundings.

This means that the particles slow down.

So, we can see here we've got an image which is showing you how particles might cool down, and as they do, they move around less, and they begin to move into a more fixed arrangement closer together.

As the substance cools, its temperature will remain constant when it changes state.

So, in the same way, during a heating curve, when the substance is changing state, there is no temperature increase, and this is represented by a horizontal section on the graph.

The same is true for a cooling curve, so when the substance is changing state, there will be a horizontal section on the graph.

Eventually the particles will no longer have enough energy to overcome the forces of attraction, so the particles come together and the substance changes state.

A substance in the gas state will condense at its boiling point and change into the liquid state.

A substance in the liquid state will freeze at its melting point and change into the solid state.

And we can see this represented on our cooling curve here.

So, we've got the boiling point; this occurs when the substance is condensing, and the melting point occurs when the substance is freezing.

The state changes that occur on a cooling curve will be shown as horizontal lines for a pure substance.

They show where the following state changes occur condensing and freezing.

The boiling point and melting point can be labelled on the graph, and this can be seen on this graph here, where the boiling point is labelled where the substance begins to condense, and the melting point is labelled where the substance begins to freeze.

Let's check our understanding.

Which section of the graph below represents when a substance is condensing? A, B, C, or D? This is B.

Well done if you got that right.

C is the other state change, but this is showing freezing.

Which section or sections of the graph below represents when the temperature is staying the same? A, B, C, or D? This is B and C.

The temperature stays the same at the horizontal sections of the graph, and this is where the state change is occurring.

So well done if you got those right.

The melting point of phenol is 41 degrees celsius.

Which of the graphs below correctly show this information? A, B, or C? This is B.

Now, we know this because the melting point is the change of states between liquid and solid.

A is showing you the boiling point where the substance is changing from a gas to a liquid, which is condensation.

We're ready now to move on to task B, and in the first part of task B, you need to identify and correct five mistakes that have been made labelling the following diagram.

Title, freezing curve.

You've got the melting point labelled on the graph.

The y-axis says temperature, and the x-axis says temperature in seconds.

And then you've got a label saying temperature not decreasing.

Give this your best go, and then press play when you're ready for me to go through the answers.

Let's see how we did.

So, the title is cooling curve.

The temperature had missing degrees celsius, so there was no unit for the temperature.

It was labelled as the melting point, whereas actually it was the boiling point, showing you the change of state from gas to liquid.

And the melting point is lower down on the graph.

We needed to label the first state of change at the higher temperature as condensing, and then the other state change was freezing.

And the temperature is not decreasing at the two points in the graph where it is horizontal, not where there is a sloped line going downwards.

Fantastic job if you managed to get those right.

Well done.

Great job so far.

We know about heating and cooling curves, so let's look now at planning a heating curve experiment.

An experiment can be carried out to plot a heating curve.

So, what we would say, for example, is, how does the temperature change over different time periods for a certain substance? And doing that, you would produce a heating curve that looks like this graph.

Now in this example we would be changing the time, so therefore the time is the independent variable and the factor that we are measuring is the dependent variable, which in this case is the temperature.

So, the independent variable on the x-axis is the time in seconds, and the dependent variable on the y-axis is the temperature in degrees celsius.

The equipment needed to do this experiment would be a boiling tube filled with stearic acid in the solid state with a thermometer in.

So, it's very important that the stearic acid is in the solid state.

You would need a beaker of water to use as a water bath to allow heating of stearic acid up to 100 degrees celsius.

Using a water bath allows us to heat up in a more controlled and slow fashion, whereas if you just directly applied a Bunsen burner to the stearic acid, there's a chance that it could go over the boiling point.

You need a Bunsen burner and a heatproof mat.

You need a tripod and gauze, which is what the beaker of water is going to rest upon.

You would also need a clamp, boss, and stand and a stop clock.

And then you would set up the equipment as shown.

So, the clamp, boss, and stand are used to clamp the boiling tube in place over the water bath.

The water bath is sitting on top of the gauze, which is on the tripod, and then the Bunsen burner sits underneath the water bath to heat up the water, which will then heat up the stearic acid.

So, the stearic acid is inside the boiling tube, and the water is inside the beaker.

You also need a stop clock, and we have to make sure that the thermometer is inside of the boiling tube.

Let's check our understanding.

Which of the pieces of equipment below would you use to heat the substance in a boiling tube up to 100 degrees celsius? A, B, or C? This is B.

The water is acting as a water bath, and you would place the boiling tube of stearic acid into the water bath, and this makes sure it doesn't get heated up too quickly.

So, well done if you recognise that.

Which of the pieces of equipment below would you use to measure the dependent variable? This is C, the thermometer.

Now this is quite a tricky question because you had to remember what the dependent variable was.

The dependent variable is the thing that you are measuring, which in this case is the temperature.

And remember the dependent variable is found on the y-axis of the graph, which we know is the temperature in degrees celsius.

So great job, if you remember that.

B is wrong because B is representing the time, and that is the independent variable.

Which of the following are safety precautions that should be taken when using a Bunsen burner for heating? A, don't tie long hair back.

B, wear eye protection.

Or C, sit down in front of your equipment.

This is B, wear eye protection.

Remember, when using a Bunsen burner, you should always tie long hair back and you should always stand up to perform your practical experiments.

True or false? A Bunsen burner will quickly heat the water bath and sample stearic acid to 120 degrees.

True or false? This is false, and justify your answer.

A, using a Bunsen burner to heat the water bath ensures the sample is heated to no higher than 100 degrees.

And B, using a Bunsen burner to directly heat the sample ensures that the sample is slowly heated to no higher than 100 degrees celsius.

This is A, remember, we always use the water bath to ensure that the sample is heated slowly and to no higher than 100 degrees celsius.

Great job if you got that right.

We're ready now to do task C of the lesson, and in the first task, you need to draw and label the missing equipment in place and add the missing labels.

So, some parts of the apparatus have already been labelled for you, but you need to fill in the missing equipment and add any missing labels.

Now just a reminder you might need to draw some bits of missing equipment as well as labelling so pause the video, give this your best go, and then press play when you're ready for me to go through the answers.

Let's see how we did.

So, first of all, we've got a Bunsen burner here drawn in, so that was one part that was missing, and then if we look more closely at this section here, you've got the thermometer labelled, the boiling tube, and then the stearic acid is inside of the boiling tube.

Then the beaker and the gauze need to be labelled and then that there is the Bunsen burner.

We've also got the tripod, so great job if you managed to add those labels.

For part two of task C, match the description to the piece of equipment used.

So, these are the descriptions of what the equipment do and then on the other side we've got the pieces of equipment, so you need to match the description to the correct piece of equipment.

So, give this a go, and then press play when you're ready for me to feed back.

Let's see how we did.

So, the container used to hold the stearic acid is the boiling tube filled with stearic acid in the solid state.

Holding equipment when heating is the clamp and stand.

Measuring the temperature is a thermometer.

Heating the water bath is the Bunsen burner and heatproof mat.

Gently heating the substance is the beaker of water to use as a water bath.

Supporting equipment over a flame is the tripod and gauze, and measuring the time is the stop clock.

So, fantastic job if you managed to get those right, well done.

And now for the final part of task C, number three.

Plan an experiment to create a heating curve to determine the melting point of stearic acid.

Its melting point is less than 100 degrees celsius.

In your plan, you need to include, a, step-by-step method, b, a table to record your results, and c, suitable safety precautions you might need to take to keep yourself and others safe.

So, please go into as much detail as you can.

Make sure you include all of a, b, and c in your answer.

Give this your best go, and then press play when you're ready for me to go through the answers.

Let's see how we did.

So, in your plan, you needed to include the step-by-step method, which was a, and you should have had something like this.

So, first of all, record the initial temperature of the substance.

Number two, start heating the water bath and start the stop clock.

And number three, record the temperature every 30 seconds for 20 minutes.

You might have also drawn a diagram to help you represent this.

And then in part b, the table to record your results.

You should have had time first in the first column in seconds, and if it was every 30 seconds, it should be 0, 30, 60, 90, and so on going down.

And then you should have in the next column the temperature of the sample in degrees celsius.

Really important to make sure you've got the units in your tables as well.

And then for part c, suitable safety precautions.

You might have included some of the following precautions.

Wear eye protection or wear goggles, tie long hair back, place equipment in the middle of the bench, stand up during the experiment, and tuck stools under benches and hang up bags.

Really fantastic job.

You've done a really great job so far.

If you need to pause the video, do add in any extra detail, and then come back because we're going to summarise what we've learnt this lesson.

So today we've been talking about heating, and cooling curves, and we've said, that an experiment can be planned to measure the temperature change of a substance during the process of being heated or cooled.

When a substance that is being heated changes state, energy is required to overcome the forces of attraction between its particles, so the temperature does not increase.

The change in state of a pure substance on a heating, or cooling curve is shown by a horizontal line.

There is a change of energy, arrangement, and organisation of particles in a pure substance at each stage of heating or cooling.

I really enjoyed today's lesson.

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

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The following statements are about some of the sections (e.g. ‘CD’ means the sloping part from C to D). Which of the statements are correct?", "image": "https://www.thenational.academy/_next/image?url=https%3A%2F%2Foaknationalacademy-res.cloudinary.com%2Fimage%2Fupload%2Fv1700470968%2Fn7l8o8vq6qxvuvn6wkpk.png&w=640&q=75", "imageKey": "4b94919c-384e-21b4-adea-77b60ed309ff/question/b5da4248cf592f3b78e3d44290e7cfdf.png" }, "questionType": "Multiple choice", "correct": [ { "text": "AB: temperature of pure solid substance increases as energy is transferred to it" }, { "text": "AB: particles remain in fixed position but vibrations increase" }, { "text": "BC: temperature does not change but particles start to move past each other" }, { "text": "DE: temperature does not change but particles spread apart from each other" } ], "choices": [ { "text": "AB: temperature of pure solid substance increases as energy is transferred to it" }, { "text": "AB: particles remain in fixed position but vibrations increase" }, { "text": "BC: temperature does not change but particles start to move past each other" }, { "text": "BC: temperature does not change but particles spread apart from each other" }, { "text": "DE: temperature does not change but particles spread apart from each other" } ], "id": "ef3baf7d-47a3-9f87-3247-4f289a3e5e35", "lessonId": "41d1b8c1-23ee-29af-16ff-c61c0bfa08b3" }, { "type": "Question", "question": { "text": "The image shows three cooling curves. What do the ringed sections represent, in order from left to right?", "image": "https://www.thenational.academy/_next/image?url=https%3A%2F%2Foaknationalacademy-res.cloudinary.com%2Fimage%2Fupload%2Fv1725394407%2Fjzxhs6e7positzspyhjf.png&w=640&q=75", "imageKey": "4b94919c-384e-21b4-adea-77b60ed309ff/question/4ff1a8d61f0e03a0b83f4bc3d7ff32e4.png" }, "questionType": "Multiple choice", "correct": [ { "text": "gas state; liquid state; freezing point" } ], "choices": [ { "text": "condensing point; liquid state; freezing point" }, { "text": "condensing point; liquid state; solid state" }, { "text": "gas state; liquid state; freezing point" }, { "text": "gas state; liquid state; solid state" }, { "text": "gas state; melting point; freezing point" } ], "id": "5491b7a1-cd70-8fe9-298d-1439074dde26", "lessonId": "41d1b8c1-23ee-29af-16ff-c61c0bfa08b3" }, { "type": "Question", "question": { "text": "Sort the following stages of a heating curve into the correct order, starting from a statement about the solid substance:" }, "questionType": "Ordering", "elements": [ { "order": { "text": "1" }, "item": { "text": "Temperature of the solid substance increases until it reaches its melting point." } }, { "order": { "text": "2" }, "item": { "text": "Melting point: transferred energy is used to overcome forces of attraction." } }, { "order": { "text": "3" }, "item": { "text": "At the melting point, the temperature stays the same." } }, { "order": { "text": "4" }, "item": { "text": "Once all the substance is in the liquid state, the temperature rises again." } }, { "order": { "text": "5" }, "item": { "text": "Boiling point: transferred energy is used to overcome forces of attraction." } }, { "order": { "text": "6" }, "item": { "text": "At the boiling point, the temperature stays the same. " } }, { "order": { "text": "7" }, "item": { "text": "Once all the substance is in the gas state, the temperature rises again." } } ], "id": "79346ed3-601b-75f4-309a-46af8576e6c3", "lessonId": "41d1b8c1-23ee-29af-16ff-c61c0bfa08b3" }, { "type": "Question", "question": { "text": "In a heating curve experiment, the dependent variable is temperature, and the independent variable is ______." }, "questionType": "Short answer", "correct": [ { "text": "time" } ], "id": "5934755f-4898-7b4f-c768-a24f0a612049", "lessonId": "41d1b8c1-23ee-29af-16ff-c61c0bfa08b3" }, { "type": "Question", "question": { "text": "Which of the following pieces of equipment would you use to measure the dependent variable in the heating curve experiment?" }, "questionType": "Multiple choice", "correct": [ { "text": "thermometer " } ], "choices": [ { "text": "beaker" }, { "text": "boiling tube" }, { "text": "Bunsen burner" }, { "text": "stopwatch" }, { "text": "thermometer " } ], "id": "4941fc9f-9b5a-d950-7886-ee123c2bad06", "lessonId": "41d1b8c1-23ee-29af-16ff-c61c0bfa08b3" }, { "type": "Question", "question": { "text": "The image shows a curve for oxygen. This is a ______, although it starts at the melting point rather than with the substance in the solid state.", "image": "https://www.thenational.academy/_next/image?url=https%3A%2F%2Foaknationalacademy-res.cloudinary.com%2Fimage%2Fupload%2Fv1725392994%2Ffjagl65glbkszu6g10fd.png&w=640&q=75", "imageKey": "4b94919c-384e-21b4-adea-77b60ed309ff/question/8ec73439420b86bb4f4a3f1ce60557cb.png" }, "questionType": "Short answer", "correct": [ { "text": "heating curve" } ], "id": "98064395-94c1-704d-9f54-6a2edf80cc4d", "lessonId": "41d1b8c1-23ee-29af-16ff-c61c0bfa08b3" } ], "lessonId": "41d1b8c1-23ee-29af-16ff-c61c0bfa08b3", "meta": { "gradeStageId": "ks4", "subject": "Chemistry", "tier": "Foundation", "examBoard": "AQA", "grade": "Year 10", "topic": null, "unit": "States of matter", "lessonCount": "7", "age": [ 14, 15 ], "gradeId": "age_14_15", "gradeStage": "GCSE", "lesson": "Heating and cooling curves" } } ] }