Lesson video

- Hi, I'm Ms. Barrett, from the Oak National Academy.

Today's lesson is all about conservation of mass from the atoms and period table unit from Key Stage 3.

I would recommend you have a calculator with you today because we are gonna be doing some calculations.

Let's get started.

So in today's lesson we are gonna be describing what is meant by conservation of mass and applying this to chemical and physical changes.

You might not have heard of conservation of mass at the moment, but don't worry, by the end of this lesson you are gonna be an expert.

So we're just gonna check our understanding of some keywords 'cause these are gonna help us for today's lesson.

Mass is the measure of the amount of matter in an object.

So when we say what is the mass of something, we're essentially saying how much matter does that object have? So if I was to measure the mass of myself, that might be in kilogrammes, for example.

A substance's state is whether it is a solid, liquid or a gas.

A chemical reaction is a process where substances are converted into new substances.

So during this lesson, we've got three parts.

The first part, we're gonna be identifying what is conservation of mass? Then we're going to look at why mass might appear to change in chemical reactions.

And then we're going to have a look at an experiment that involves conservation of mass.

So, let's start with what is conservation of mass? So first of all, here are some ingredients to make a strawberry cheesecake.

What I want you to do is I want you to work out what the total mass is of these ingredients.

So I'll give you a few moments to work that out.

You're welcome to use a calculator.

Did you get it? So the total mass of these ingredients is 1,450 grammes.

Now, if we take these 1,450 grammes of cheesecake ingredients, we can then turn that into our strawberry cheesecake.

So what do you think the mass is of the product? So what's the mass of that strawberry cheesecake? And you might be wondering, well I don't know.

How do I know without weighing it? How do I know without recording the mass? Well the mass of the strawberry cheesecake will also be 1,450 grammes.

So the reason for that, is because the total mass of our ingredients has to be equal to the mass of product.

This is known as conservation of mass.

And this is because we haven't created any new ingredients along the way and we haven't lost any along the way.

As long as you didn't eat some of the ingredients while you're making it, then theoretically, all of those ingredients should end up in the cheesecake.

So the mass should be the same.

So mass is conserved during physical changes.

So for example, when substances change form, so here, I've got a wine glass and if this wine glass has a mass of 300 grammes and then the wine glass smashes, how much do you think that wine glass, the smashed wine glass, what's the mass of that? So that will still be 300 grammes.

So even though the glass will have broken up into tiny, tiny bits and you might technically lose some on the floor, for example, if you were to add up every single glass particle, it will still add up to 300 grammes.

So mass is also conserved when substances change state, which is another physical change.

So here I've got an ice cube and the mass of this ice cube is 10 grammes.

So when the ice cube melts and just turns into a puddle of water, what is the mass of that water? So that is also 10 grammes.

So again, we haven't lost anything along the way and we haven't suddenly created anything new.

The water particles that were in the ice are still the same as the water particles that are in the puddle.

So mass is conserved, it's still 10 grammes.

And that is because when a substance changes form or state, the total number of particles remain the same.

So in our wine glass, originally all the glass particles were part of one piece and then the glass particles are now in many pieces, but the total number of particles hasn't changed.

And in the ice cube, we have the particles all arranged as a solid and then the ice cube melted and we have the exact same number of particles, but they're just now arranged as a liquid.

So in terms of particles, nothing has changed and this is why mass is conserved.

So we've talked about physical changes.

Now we're gonna talk about chemical reactions.

So chemical reactions involve rearrangement of atoms to make new products.

So for example, hydrogen and oxygen react together to form water.

So here's the balanced equation.

We've got two hydrogen molecules reacting with one oxygen molecule to form two molecules of water.

And here's the particle diagram to display that.

So if we have a look at the total number of atoms of hydrogen and the total number of atoms of oxygen on both sides, you'll see that the total number is the same.

We've got four hydrogen atoms on the left and four hydrogen atoms on the right, two oxygen atoms on the left and two oxygen atoms on the right.

So no atoms were created, no atoms were destroyed.

The only thing that is happening during a chemical reaction is particles are, or atoms are rearranged.

So this is why our equations have to balance.

This is why it wouldn't make sense if we didn't have that big 2 in front of the H2O at the end because then we would have some hydrogen and oxygen atoms left over.

So Antoine Lavoisier theorised conservation of mass in 1789.

And he said, "During a chemical reaction, no atoms are made or destroyed." So the mass of the products is the same as the mass of the reactants.

So here's an example.

So if I took 50 grammes of this purple solution and I added it to 10 grammes of this green solution, the product it makes would have to be equal to 60 grammes.

We can't lose anything along the way, or we can't create anything extra along the way.

So, back to the cheesecake.

This time we've got a different cheesecake and the total mass of the final product cheesecake weighed 1,600 grammes.

So, what mass of strawberries was used? Now remember, the mass of our ingredients or the reactants, has to be equal to the mass of the product.

So this means we can work out one of the missing ingredients.

Have a go at that, use a calculator if you need to.

So do you think you've got it? So we can use conservation of mass to calculate reacting masses.

So, mass of the cheesecake, as we said, must be equal to the mass of the biscuits, the butter, the cheese, the sugar and the strawberries.

So you put that into an equation and we add up the 250, the a 100 grammes, the 600 grammes, and the 100 grammes.

We can work out what is the missing one for the strawberries and that should have equaled 550 grammes.

So the mass of the product must be equal to the mass of the reactants.

So, baking technically is chemistry, but we're gonna move on to some laboratory chemical reactions now.

So 20 grammes of calcium reacts with 25 grammes of sulphur to produce calcium sulphide.

So here's my particle diagram.

You can see that the atoms are balanced.

I've got the same amount on both sides.

So, as we know the mass of both reactants, we can work out the mass of the product.

So what do you think the mass of calcium sulphide is? So what I would do, is I would add the 25 to the 20 and that would give you 45 grammes.

So that must be the mass of the product.

So calcium and sulphur react together to produce a 100 grammes of calcium sulphide this time, and 65 grammes of calcium was used.

So, same equation, but this time we need to work out the missing reactant.

So how would I do that? So what you would do, is because we know the mass of the product, we can take away the mass of one of the reactants from the mass of the product and that would give us 35 grammes.

And you can check that it makes sense if you add together 65 and 35, that equals 100.

So we've proven here that the mass is conserved.

Okay, so let's check how much we've understood so far.

So conservation of mass is where the products always weigh more than the reactants.

Is that true, or false? So that one is false.

Well done if you got that.

So why is that? Well remember, conservation of mass means the mass of the reactants must be equal to the mass of the products.

Okay, let's have a look at this next question.

Six grammes of hydrogen react with 10 grammes of oxygen to produce how many grammes of water? So have a think, if you need to use a calculator, that's absolutely fine.

But I can imagine you'd be able to do that one in your head.

So the mass of the water is 16 grammes.

Did you get it? Well done if you did.

So don't let the fact that there are two molecules of water confuse you and think that that must mean you need to times it by two.

You don't, all we're doing here is just working out the total mass by adding together the mass of the reactants.

Alright, let's have a go at this next question.

So 20 grammes of N2O4 breaks down into 12 grammes of O2 and how many grammes of N2? Now just a note, don't let the molecules or the atoms, or whatever it is that's being reacted put you off.

That doesn't matter.

All we need to think about here are the numbers.

Have a go.

So for this one, if we're starting off with 20 grammes and one of our products is 12 grammes, that must mean the other product is eight grammes.

Well done if you got that.

So let's move on to the task and really test your understanding of conservation of mass.

So in the first part, you're going to draw a particle diagram storyboard to show how mass is conserved in water boiling, and then the reaction between carbon, which is a C, oxygen, which is O2 to make carbon dioxide.

Now you can do this in any way you like.

If you want to have a sort of before and after picture, or maybe you might have many steps in your storyboard, that's totally fine, whichever way suits you.

But I will show you an example after.

And then part B, you're going to identify out of the water boiling, and the carbon dioxide reaction, which is the physical change and which is the chemical reaction? Pause the video here and have a go and I will see you when you get back.

Right, how did you get on with that? So we'll start off with water boiling.

I'll show you my example, but you might have done something a little bit different.

So here I've got a beaker of water boiling and I've showed the particles as a liquid and then I've drawn an arrow to a second part of the storyboard that shows some of those particles are turning into a gas.

So that shows water is boiling.

As I said, you might have drawn more steps in your storyboard, but as long as you have shown somewhere the particles as a liquid and then the particles as a gas, that is the most important part.

So is this one the physical change or is this one the chemical reaction? So water boiling is a physical change.

Remember, that it's still water, as like it's just now a gas.

So the only difference here is that it's changed state.

So, next one, I'll show you my storyboard for the reaction of carbon dioxide.

So I drew a particle of carbon and molecule of oxygen and then I've shown the carbon and oxygen's bonded together as carbon dioxide.

So as long as you've shown something along those lines that shows the rearrangement of those atoms, then that is totally fine.

Now is this a physical change or is this the chemical reaction? So this one is a chemical change, or a chemical reaction.

So remember, a chemical reaction is where atoms break bonds, rearrange and then form new bonds.

Really well done if you got those.

So part C, for each reaction you're going to write the word equation and then you're gonna calculate the missing mass.

So it might be a product, it might be a reactant, it doesn't really matter, there's just one mass that is missing.

So have a read through those, have a go.

Use a calculator if you need to.

Pause the video and come back once you've completed it.

Welcome back.

So let's go through the answers.

So 12 grammes of hydrogen is completely reacted to six grammes of fluorine.

What mass of hydrogen fluoride is formed? So the reason why I've got you to write out the word equation first, 'cause this can help me visualise the calculation.

So the word equation is hydrogen plus fluorine gives hydrogen fluoride.

Now what I would do next, is I would write in the masses underneath the equation of the things that you know.

And this makes really clear what is the missing mass.

So we've got 12 grammes of hydrogen and six grammes of fluorine.

So we know we need to add those together to work out the mass of the product, which equals 18 grammes.

So give that a tick if you got it right.

Okay part 2, 40 grammes of calcium oxide is formed from 15 grammes of calcium reacting with oxygen.

What mass of oxygen reacted? So again, we'll give the word equation.

That is calcium + oxygen gives calcium oxide.

And then if we write in the masses that we know, we've got 15 grammes of calcium, which is a reactant and 40 grammes of the product, the calcium oxide.

So we need to rearrange that to work out the missing reactant.

So if we do 40 grammes of product minus the 15 grammes, that gives us 25 grammes of oxygen.

So the third one.

1.

5 grammes of methane reacted with how many grammes of oxygen if the total mass of carbon dioxide and water was 3.

6 grammes? Now I think you might have found this a little bit more confusing because we've got two products and two reactants here.

But if you write out the word equation, that should give us methane + oxygen gives carbon dioxide + water.

Now we'll write in our masses that we know.

We've got 1.

5 grammes of methane and it doesn't really matter that we don't know the carbon dioxide and water individually.

We do know the total mass of the product was 3.

6 grammes.

So we can still do this calculation.

So if we do 3.

6 minus 1.

5, that gives us 2.

1 grammes for oxygen.

Now those were quite difficult, so a really big pat on the back if you've got those right.

Let's move on to the second part of the lesson.

So our second part of the lesson is why does mass appear to change? So in some experiments it appears that mass is not conserved.

So for example, if we had a birthday cake with a birthday candle, what happens to the birthday candle over time if you just leave it to burn? So yes, that's right.

What you should see, is that the candle wax starts to go down.

But does that mean the wax atoms are just being destroyed or they are just disappearing into thin air? Well actually, no.

What's going on here is a chemical reaction.

So what's happening is candle wax is reacting with oxygen to give us carbon dioxide and water.

And the products here, the carbon dioxide and water are both gases.

And these gases have been lost to the surroundings and are very difficult for us to observe and weigh the mass of.

However, if we were to try and capture all that gas and measure the mass of it, then mass would still be conserved.

So just because we can't see the products, doesn't mean that mass is not conserved.

So here I've got a little animation to show you what's happening here.

So as you can see, as the candle is burning, those particles are coming from a solid and turning into a gas.

And when they are a gas, we can't see them and we can't really measure their mass, but they are still there.

We're gonna have a look at another chemical reaction now.

So to start a campfire, you use a pile of wood.

And as that campfire burns, that wood shrinks.

And what you might be left with at the end is just a big pile of ashes.

So what happened to the matter that we started with? Where's that big pile of wood gone? So I imagine that the pile of wood is probably a lot heavier than that little pile of ashes that we end up with.

So, where did it go? Well, this is another chemical reaction.

So wood reacts with the oxygen in the air to once again produce carbon dioxide and water and carbon.

So the carbon dioxide and the water, just like before, they are both gases.

So they are not possible for us to measure, but they are still there.

They are just difficult to see.

So just like the candle, mass is still conserved.

If we were to add up all of the mass of the particles of the carbon dioxide, water and carbon, it will add up to the mass of the wood and the oxygen that we started with.

We're gonna have a look at another example now.

On a cold day, you might wake up to find your windows are wet.

So I've got a little picture on the right-hand side to show you an example here of the condensation.

So where have these water molecules come from? Did they just appear overnight? Were they created from nothing? Well actually, this is what happens.

So the water particles were originally a gas, which is water vapour.

And this would've just been inside the room floating around that you can't see, and therefore it's not possible for us to measure the mass of the water vapour that's in the air.

So here on the diagram, you can see that I've got the particles drawn as a gas.

As the particles touched the cold window, they cool down and they turn back into a liquid, just like this.

So now, this means that they are visible.

So the total number of water particles that we started with and what we ended with is exactly the same.

Mass is still conserved.

So why does mass appear to change then? Well, if gases are involved in the chemical reaction, this will change our observable mass.

So if the observable mass is increased, this is usually because some of the reactants are gaseous and the products are a solid or liquid.

So some of the reactants weren't possible to measure in the original measurements, but all of the products were possible to measure.

So why might our observable mass decrease? Well, this would be the other way around.

So this would be where some of the products are gaseous and the reactants are solid and liquid.

So we were able to measure all of the mass at the start and then as the reaction progressed, we appeared to lose some mass because some of those reactant particles turned into a gas.

So state symbols can help us identify if the reactants and products are a solid, liquid, or a gas.

So the state symbols you might come across, solid is written as an s in brackets.

Liquid is written as a little l in brackets.

Gases is written as a lowercase g in brackets.

And you might come across this one, an aq, which stands for aqueous.

And this just means it's a solution.

So this is where you might have a solid that's been dissolved in a liquid.

For example, a solution of sodium chloride.

So for example, lithium reacts with water to produce lithium hydroxide and hydrogen gas.

So here's the equation.

We've got lithium, which is a solid, so we write a little s in brackets.

Water is a liquid, so it's got an l in brackets.

Lithium hydroxide is a solution.

So we write aq in brackets and hydrogen is a gas, so we write g in brackets.

Will the mass of coal in a barbecue appear to increase or decrease whilst it's burning? That's right, it will decrease because the coal is burnt and turned into a gas.

When something burns, the products are usually carbon dioxide and water.

Both of these end up being gases, so therefore the mass will decrease during a burning reaction.

Calcium carbonate was added to hydrochloric acid in a test tube.

Calcium chloride, water and carbon dioxide gas were produced.

Will the mass in the test tube increase or decrease? So once again, that will appear to decrease, again, because a gas is produced.

Calcium carbonate was added to hydrochloric acid in two test tubes.

Calcium chloride and water and carbon dioxide gas were produced.

In which test tube will mass appear to be conserved? Now look very, very carefully at test tubes A and B.

So you might see that B has a bung in the test tube.

This means that the bung is gonna stop any gas escaping.

So when you remeasure that test tube, mass will appear to be conserved because you're still able to measure those gas particles.

Okay, true or false? Iron reacts with oxygen to produce solid iron oxide.

During the reaction it appears the mass decreases.

Is that true or false? So that one is false.

And that is because it will appear to increase because one of the reactants is a gas.

So we are reacting with something in the air, oxygen, that will increase the mass, or appear to increase the mass over the course of the reaction.

State whether the mass would appear to increase or decrease for the following reaction.

Now look carefully at those state symbols.

So mass will appear to decrease because one of the products is a gas and it will be lost to the surroundings.

So let's go on to Task 2.

So the first part, you're going to explain why mass appears to change in chemical reactions.

So write in your own words, a few sentences to explain why mass appears to change.

Pause the video here and come back once you've done it.

Okay, how did you get on with that? So I'm gonna show you an example of things that you could have included in your answer.

So mass may appear to decrease or increase.

An increase is due to a gaseous reactant giving solid or liquid products.

A decrease is due to solid or liquid reactants giving a gaseous product that is escaping to the surroundings.

So you might have written slightly different words, but as long as you've got words to that effect, then that's great, give it a tick.

So let's move on to part B.

For the following reactions, tick to show whether the mass will appear to increase or decrease.

Pause the video here, have a go and come back once you've completed it.

Right, let's go through the answers.

So for the first one, we had a chemical reaction with a gas reactant and a solid product.

So for that one, mass will appear to increase.

For the second one, if you have a look, the gas is on the product side, so mass will appear to decrease.

And I'll show you the rest of them.

Third one, mass will appear to decrease.

Fourth one, mass will appear to increase.

And the last one, mass will appear to decrease.

Good job if you got that.

Okay, so we're gonna move on to the final part of the lesson.

Conservation of mass experiment.

So when magnesium burns, it reacts with oxygen in the air to produce solid magnesium oxide.

So here's the equation, magnesium + oxygen gives magnesium oxide.

And I've drawn a little diagram to show you what's happening here.

So we are gonna find out if mass will increase or decrease during this reaction.

So what we need to do first, is we need to record our starting mass.

So we take a crucible, which looks like this, and we weigh the mass of that crucible when it is empty and that weighed 25 grammes.

Then we put the magnesium strip in the crucible and re-weigh it.

And that weighed 31 grammes.

So what is the mass of the magnesium? So, if you do a little calculation, you should find out that the mass weighs six grammes.

So then we're going to heat the magnesium and it will react with the oxygen in the air to produce magnesium oxide.

So we're gonna heat that on a blue flame over a Bunsen burner.

And over time it'll turn into our product, magnesium oxide, which is a white powder.

So we need to re-weigh the crucible with the magnesium oxide.

So now the crucible with the product weighs 40 grammes.

This is our third mass.

So how do we find out how much product we have produced? Well? What we do, is we take the final mass of the product in the crucible and we subtract the original mass of the crucible.

So we do 40 take 25 and that gives us 15 grammes of magnesium oxide.

So that's the mass of our product.

So did the mass increase or decrease during the reaction? Well the mass increased by nine grammes during this reaction.

It went from 31 with the crucible and reactant and 40 with the crucible and the product.

So why was that? So when magnesium burns, it reacts with oxygen in the air to produce solid magnesium oxide.

Now, potentially you've just worked out the oxygen in the air is a gas.

So it will appear as if the mass increases during the reaction.

But, no atoms were created here.

Remember the gaseous oxygen atoms, we couldn't weigh those at the start.

We couldn't put those into the crucible because they are just part of the surroundings.

And as the reaction progresses, they become part of that solid product, which means their mass could then be weighed.

Okay, what piece of equipment do you use to measure mass? So that one is a balance.

Sometimes it's referred to as a scales, but really in science we call that a balance and it looks like this.

What state is magnesium at room temperature? Is it a solid, liquid, or a gas? So magnesium is a solid, it is a metal.

What state is oxygen at room temperature? Oxygen is a gas.

What state is magnesium oxide at room temperature? So that was our product and that one was a solid.

As magnesium and oxygen react, the mass in the crucible, did it decrease, increase or stay the same? So the mass in the crucible increased over the course of the reaction.

Okay, so for Task 3, you're going to rewrite the paragraph and correct the mistakes.

So give it a read through and read it very, very carefully because I've changed some things to make it wrong.

You're going to rewrite it out to make the entire paragraph make sense.

Good luck, pause the video.

I'll see you once you're done.

Okay, are you ready for the answers? I'm gonna show you the correct paragraph now, make sure that you've got all of the bits changed that were wrong.

So magnesium is a solid, not oxygen.

And when magnesium reacts with oxygen gas from the surroundings, it forms magnesium oxide.

Magnesium can be heated in a crucible over a Bunsen burner.

The mass of the crucible would increase over the course of the reaction.

This is because the magnesium oxide is a solid.

The word equation for this reaction is magnesium + oxygen, gives magnesium oxide.

Really good job if you spotted all those mistakes 'cause some of those were a little bit sneaky.

So that is all of our tasks completed for today.

So really, really big well done for getting to this point of the lesson.

So let's summarise everything we've learned today.

Conservation of mass is where the total mass of the reactance is equal to the mass of the products, as no atoms can be created or destroyed.

Reacting masses and mass of products can be calculated due to the law of conservation of mass.

Mass may appear to increase if one of the reactants was a gas and the product is a solid or liquid.

Mass may appear to decrease if the reactants are solid or liquid and one of the products is a gas that escapes to the surroundings.

So, fantastic work today, you are now an absolute expert on conservation of mass.

So go away and have a go at that exit quiz and I'm sure you will smash it.

So thank you for using the Oak National Academy for your lesson today, and I'll see you next time.