Hello, my name's Mrs. Nivin, and today we're going to be talking about conservation of mass as part of our topic on calculations involving masses.

Now, you may have some experience of this from your previous learning, but what we do in today's lesson will not only help us to answer that big question of what are substances made of, but will help us to understand better what's actually going on during a chemical reaction, specifically when we look at the masses of the chemicals involved in those reactions.

So by the end of today's lesson, you should be able to describe what happens to the atoms of reactants as they form products in a chemical reaction.

Now throughout the lesson I'll be using some keywords and these include: atom, reactant, product, closed system, and conservation of mass.

Now the definitions for these key terms are given in sentence form on the next slide, and you may wish to pause the video here so that you can jot down the definitions for reference later on in the lesson or later on in your learning.

So today's lesson is broken up into two parts.

Firstly, we'll look at atoms in a chemical reaction and then we'll take a closer look at what we mean by this term of conservation of mass.

So let's get started by looking at what's going on with atoms during a chemical reaction.

Now, I am a massive fan of toy kits, and if you've ever had one yourself, you could see that some of them are actually composed of lots of different parts that can be assembled in different ways to create something different.

Now, atoms are very similar.

In chemistry, the parts that are rearranged and then reassembled into something new are the atoms of the reactants.

Now, how atoms are rearranged during a chemical reaction is represented using a chemical equation.

And the generic form that we have is reactants on the left, an arrow, and then the products on the right.

So if we look at a chemical reaction here, we've got magnesium reacting with some oxygen to form magnesium oxide, and we can see then that the reactants are all on the left, the products are all on the right, and when we remove those circles, we have then our chemical equation that tells us what's going on with these atoms. Magnesium adding to oxygen to form magnesium oxide.

So chemical equations can actually represent a chemical reaction in different ways.

So we have here still, the same reaction we talked about a moment ago, and we can represent that using our word equation.

Now a word equation simply refers to the chemicals by their chemical name.

So we've got magnesium plus oxygen is making magnesium oxide.

But another way we can show this chemical reaction is using what's called a symbol equation.

And I've shown that here.

Now what a symbol equation does is that it changes the chemical names into the chemical formula for both the reactants and the products.

But if we look a little bit closer at this symbol equation, it actually provides us with a lot more information than simply the chemicals that we started with and ended with.

So we also are told the physical state of our chemicals.

So we can see with the state symbols here, my magnesium and my magnesium oxide are solids and the oxygen is a gas.

It also tells us which elements the chemicals are actually composed of.

So we can compare these symbols to our periodic table to identify the elements that are involved.

And finally, it will also tell us the ratio or how many atoms we can actually find in each of our reactants and each of our products.

So a symbol equation gives us a lot more information about this chemical reaction than a word equation might.

Let's take a moment now to take a look at a few chemical reactions and see how we can discuss the atoms and what's going on in these reactions.

So the first example I have here is when iron is heated with sulphur, it forms iron sulphide.

So if I look at those individual substances, so I've got my iron filings, my yellow sulphur, and then the iron sulphide on the side.

I could represent this using a word equation or I could represent it using a symbol equation.

But remember we're talking about the atoms in these reactions.

So I'm going to add another layer here and I'm gonna represent these atoms using a diagram.

So I've got my iron atom and I'm gonna add it to a sulphur atom, and I get this formula unit of the iron sulphide that is composed of one iron and one sulphur.

Let's look at another example.

When calcium carbonate is heated, it decomposes or breaks down into calcium oxide and carbon dioxide.

So again, I can show that with my word equation or it can show it with my symbol equation.

Now if I dig down a little deeper, I can still show that with my diagrams of the atoms. So in my calcium carbonate, I have one calcium, one carbon, and three oxygen atoms. My calcium oxide then has one calcium and one oxygen, and my carbon dioxide has one carbon and two oxygen atoms. And so we can represent our substances taking from that assembly equation and start to look a little bit more closely at the atoms that are involved.

Let's stop here for a quick check.

What information can be obtained from a chemical symbol equation? Now, there's a few different options here, so you may wish to pause the video so you can discuss them and come back when you're ready to check your answer.

Well done if you said all of them.

You can find the physical state of the chemicals, what elements those chemicals are made of, which ones are the reactants and which are the products, and even the ratio of the atoms of each element that you have in your reactants and products.

So a lot of information is provided from a symbol equation.

Well done if you managed to choose all of them.

Great job, guys.

Fantastic start.

Now, once we know what atoms are actually involved in a chemical reaction, we need to remember that they don't change into different atoms during a chemical reaction.

What's actually happening is they're acting a little bit like that toy kit, okay? The atoms are rearranging themselves, reorganising to form the products.

So let's look at another example here.

So if we go back to our reaction of the calcium carbonate being heated and broken down into calcium oxide and carbon dioxide, I'm gonna show the symbols for each of the atoms involved below those chemical formulae.

And when we take a closer look, we can see that the calcium and oxygen from our calcium carbonate on the left has formed a calcium oxide on the right.

And then if we look at the carbon in two oxygens that are left over in my calcium carbonate, they've combined to create the carbon dioxide that is one of my products.

So they've simply rearranged, reorganised from what we started with in our reactants, to what we've ended up with in our products.

So what that means then is if you want to make a particular product, you need to have the right atoms available in your reactants to make that product.

Just like if you are making a particular toy, you need the right pieces in your kit to make that toy.

So if we look at a symbol equation that looks like this and we draw our atom diagrams below, we can see that in our product, we don't have the correct atoms in our reactants.

Our product needs are chlorine atoms, but we've only got chlorine atoms available in my reactants, and therefore this would not work.

So we'd need to make sure that whatever we have in our reactants is rearranging to make those products.

And we can see here then if we've got our barium and our chlorine atoms in my reactants, they're gonna combine to make my barium chloride in my products.

Let's stop for another quick check.

True or false? Atoms can change into different atoms during a chemical reaction.

Well done if you said false.

But which of these statements best supports that answer? Well done if you said B.

Atoms are only rearranged during a chemical reaction, they cannot be created or destroyed during it.

They simply are broken apart and rearranged into our new products.

So well done if you managed to get that correct.

Great job, guys.

Really impressed.

Okay, it's time for the first task of today's lesson.

And the first thing I'd like you to do is to match each of these key terms to the best descriptions.

So you may wish to talk it over with your neighbours, have a little think about what we've learned.

So pause the video here and then come back when you're ready to check your answers.

Okay, let's see how you got on.

So the word equation is going to be the third description down where it says sodium plus chlorine makes sodium chloride, and that's because it's using the chemical names for our chemicals involved in that reaction, and that's what makes it a word equation.

Now the reactant is what we start with, and that's gonna be found to the left of an arrow in that chemical reaction, which is our bottom description.

The state symbol then is indicating the physical state of the substances in that chemical reaction.

So that was the red flag in the description for me that we've got our state symbol is the physical state.

The product then is what is formed in the reaction.

So that's the second description down, meaning that our symbol equation then is going to be representing a chemical reaction using formulae in that equation.

So very well done if you manage to get those correct.

Great job, guys.

Keep it up.

Okay, this next part of the task is a little bit more involved.

You have three reactions here, and for each one I want you to do three things.

The first thing you need to do is identify the reactants and products and their physical states for each reaction.

I might recommend that you circle the reactants in one colour and the products in the other colour.

That might help you just to do it a bit a bit more quickly.

Alternatively, you could maybe create a table where you have reactants and products listed there.

Then I want you to use that chemical equation to identify the elements that are involved in the reaction.

You may wish to have a periodic table handy to help you identify what that symbol actually represents for the element's name.

And then I'd like you to tell me how many atoms do we actually have involved in each of our reactions? Okay? Now this is gonna take a little bit of time.

You might wish to work with a partner, maybe check your answers with another group and then come back when you're ready to check your work.

So pause it here and come back when you're ready.

Okay, let's see how you got on.

Now, there were quite a few things I asked you to do per reaction.

So we're gonna look at each reaction individually.

And I mentioned earlier about maybe circling or highlighting your reactants and your products using different colours.

And that's what I'm going to do as part of my feedback.

So for the first reaction, we had CuBr2, makes Cu plus Br2.

And the reactant then is what is on the left of the arrow, and that is CuBr2 or copper bromide.

And that is in the solid state because I have the S in brackets.

My products then are on the right hand side of the arrow, and that means I have copper, which is as a solid, and the bromine is a liquid because it has the L state symbol.

So the elements that are involved is copper, and I have one atom there, and bromine and I have two atoms that are present, and that's because it has that subscript or small number 2 next to it.

So well done if you managed to get that first equation correctly identified with all those different aspects I asked you to do.

So let's crack on and see how you got on with part B.

So I'm gonna stick with that colour coding that I mentioned before.

And so my reactants for this equation are, again, still on the left hand side of the reaction.

The zinc, the Zn is in the solid state, but my H2SO4 is in the aqueous, so that's aq, meaning dissolved in water.

My products then are to the right of that arrow and the hydrogen, H2, is a gas and the zinc sulphate or ZnSO4 is again in the aqueous or dissolved in water state.

So the elements that are involved and the number of atoms, I have zinc and one atom of them, I have hydrogen and two atoms of those, I have one atom of sulphur and four atoms of oxygen.

Again, using those small numbers to represent the number of atoms that are present.

And remember, if there is no number involved, just by having the symbol there, it tells you you have at least one atom.

So great job, guys.

You're doing really well.

Let's see how you got on that last part of part C.

Now this one was a little bit more complicated.

We've got lots of different substances involved, but we are gonna take a really systematic approach to how we answer this question.

So sticking with that colour coding, my reactants remain always on the left of the arrow.

The Na2CO3 is in the solid state because it has that lowercase s in brackets.

And my H2SO4 isn't aqueous or dissolved in water.

My products then, as always, are gonna be on the right hand side of that arrow.

The Na2SO4 is solid state for the S, the CO2 is in the gas state because it has lowercase g and the H2O is going to be a liquid because it has the lowercase l.

Now, there were a lot of elements and atoms involved.

So I have two sodium atoms, that's the Na represents sodium.

I have one carbon atom, seven oxygen atoms, and that's because you needed to count up all of the oxygens that are present on either the reactants or the products.

And so sometimes these atoms are gonna be split between the different reactants and you need to count them all up on that one side, okay? Hydrogen then.

We've got two atoms of those.

And we have one atom of sulphur.

So very well done if you managed to identify the correct elements involved.

And incredibly well done if you managed to correctly identify the number of atoms, specifically the oxygen, because that was a little bit of a tricky one.

I am so impressed with the start that you've made to this lesson, guys, really, really well done.

Now that we're feeling a little bit more comfortable talking about the atoms that are involved in reactions and what happens to those atoms during a chemical reaction, let's move on to this idea of conservation of mass.

Now, you may have heard of conservation or to conserve something previously, but essentially what it means is to preserve it, to keep it from changing.

So we might have talked about the conservation of animal populations like elephants or you may have heard about the conservation of plants, like that that's taking place at the Eden Project in Cornwall or Kew Gardens in London.

But when we're talking about a chemical reaction, atoms are what are conserved.

They are unchanging.

So what that means then is that all of the atoms and the reactants are reorganised to form all of those products.

And we saw that earlier when we looked at that reaction between iron and sulphur to form iron sulphide.

Okay, no atoms are lost, no atoms are gained.

They simply all rearrange from the reactants to make those products.

Now, sometimes when discussing a chemical reaction, chemists refer to what's known as an observed environment.

Now, an observed environment is simply where this reaction is taking place and the chemicals involved in that reaction.

So we can see here we've got two different reactions that are taking place, and the observed environment then is what is outlined here in green.

You'll notice on the left hand side that the observed environment is not the entire beaker, it's simply where the chemicals are involved, okay? So it's really important when we're talking about the observed environment, that we are being really specific about where this reaction is taking place.

Now, if we think back to that reaction we saw earlier with producing iron sulphide, it actually takes place in what's known as a closed system.

Now, a closed system is simply an observed environment, so that's where the reactions taking place and the chemicals involved, but it's an observed environment where nothing can enter or escape from that environment.

So if we take a closer look at what that reaction actually is, we've got our iron sulphide being produced by heating the iron and sulphur at the bottom of this boiling tube.

It makes it a closed system because the apparatus contains a plug.

It prevents anything from entering or escaping from that environment.

The same thing can be seen in this precipitation reaction.

Nothing is entering or leaving this observed environment, that space within the reactant chemicals.

Not the entire beaker, just where that reaction is taking place within those solutions.

Logically then, it stands to reason that a non-closed system or an open system is one in which substances can either enter or leave that observed environment where the reaction is taking place.

So if we look at burning a match in air, it's completely open to the air.

And because it's open like that, it allows oxygen from the air to enter that observed environment and burn along with that match, allows that oxygen to enter that reaction location.

Another example might be heating magnesium in a crucible.

When that lid is opened, that allows any gases to enter into the crucible, into that observed environment or any substances to exit that observed environment where that reaction is taking place.

Let's stop here for another quick check.

True or false? Combusting wood on a bonfire is an example of a closed system.

Well done if you said false, but which of these statements best supports that answer? Well done if you said A.

Now combusting wood on a bonfire is very similar to burning a match.

It is completely, it's an open system and that's because air is able to enter and any gases are able to exit that observed environment where that bonfire, where the wood is combusting.

A closed system is one in which substances can't enter or exit, and we're looking for something that has a plug or a lid on it, and bonfires do not have plugs or lids on them.

So well done if you managed to get that correct, guys.

You're doing really well.

Now, regardless of whether or not a reaction takes place in an open or a closed system, mass is always conserved.

We've got here two colourless liquids that were on a balance and had a starting mass of 182.

52 grammes.

And when they're mixed together, we know a reaction has taken place because we have a colour change.

And the final mass then is 182.

52 grammes.

And this is further evidence then that we haven't lost any atoms or gained any atoms. They've simply rearranged from those starting materials, our reactants, into our finished products.

Now, conservation of mass simply states that the combined mass of the starting reactants is going to equal the combined mass of the products that are formed.

Now that's a bit of a lengthy statement and it makes me think that maybe there's a mathematical relationship here because I've got that word equals.

So if I wanted to, I could revamp this definition into an equation.

So conservation of mass means the total mass of the reactants equals the total mass of the products.

And we can see that when we look at some of our reaction examples here.

So I have my precipitation reaction before with my two solutions forming a solid product and an aqueous product, and I have the masses for each of those listed directly below them.

What I'm going to do here then is, notice where the arrow is.

That is distinguishing my reactants on the left from my products on the right.

Now conservation of mass is comparing my reactants to products.

So directly below that arrow in my reaction equation, I'm going to draw an equals sign to remind me of that conservation of mass reactance equals products.

Okay? The other thing I'm gonna do then is go back and every time I see my adding sign, so I've got my reactants that have combined together to create my multiple products, I'm going to bring those adding signs down, okay? So any of the symbols that you have within your reaction equation, you are including then down into your mathematical relationship for conservation of mass.

What I have then is if I add those masses together from my reactants, I get 1.

17 grammes.

And if I do the same for my products, again, I get 1.

17 grammes.

And what this tells me then is that mass has been conserved throughout this reaction.

Let's stop here for a quick check.

I have these two reactants placed on a balance, this lead nitrate solution and the potassium iodide solution.

And when they're mixed together, we get a yellow precipitate forming.

So we've got that precipitate of lead iodide solid and the potassium nitrate as a solution.

Okay? What do you think is the mass of the products? What should it say on that balance? Well done if you said 148.

2 grammes.

The diagram that we started with at the top here showing my two reactants that were placed on the balance, when they are combined, mass is conserved and it should show the exact same mass because nothing has been lost, nothing has been gained, and the mass of my reactants should equal the mass of my products.

So, well done if you chose the correct mass.

If a reaction takes place in an open system so substances could enter or leave that observed environment where the reaction's taking place, any products that's a gas could be lost to the surroundings.

So if we look at this example here of reacting some magnesium and acid and we look at the balances, we can see that there's a difference of 1.

47 grammes.

And looking at that reaction equation, I can see that a gas was produced.

I can see the bubbles in my diagram on the left, but my reaction equation also shows hydrogen as a gas is produced on the right, that tells me that 1.

47 grammes of hydrogen has escaped this system into the surroundings.

So mass was still conserved, it's just the gas has been lost.

Similarly, if you open the lid or remove a plug for what was a closed system, you now allow gases to enter that observed environment.

You're taking a closed system and turning it into an open system.

And that's an example here with the magnesium and oxygen reacting.

Now, if I was to put the solid substances on a scale at the start of my reaction and the end of my reaction, I get these two masses.

I can use this idea of conservation of mass to tell me a little bit more about the gas that was involved, okay? Because the mass of my reactants needs to equal the mass of my products, so I can take those values, subtract them, and I get 0.

08 grammes.

That tells me that 0.

08 grammes of oxygen was gained from the surroundings when that lid was lifted, changing my closed system into an open system.

So a gas is involved and it's changed the mass here, but still, mass is conserved.

The mass of my reactants equals the mass of my products.

We can see this a bit more clearly if we actually react something on a scale here.

So we're reacting magnesium with acid and that produces hydrogen gas.

So if we are able to create a closed system, we can actually see the conservation of mass because the gas can't escape into the atmosphere, it's just escaping into our balloon.

So the magnesium's been added into our acid.

The gas is produced and it starts to inflate that balloon.

And what that tells me is my starting mass was 123.

9 and my end mass remains 123.

9 grammes.

So it shows even though the gas is lost because I've put that balloon on it and created a closed system, I've gained some evidence to support the idea that even though the gas is being produced in that closed system, I can show that the mass has been conserved.

The mass of my reactants is equaling the mass of my products.

Now we can use this idea of conservation of mass, that the mass of the reactants equals the mass of the products to find an unknown mass, to calculate an unknown mass.

So let's go through an example together.

Three grammes of hydrogen reacted with 24 grammes of oxygen to form water, and I wanna know what mass of water was produced, assuming that mass is conserved in this reaction.

So the first step I'm gonna do is write a word equation, okay? It's the simplest form.

I don't need to know any formula.

It's really easy to do 'cause I can just copy stuff straight out of the sentence that I was given and below each of those substances then, I'm going to write the mass that I've been provided.

Now I know I have three grammes of hydrogen, 24 grammes of oxygen, and I need to know how much water to form.

That's not a mathematical relationship though.

So I'm gonna find my pluses and drop them down into my mathematical equation.

And for my arrow, I'm going to change to an equals sign.

I now have my mathematical relationship from conservation of mass, 3 + 24 is going to tell me the mass of the water and that is 27 grammes.

What I'd like you to do now is have a go at using conservation of mass on your own.

So we've got here, what mass of barium reacted with 3.

3 grammes of chlorine if 12.

1 grammes of barium chloride was produced.

And I want you to assume that mass is conserved in the reaction.

So use my example on the left to help you go through and have a go at calculating the mass of barium in your example.

You may wish to pause the video here, get out that calculator, there's no marks for mental maths here, and then come back when you're ready to check your answer.

Okay, let's see how you got on.

So the first thing you need to do is to write your word equation.

So it should be barium plus chlorine gives barium chloride.

We're going to put the masses directly below those substances and then use our reaction equation to write my mathematical equation.

So dropping down those plus signs and changing my arrow into an equals sign.

So I have my equation of something plus 3.

3 equals 12.

1.

If I then subtract 3.

3 from 12.

1, I get the mass of my barium and that should be 8.

8 grammes.

Well done if you managed to do that and much better, well done if you also showed your working.

Good job, guys.

Okay, let's move on to the last task for today's lesson.

For this first part, we've got five students who are discussing what the final mass will be when a reaction between calcium carbonate and hydrochloric acid finishes.

I want you to read through their ideas on the next slide and I want you to decide which of them do you think is correct and why? So when discussing this reaction, Sam thinks that the mass will decrease because one product is a gas and therefore it doesn't weigh anything.

Jun, on the other hand, says the mass will stay the same because the products are made of the same atoms as the reactants.

Aisha thinks the mass will stay the same because nothing goes in or out of the containers.

Jacob reckons that the mass will decrease because one product is a gas and can escape out of the top of the flask.

And Sofia reckons that the mass will increase because there are three products, but only two reactants.

Now there's a lot to digest here and you may wish to discuss your ideas with the people nearest you.

So I reckon you should pause the video here and come back when you're ready to check your answers.

Okay, guys, let's see how you got on.

Now you were asked to discuss five different statements about this reaction and decide who you agreed with the most and why.

So I'm looking for that because clause to support your decision of who you agreed with.

I'm gonna go through each of their statements separately and let's see how my discussion is compared to yours.

So we start with Sam, who reckoned that the mass will decrease because one product is a gas and doesn't weigh anything.

Now, they're not wrong.

Sam has correctly identified that a gas is formed, but sadly, thought that it doesn't have mass and all gases have mass, so they are incorrect.

Jun, on the other hand, thought that the mass will stay the same because the products are made of the same atoms of the reactants.

And again, Jun is not wrong.

He has correctly stated that the atoms and the reactants and products are the same.

But what he hasn't recognised is that this is an open system, so there's no lid, no plug on that conical flask or beaker and that a gas has been produced.

So if you look more closely at that reaction equation, on the right hand side we have CO2 as a gas being formed.

So Jun, sadly, is incorrect.

Now Aisha mentioned that the mass will stay the same because nothing goes in or out of the containers.

Now what she's not noticed is the same thing as Jun.

This is an open system and therefore, the carbon dioxide gas can escape it.

So she's incorrect, I'm afraid.

And Sofia suggested that the mass will increase because there are three products and only two reactants.

Now, unfortunately, Sofia hasn't realised that the number of products is completely irrelevant to the total mass, okay? It doesn't matter how many products or reactants you have, what matters is their mass and they need to be equal to each other.

So sadly, Sofia is also incorrect.

Now, Jacob had mentioned that the mass will decrease because one product is a gas and can escape out the top of the flask and he is completely correct, okay? The CO2 gas is going to escape because this is an open system because that conical flask does not have a plug or a lid or anything to keep that gas contained on top of the balance.

So very well done if you chose Jacob and supremely well done if you were able to support your decision with that because clause.

Good job, guys.

Okay, for the second part of this task, we're going to apply our understanding of conservation of mass, okay? So for each reaction I want you to, first of all, write a word equation for the reaction and then use your conservation of mass understanding to answer the question.

And crucially, I want you to show your working, please.

It's a very good skill to have, especially it helps us to identify where you have gone wrong, if you do go wrong.

And if not, then it is really useful to look back at to remind yourself of how to do a process as we go through.

So always show your working.

Now this is maybe gonna take a little bit of time, so I'd like you to get your calculators out, pause the video and come back when you're ready to check your answers.

Okay, let's see how you got on.

So for part A, we had 0.

48 grammes of magnesium reacts with 1.

52 grammes of chlorine.

And you were asked to find the mass of magnesium chloride that is formed.

So we write out our word equations, which should look like this, magnesium plus chlorine, arrow, or makes, magnesium chloride.

We put our masses that were given to us below, change it into our equation so we get 0.

48 plus 1.

52 gives us the mass of my product, which should be 2.

00 grammes.

So well done if you managed to get that correct.

In part B, we have eight tonnes of hydrogen reacts with 31 tonnes of nitrogen.

And I want to know what mass of ammonia will be produced.

So writing my equation again, I have hydrogen plus nitrogen makes ammonia, so the makes changes into an arrow.

I'm going to write the masses below of what I've been given and what I need to find.

And then using my conservation of mass understanding, the reactants must equal the products.

So 8 + 31 gives me an answer of 39 tonnes of ammonia would be produced here.

So well done if you managed to get that.

Now part C was a little bit trickier.

Let's see how you got on.

Writing my word equation, I should have then sodium hydroxide plus nitric acid makes, or arrow, sodium nitrate plus water, putting my masses below those substances, again, including the one I'm trying to find.

I have then this equation of 0.

3 plus my unknown equals 8.

9 + 1.

7.

If I rearrange this equation to make the question mark my subject, I get my unknown is equal to 8.

9 + 1.

7 - 0.

3.

And that tells me that I need 10.

3 grammes of nitric acid for this reaction to occur.

So very, very well done if you've managed to get this correct.

This is a practise makes perfect, and the more you do it, the faster you get.

But the main thing here is that you should be writing out your working so we can identify where you've gone wrong if you've gone wrong, okay? Really, really well done.

The final part of this task then is a practical.

Yeah! So what I'm gonna ask you guys to do is to carry out a method for reacting magnesium with oxygen.

Okay? And I've given you the reaction equation here.

This is the setup that you're going to use.

You're going to be using a crucible with a lid with magnesium ribbon inside it.

You're gonna need to use a clay triangle, tripod, bunsen burner and tongs.

And what I'm gonna recommend you do is practise using the tongs to lift that crucible lid a few times before you actually carry it out because it can be a little bit tricky.

So just make sure that you're practising a little bit more.

It may be that one person in your team is a little bit more confident, that's absolutely fine.

But practise before you actually start heating up your magnesium in the crucible.

Now what you're going to do is you're gonna record the masses as outlined in the results table I'm going to show you in a moment, and you're gonna heat this magnesium ribbon really strongly.

So we want that on the highest blue flame.

And whilst that's heating, you're gonna ever so slightly, occasionally open up that crucible lid.

And what you should see is the magnesium start to glow.

And when that happens, leave it for a moment or two, not looking directly at it, okay, that's rather dangerous.

Don't look directly at the magnesium, and then you're going to put the lid back down on it for a little bit and just keep opening and closing it ever so slightly.

So you're creating this closed system into an open system.

And after about five minutes of heating, you just want to turn the bunsen burner off, then leave that crucible to cool and then you're gonna record that final mass in your table.

Now if you are unable to actually carry out the practical, you could watch a video of it, which I'll show you in a moment.

So I told you I'd show you the table that I wanted you to record your masses in.

And this is it here.

So you may wish to pause the video to record it or use the worksheet to record it.

And once you finished your practical and recorded all your masses, you're going to use them and your understanding of conservation of mass to calculate the mass of oxygen that's reacted.

So at this point, you may wish to pause the video here so you can carry out your reaction and record your masses.

You may wish to go back to the previous slide that shows the instructions on how to carry out that reaction in the first place.

Just to remind you to open it and be very careful as you do that.

But pause the video then and come back when you're ready to check your answers.

Okay, so we've got all of our equipment ready for our our practical, and we moved the crucible and the lid onto our balance to get that first mass, which is 23.

94 grammes.

And next what we're going to do then is to move our magnesium strip into the crucible, and that mass then is 24.

20 grammes.

Now that magnesium strip has been curled up a little bit to make sure that it can fit into our crucible easily and that the lid will fit on it closely as well to make sure that we've got a closed system here.

Now we put it in our clay triangle on top of our bun, sorry, on top of our tripod, move our bunsen burner then to that roaring blue flame and put it underneath our crucible to let it heat quite strongly.

Now remember, we're gonna be lifting that lid ever so slightly, changing our closed system into an open system and we can see that it's definitely reacting with that bright glow that's forming there.

And we're gonna keep doing this, every so often, double checking up, it's still glowing, which means it's still reacting.

And we're gonna close that lid again to make sure that none of the products that are forming are escaping as well.

Keep checking it now.

And every time it glows, we know that it's still reacting, put that lid back on and we're gonna keep doing that until it stops glowing.

Now this one then has finished reacting and it has now cooled down.

So I'm removing it from the clay triangle and I'm gonna put it onto the balance once it's been teared.

So it should say 0.

00 grammes.

Once that happens, you can now put your crucible with our product and we know it's product 'cause of that white powder we can see within it, put the lid on as well.

And we now have our final mass of 24.

33 grammes.

Okay, let's see how you got on.

Now, depending on whether or not you carried out the practical yourself or watched the video, these values that we have for the masses and our calculations are going to be different.

So if you've done the practical yourself, you are going to want to double check the processing of the values that we have.

And if you did the masses based on the practical video, this is what you may have got.

So the crucible in the lid was 23.

94 grammes.

When the magnesium ribbon was inside it before heating, it was 24.

20 grammes.

And after heating that crucible lid and the magnesium ribbon had a mass of 24.

33 grammes.

Okay? Now the next thing I asked you to do then was to use your results and the understanding of conservation of mass to calculate the mass of oxygen that reacted.

So that was our unknown.

And again, if we're using those results from the practical video, the start had 24.

20 grammes.

And this is a value that may be different for you if you did the practical yourself, okay? At the end of the video, the mass was 24.

33 grammes for our magnesium oxide, crucible and everything.

Again, this is a value that might be different if you did the practical yourself.

But using our conservation of mass, what we get then is 24.

20 plus our unknown mass of oxygen, should equal 24.

33.

So if we rearrange our equation to find for our unknown, we get 0.

13 grammes.

And that then, is the mass of oxygen that added to our magnesium throughout this reaction.

Wow.

Well done, guys.

Now we have done a lot in today's lesson.

So let's take a moment to summarise what we've learned.

What we reminded ourselves that atoms are the building blocks of all the chemicals.

And during a chemical reaction, the atoms of the reactants do not change into different atoms. They are simply rearranged to form all the products of that reaction.

And it's impossible to make a particular product unless you have the atoms that are needed to make those products, okay? Now, because atoms are not lost or gained during reaction, the mass is conserved.

So simplifying it down into that mathematical relationship, the total mass of the reactants is equal to the total mass of the products.

And the law of conservation of mass can be applied to both a closed system and an open system that react with or produce gases.

So it doesn't matter if a gas is involved, you can still use conservation of mass.

So a lot that we got on through today, I hope you had a good time learning with me.

I certainly had a good time learning with you, and I hope to see you again soon.

Bye for now.