Hello, my name's Mrs. Clegg.

I'm pleased to be learning about conservation of mass and how to balance symbol equations with you today.

We will also learn about reactants and products, and what occurs during a decomposition reaction, as well as how to balance those equations.

This lesson is part of, Understanding Chemical Reactions.

So let's get started.

So here are today's outcomes.

By the end of our lesson, you should be able to explain the conservation of mass and be more confident in writing balanced chemical equations.

You'll understand how the atoms of reactants in a chemical reaction are rearranged to form new products.

Here are the key words for today's lesson that I want you to listen out for, and be able to use by the end of the lesson.

Chemical formula, word equation, balanced symbol equation, and coefficient.

Here are those words written into a sentence.

So we'll be using these key words today, so watch out for them.

You might like to pause the video, and take notes here to refer back to.

Today's lesson is divided into two, conservation of mass, and chemical equations.

So let's get started with conservation of mass.

During a reaction, no atoms are created or destroyed, and it's really important to be thinking that.

The atoms are rearranged to make new substances, or new products.

The mass of the products that we make is equal to the mass of the reactants that we start with in a reaction.

And we call this, conservation of mass.

So the mass of magnesium and oxygen at the start of the reaction in the video here will be equal to the mass of the product, which is magnesium oxide, which is formed at the end of the reaction.

And you can see magnesium oxide being formed there, which is like a white ash, or powder.

So let's have a look at this reaction more closely.

On the left hand side we have the reactants, and on the right hand side we have the products.

So magnesium and oxygen are the reactants, and magnesium oxide is the product.

And this is the chemical equation.

This is a word equation.

And if we read what that chemical equation says, basically it's magnesium adding oxygen, changes into or forms magnesium oxide.

This plus symbol means adding.

The magnesium is added to the oxygen.

And the arrow shows what the reactants are changed into, or what is formed.

So the arrows in a chemical equation represent the words changes into.

So reactants change into products.

And word equations are just a simple way of describing chemical reactions using only words.

Here we go, magnesium and oxygen are the reactants, and they're changed into magnesium oxide, the product.

And because we're adding oxygen here, this is an oxidation reaction, and it's an example of a chemical reaction.

The Greek symbol here, the sigma, is a shorthand way of saying the sum off.

So the sum of the mass of the reactants will be equal to the sum of the products.

And here's an example, 127 grammes of copper metal, reacts with 32 grammes of oxygen, so the mass of reactants, 127 plus 32 is 159 grammes.

And there we've got the copper, and the oxygen on a balance, and we can see that the display reads 159.

These react together to form copper oxide, black copper oxide.

So another example of an oxidation reaction here.

And so the mass of the product is also 159 grammes.

So this is the same mass as the sum of the reactants.

Let's have a quick check.

So which of these statements is correct? Well done if you said B.

The sum of the mass of the reactants equals the sum of the mass of the product.

Here's another check.

A metal reacts with oxygen in a large sealed container.

The contents had a mass of 150 grammes of reactants, that was the metal and the oxygen, what will the mass of the product, the metal oxide be? Well done if you said B, 150 grammes.

So remember, the sum of the mass of the reactants is equal to the sum of the mass of the product.

Now sometimes we only have one reactant, and this breaks down when it is heated to form several products, and we call this decomposition.

The reactant is breaking down into simpler substances.

Copper carbonate here, the green copper carbonate powder there is being heated.

Nothing's been added other than heat, and it is breaking down, and eventually you can see it's forming a black solid.

This is called thermal decomposition, because heat was what was needed to start the thermal decomposition, the breakdown happening.

The chemical word equation is here, copper carbonate changes into copper oxide, and carbon dioxide.

So the copper carbonate was the reactant, and copper oxide and carbon dioxide were the products.

So what did the sigma symbol mean? Yes, the sum.

Sum of.

So the total mass, the sum of the mass of all the reactants is the sum of the mass, or the total mass of products.

So let's have a look at this example.

We've got 247 grammes of copper carbonate, we've heated it and it decomposes to form products that are 159 grammes of copper oxide, and 88 grammes of carbon dioxide.

We put that on the balance.

It will be 247 grammes of copper carbonate, so what will be the product? 247 grammes.

Here's the reaction.

And we can just put the numbers underneath there, and you can just see that the products again have the same mass as the sum of the reactants.

That's called conservation of mass.

Let's have a look at another example.

Here in this reaction, the display on the balance shows a hundred grammes throughout showing that the mass of the reactants and the equipment is equal to the sum of the mass of the products and the equipment.

So we've got a conical flask and we've put some calcium, solid calcium carbonate into it with hydrochloric acid, and we've put a balloon on the top to catch any gas.

And the gas that's being given off here is carbon dioxide.

And so the mass of the equipment, and the reactants is 100 grammes.

And at the end of the reaction, the mass of the equipment and the products is also 100 grammes.

And you can see that the balloon has inflated there, and the carbon dioxide gas will have been collected inside it.

You'll notice that the solid calcium carbonate has disappeared, but it hasn't disappeared, it's reacted and the atoms have been rearranged.

Let's have a quick check.

So a reaction between two chemicals takes place to produce a total of 256 gramme of product.

What was the total mass of the reactants? Well done if you said B.

If it was 256 grammes of product, then there must have been 256 grammes of reactant.

So in this check, what reading would you expect to see on the balance at the end of the reaction? And mass of the reactants and equipment has a mass of 225 grammes.

Well done if you said B.

The mass of product at the end of a reaction is 300 grammes.

There were two reactants this time, one reactant had a mass of 120 grammes, so what was the mass of the other reactant? Well done if you said B.

So 180 grammes plus 120 grammes that we already knew about is 300.

Well done.

Some pupils are talking about atoms and their reactions.

Now some of these statements are not correct, so what I'd like you to do is pause the video, and rewrite their statements so that they are correct.

Join us when you've done that.

Okay, how did you do? Let's have a look.

So Jacob, we've changed the statement to say, "No atoms are created or destroyed.

When a reaction takes place." For Izzy, "New substances are formed from the reactants when atoms are rearranged during a reaction." And Sam, "The mass of the product is always equal to the mass of the reactant." Well done if you've managed to do all of those.

Well done.

So I think we're ready for that question number two.

Complete the table below.

Don't write in the shady boxes, the ones that are coloured grey, you don't need to put anything in there.

You can use a calculator if you want to.

So in the first column, we've got the mass of reactant one, and we've got the mass of reactant two, and then we've got the total mass of the reactants.

Then we've got mass of product one, mass of product two, and the total mass of the products as well.

So see if you can work all those out, pause the video and come back when you completed the table.

Let's see how you've done.

So we've got two reactants, a hundred and 150.

So the total mass of the reactants is 250 grammes.

Therefore there's only one product formed, so that must be 250 grammes, so the total mass of the must also be 250 grammes.

Second line, we've got two reactants again, 135 plus 85, so the total mass of reactants must be 220.

And then we've got two products being formed, and we know the first product weighs has a mass of 65 grammes, and so the mass of product two must be 155, and so the total mass must be 220.

So the third line, there's only one reactant here, so 950 grammes.

So the total mass of reactants is 950 grammes, so the total mass of the products we know must be 950.

We've only got 525 grammes of product two, so product number one must be 425.

950 minus 525 gives you 425 grammes.

And the final box, final row, 127 grammes of reactant plus 84 grammes of reactant two, gives a total mass of 211.

So we know the total mass of the products must be 211.

Product two is 12 grammes, so if we do 211 minus 12, we get 199 grammes.

Absolutely fantastic if you've got all of those correct, well done.

So let's move on to the second part of the lesson, chemical equations.

The chemical formula of a substance represents the number of each type of atom that are present.

So in water, H₂O, if we look at the little subscript number there, the two, that tells us there are two atoms of hydrogen.

So it's the atom in front of the number that it refers to.

And if we have a look at the structure, we've got two hydrogen atoms, and one oxygen atom.

Now oxygen you might notice, has no subscript number, and we don't bother with subscript numbers if there's only one atom of that type.

So if there's no number, it's one.

Let's have another example.

So this is the chemical formula of a substance called ethane.

Now, if we have a look at the subscript number here, six, it only multiplies the element it directly follows.

So there are six hydrogen atoms in this compound.

And if we look at the breakdown of ethane, we can see that there are two carbon atoms, and there are six hydrogen atoms. Which of the particle diagrams below matches the chemical formula for carbon dioxide, which is CO₂? Well done if you said B, because there's one carbon, there's no subscript number, so we know that that's a one.

And we've got two oxygens.

How many and what type of atoms are there in a substance called copper sulphate? So which of these statements is correct? Well done if you said A, there is one copper, there is one sulphur, and four oxygens.

Remember that little subscript four there only applies to the oxygen, the element directly in front of it.

There's no number after the copper, so that's a one, and there's no number after the sulphur, so that's a one.

Phosphoric acid contains three hydrogen atoms, one phosphorus atom, and four oxygen atoms. Complete the blanks to write the formula for phosphoric acid.

Okay, have a go at that.

Okay, let's have a look.

So there's three hydrogens, one phosphorus, and four oxygen atoms. So it should look like this.

Citric acid contains six carbon atoms, eight hydrogen atoms, and seven oxygen atoms. Complete the blanks to write the formula for citric acid.

Well done.

C₆H₈O₇.

Another one.

Copper carbonate is made of one copper atom, one carbon atom, and three oxygen atoms. Write the formula for copper carbonate.

Okay, let's have a look.

So one copper, one carbon, and three oxygens.

That's the formula for copper carbonate.

Let's have another one.

Zinc sulphate is made at one zinc atom, one sulphur atom, and four oxygen atoms. Write the formula for zinc sulphate.

So one zinc atom, one sulphur atom, and four oxygen atoms. Well done.

Hopefully you're getting the hang of this now.

Okay, let's have another go.

How many, and what type of atoms are in the compound glucose? Well done if you said C.

Here's another one.

Sulfuric acid contains two hydrogen atoms, one sulphur atom, and four oxygen atoms. Which of the formula represents sulfuric acid? Well done if you said C.

It can't be any of the others.

Okay, if we just have a quick look at, for example A, that's got two hydrogens, which is correct, but actually it's got four sulfurs, so that's not correct, and it's only got one oxygen atom, so it can't be that one.

So we're gonna have a look at symbol equations now.

Up to now we've been using words.

So symbol equations are chemical equations that you use formulae, and they represent how atoms in the reactants rearrange the form products.

They indicate the ratio of reactants required and the products formed.

And that can be really useful when you're trying to make a certain amount of something.

So if we have a look at magnesium and oxygen again, here you can see the symbol equation for this reaction.

And you'll notice this time that there is a two in front of the magnesium.

And this means we need two atoms of magnesium to react with every one molecule of oxygen, and we will produce two molecules of magnesium oxide.

This is known as the substance diagram, and helps us to see what's happening.

When we balance equations, we need to remember that all the reactant atoms are rearranged to form the products.

Atoms are not lost or gained.

And the number of reactant atoms will equal the number of product atoms. So here in this chemical equation, we've got one atom of magnesium, one molecule of oxygen, we've made one molecule of magnesium oxide, and then we've got this sort of leftover oxygen atom.

And that isn't what would happen.

What would happen is that we would have two magnesium atoms react with one molecule of oxygen to produce two molecules of magnesium oxide.

So in this reaction, twice as much magnesium as oxygen is needed to react with all of the oxygen.

When we write a symbol equation, we need to remember that the ratio of the reactants and products are shown with coefficients, the big numbers that are placed in front of the symbols.

The process of calculating the coefficients for a symbol equation is called balancing.

So we talk about balancing the equation.

So here this is already balanced, this is a balanced symbol equation.

We've got sodium and bromine reacting to form sodium bromide, and we're using two atoms of sodium, to one molecule of bromine, and we're producing two molecules of sodium bromide.

Now remember here where there is no coefficient, no big number shown, we assume it's just one lot of the substance.

Let's look at some of the rules about formula and equations.

A substance's chemical formula can't change, it mustn't change.

The little numbers, the subscripts in a formula can't be changed or added to.

So water, H₂O, there is nothing we can do with that little subscript two, because that's what makes water, water.

If we did this, and put a two on the end there, that makes a totally different compound called hydrogen peroxide.

Water is safe to consume, but hydrogen peroxide would be dangerous.

So coefficients, the big numbers, are always found in front of the formula and they can change.

So here we've got sulfuric acid, H₂SO₄, and we could have nothing in front of it.

So the first one is like one molecule of sulfuric acid, or two or three.

So we can change those big numbers.

And if a chemical formula has no number in front of it, we always assume it's one, the coefficient is one.

So what is the coefficient of sodium in the following equation? So look at that equation, what's the coefficient for Na, which is sodium? Well done if you said four.

So look in the reactants, and we can see four Na.

So the coefficient of sodium is four.

What is the coefficient of oxygen in the following equation? Look at the equation, what's the coefficient of oxygen? Well done if you said one.

And another question, only the subscripts, the little numbers, are changed to balance an equation.

Is that true or is that false? Well done if you said false, and can you justify your answer? So here's two statements.

Which of those statements is the reason why? Well done if you said A, only the coefficients, the big numbers, can be changed when balancing an equation.

When balancing a symbol equation, the aim is to ensure the same number of atoms of each element is found on both sides of the reaction arrow.

So here we've got zinc and sulphur, making zinc sulphide.

So if we look at the left hand side of the equation first of all, we write out what atoms we've got, and then we work out how many of each atom we've got.

So on this side we've got one zinc atom there, and one sulphur atom.

And on this side we've got one zinc, and one sulphur.

So this is what we call a balanced equation.

It is balanced, they're the same, the number of atoms on each side of the reactionary is the same.

So laying out the equation as we've shown below, helps to keep track of the atoms involved, 'cause it can get quite complicated at times.

So here's our magnesium and oxygen equation.

We put a line there underneath the arrow to show the sides of the equation, and then we draw out our substance diagram.

So we can see we've got, and then we work out the numbers.

So we've got one magnesium atom there, and whilst we've got one oxygen molecule, we've actually got two atoms of oxygen.

And on the other side of the equation we've got one atom of magnesium, and one atom of oxygen.

So this is really unbalanced.

We need to do something about this.

So what we need to do is add the complete substance diagram to the appropriate side of the equation, and recalculate the number of atoms of the elements.

So if we added another molecule of magnesium oxide, what would happen? So now we've got two magnesium atoms, and we've got two oxygen atoms. That's still unbalanced.

So we still need to do some more work.

We continue adding appropriate substance diagrams until each element has the same number of atoms on each side of the equation.

And we need to remember to recalculate at the bottom there.

So if we added another magnesium atom there, that would give us two magnesium atoms. And if we look now, the reaction is now balanced.

And so we tally the numbers up, and that's the coefficient for each substance.

So we've got two magnesium atoms, one oxygen molecule, and two molecules of magnesium oxide.

Now, we don't usually write the number one, if there's nothing there, we know it's one.

So that is the chemical symbol equation for the reaction between magnesium and oxygen, and it is balanced.

So let's have a quick check.

How would you write calcium carbonate when counting atoms to balance an equation? So how would you do this? Well done if you said B.

Because if you look at the formula, there is one calcium atom, there is one carbon atom, and three oxygen atoms. I'm going to balance a simple equation for you now, and then I'm going to give you one to have a go at.

So oxygen plus hydrogen gives water.

Put my line down, and then do my substance diagram, and then do the numbers.

So I'm gonna count now on the left hand side, the reactant side, I have got two atoms of oxygen, and two atoms of hydrogen.

And on the product side I have got one oxygen atom, and two hydrogen atoms. So it's not balanced.

So I need to do something.

So if I add another water molecule there, what happens? Change this, I've now got two oxygens in total, and I've got four hydrogen atoms in total.

So I'm still not balanced.

What could I do on the reactant side to help? I could add another hydrogen molecule, so that will give us four hydrogen atoms. And there we are balanced.

So now I tally up, I've got one oxygen molecule, two atoms of oxygen, and two hydrogen molecules, so I've got four hydrogen atoms, will react together to form two molecules of water.

So you have a go at this reaction, it's potassium and chlorine, making potassium chloride.

Pause the video and come back when you're ready.

How did you do? Let's have a look then.

So put the line down, do the substance diagram, and then count how many atoms you've got on each side of the equation.

So it's one potassium, two chlorines, one potassium, one chlorine.

So we need to do something.

So add another atom, add another potassium chloride, change the numbers at the bottom, so now we've got two.

Still not balanced.

So what do we need to do? We need to add another potassium atom in the reactant side.

Do that, and there we've got the balanced symbol equation.

Well done if you've got that correct, because it can be quite tricky, but setting it out like this really helps.

So I think you're ready for task B now.

So I want you to match the formula to the descriptions for the substances below.

You might need to use a periodic table to look up the symbols for the elements here.

So we've got the description, which formula matches? Stop the video and come back when you're ready.

So let's have a look at the answers.

You might need to pause the video, to check yours.

Let's get ready for question two.

We're going to be balancing equations now, and I want you to show your working out like we've been doing before.

So here's a set of equations to have a go at.

So pause the video, and come back when you're ready.

Right, let's have a look.

So the first one, calcium and oxygen, making calcium oxide.

We put our line down, we do our substance diagram, and we've got one calcium and two oxygens, one calcium, one oxygen.

So we need to do something.

So let's add another calcium oxide, change the numbers at the bottom there, and then we can change the calcium on the reactant side, change the numbers, and there we've got our balanced equation.

Well done if you've got that correct.

Sodium and iodine, making sodium iodide.

Symbol diagram, numbers, need to do something.

Change the numbers, change the sodium, balanced equation.

Well done.

If I'm going to fast for you, pause the video and check yours.

Barium and oxygen, making barium oxide.

Sodium and oxygen, making sodium oxide.

So look at sodium oxide, that is two atoms of sodium, and one of oxygen.

To balance it we need another sodium oxide.

Now we've got four sodium atoms, so we need to add four on that side.

Another three on that side, sorry, to make four, and then we've got our balanced equation.

So here we've got one reactant decomposing to form two products.

So same thing as we did before.

Okay.

So same as before.

We've got one reactant decomposing to form two products.

You can see how setting this out helps us.

Change the numbers, still not balanced, we need to sort it out.

So we add another one of those, and then we're balanced.

That was a complicated one.

And last one, calcium carbonate decomposing to form calcium oxide and carbon dioxide.

And that was almost a tricky one, because it was already balanced.

So well done.

We've come to the end of our lesson.

Let's have a quick look at the summary.

So the mass of the products in a chemical reaction is the same as the mass of the reactants.

Word equations are simple way of describing chemical reactions.

Chemical formulae represent the numbers of each type of atom in a reactant or a product.

And chemical equations represent how the atoms and the reactants rearrange to form the products.

And the arrow in a chemical equation represents the words, changes into or forms. So brilliant.

I hope you feel a lot more confident about what conservation of mass means, and how to write a balanced symbol equation.

Keep practising , and keep setting out the equation in the way that we've shown you.