Lesson video

- Hi, guys.

My name's Mrs. Farrow.

For today session you're going to need a pen and a piece of paper.

And it's a good idea to put your devices on silent or switch them off to try and help avoid any distractions.

If you'd like to get those things sorted by pausing the video now and press play when you're ready to resume.

Today we're going to be looking at Group 1, this is one of my favourite groups in the periodic table.

We're gonna describe the physical and chemical properties of the Group 1 metals and the trends that we see in their reactions with both oxygen and water.

So we're in for some pretty exciting stuff today.

Some of the keywords that you're gonna come across during today's session that you can keep an eye out for include, combustion, is the process of burning.

We've got oxidised, or oxidation.

This is when an element combines with oxygen.

And density, a measure of the amount of matter or mass in a particular volume.

Okay.

Today's session has been broken down into three distinct parts.

We have cycle one, "What is Group 1?" Number two, "Physical properties and trends".

And then lastly, the "Chemical properties and trends".

So let's jump in and take a look at "What is Group 1?" Well Group 1 metals react with water to form alkali compounds.

So Group 1 is what we refer to as being the alkali metals.

These are found in the first group or column of the periodic table as shown here in the image and they consist of lithium, sodium, potassium, rubidium, caesium, and francium.

They are all quite exciting, and I'm very excited to talk to you about them today.

What kind of properties are we looking at? Well, properties can be either physical or chemical characteristics of a substance.

Physical properties, we're talking about physical properties, we talk about their density, the amount of material per volume.

This usually is an indication of whether it's going to float or sink in water.

The appearance, so what colour does it have? Is it shiny or is it dull? And the melting and boiling point, what temperature do you have to get it to in order to change its physical state? And then some of the chemical properties, we can look at the reactivity with other elements or water.

We can talk about the acidity, be that whether it has acid or alkaline properties, and it's toxicity.

So is it harmful to living substances? Let's do a quick knowledge check then.

True or false? The alkali metals are found in the first row of the periodic table.

Okay.

So the answer to this one is in fact false.

The alkali metals are found in Group 1, which is the first column, not row.

If you're not sure about columns and rows you need to have a look at our periodic table video.

Okay, next one.

Which of the following is not an alkali metal? We've got A, magnesium.

B, lithium.

C, sodium and D, potassium.

The correct answer is A, magnesium.

It's actually an alkali earth metal found in Group 2 of the periodic table, not Group 1.

So now it's over to you for a practise task.

There are two parts to this task.

The first part is asked, why are the elements in Group 1 known as the alkali metals? And in the second part we would like you to colour the part of the periodic table which shows the location of the alkali metals.

You'll need a little bit of time to do this so if you'd like to pause the video and press play when you're ready to hear the correct answers.

Let's take a look at how you got on.

This first question doesn't need to be word for word the same as mine, but it should follow the general theme that the Group 1 metals are called alkali metals because they react with water to form alkali compounds.

Well done if you have something along those lines.

Part B asks you to colour the correct column to show the location of the alkali metals in the periodic table.

And you should have coloured this section here, indicated on the diagram, which includes lithium, Li.

Sodium, Na.

Potassium, K.

Rubidium, Rb.

Caesium Cs and Francium Fr.

Good job if you managed to get those all correct.

Well done.

Tasks will get trickier as we go through.

That's what Group 1 are and where they're found.

Now let's start looking at some of their physical properties and the trends that we see.

The Group 1 metals have some typical properties of metals including them being good conductors of heat.

They are shiny and they are good conductors of electricity.

Now these are all properties we would expect them to have as metals, and are common to other metals that we see in the periodic table.

However, they do also have some quite unique physical properties which make them rather different to what we would normally expect from a metal.

Firstly, they're soft and can be easily cut with a knife.

They tend to have much lower densities than other metals and some will even float on water.

And they have low melting and boiling points and so don't have to be got particularly warm in order to change from solid to liquid or liquid to gas.

If we take a look at the melting and boiling points of the alkali metals, you can see that they decrease as you go down the group.

Lithium has a melting point of 181, so in order to get it to change from a solid to a liquid we need to heat it to 181 degrees.

We can see this number decreasing steadily as we go down the group through sodium, potassium and rubidium, all the way to caesium at 28 degrees C.

So in theory, cesium would melt in the palm of your hand, that's if it didn't explode first of course.

We see a similar trend in boiling point, starting off much higher and gradually decreasing as you go down the group.

Now let's take a look at density.

The alkali metals generally become more dense as you go down the group.

Although unfortunately as you can see from the table the pattern isn't quite perfect as potassium is slightly less dense than sodium.

Water has a density of one gramme per decimeter cubed.

And those elements which have a density less than one are going to float.

And those alkali metals with a density greater than one will sink.

Looking at the data in the table, can you predict which ones would float and which ones would sink? Let's take a look.

Lithium, sodium and potassium all have a density lower than that of water and so will float.

Whereas rubidium and caesium have a density of higher than one gramme per decimeter cubed, and so will sink when put into water.

Let's do a quick knowledge check.

Elements in Group 1, are all metals with high density, true or false? The answer to this question is false.

Group 1 metals have low densities generally, and some of them even float on water.

Next question.

What happens to the melting points and boiling points of the Group 1 elements as you go down the group? Do they A, both increase.

B, they both decrease or C, the boiling points increase but the melting points decrease.

The correct answer is they both decrease.

It becomes easier to change the Group 1 metals from solids to liquids and liquids to gases, as you go down the group.

Now it's time for you to have a practise task on your own.

We would like you to complete the sentence by using one of the words below and then answer the questions which follow.

You're going to need to give this task your full attention and spend a little bit of time doing it.

So we suggest you pause the video now and press play when you are ready to see the correct answers.

Right, let's see how you got on.

So task A says "Complete the sentence using the words below.

The boiling points of the alkali metals are something than the boiling points of other metals." And you had a choice of higher, lower, or the same as.

Let's take a look.

Boiling points of the alkali metals are lower than the boiling points of other metals, is that correct sentence.

Task B, "How do the melting points of the alkali metals compare to other metals?" Well here you could have used the sentence above to help you with the structure.

Only this time instead of boiling points, we're looking at melting.

So, the alkali metals have much lower melting points than other metals.

And question C asked you to "Describe the trend in melting points as you move down the Group 1 metals." As you go down the Group 1 metals, the melting point decreases.

Well done if you've got all of those right.

If not, don't worry, you can always go back and re-watch this section of the lesson again.

So we've had a look at some of the physical properties and trends, now it's onto the really exciting part.

The chemical reactions of the Group 1 metals and the trends that we see as we go down the group.

Let's take a look at the reactions with oxygen.

The reaction between an alkali metal and oxygen is called oxidation, because oxygen is reacting with the alkali metal.

A general word equation would look something like this.

An alkaline metal plus oxygen gives an alkali metal oxide.

All of the reactions of the alkali metals with oxygen follow this same pattern.

Let's give you an example of a real reaction.

Lithium, an alkali metal, reacts with oxygen to form lithium oxide.

It's important to realise that we can also represent this reaction as a symbol equation, although at this point you're not expected to be able to use them.

It's worthwhile getting familiar with some of the symbols that are commonly used.

So for this reaction it would be Li plus O 2 gives Li 2 O.

Let's have a little practise at this, shall we? Here's our general word equation for reference.

Alkali metal plus oxygen are our reactants and so found on the left hand side of the arrow.

The alkali metal oxide is our product and so found on the right.

Let's take a look at another example together.

In this example we're going to look at sodium reacting with oxygen.

When sodium reacts with oxygen the alkali metal oxide formed is sodium oxide, and this is our product.

It's your turn to have a go.

I'd like you to write the word equations to the reactions of potassium, rubidium, and caesium with oxygen.

This is going to take you a little time and you'll need to think carefully about it.

So pause the video and press play when you're ready to check your answers.

Okay, time to see how you got on.

Let's have a look.

So potassium and oxygen react to form potassium oxide, rubidium and oxygen react to form rubidium oxide, and caesium and oxygen react to form caesium oxide.

Now let's have a look at the trends that we see when each of these metals react with oxygen.

When the alkali metals are cut, they do appear shiny at first, but they quickly become dull and white.

This is due to the reactions with oxygen from the air.

We actually keep the alkali metals under oil in order to prevent this from happening, and to stop them from getting wet.

Which you'll understand why we do later on in today's lesson.

This changing of colour due to the reaction with oxygen is known as tarnishing.

Lithium tarnishes quite slowly, sodium more quickly and potassium the fastest.

And so we can see the reaction getting much faster as we go down the group.

When we heat the alkali metals in oxygen, they will actually combust or burn and a chemical reaction begins to take place.

Lithium burns the least vigorously with a pretty red flame.

Sodium burns slightly more vigorously giving an orangey flame.

And potassium, my favourite, burns most vigorously with a lilac flame.

Now let's take a look at the reactions of the alkali metals with water.

We know that they react with water to form alkali compounds because we said that this is where their name comes from.

The general word equation is that an alkali metal will react with water to form an alkali metal hydroxide plus hydrogen.

If we look at an example, we can take lithium, for instance, lithium will react with water to form lithium hydroxide plus hydrogen gas.

We know it's hydrogen gas 'cause if collected it makes a squeaky pop sound.

Again, we can show these reactions as a symbol equation.

You're not expected to be able to write these yet yourself but we'll take a look anyway.

Lithium, Li, plus water, two lots of H2O, react to give two lots of lithium hydroxide 2LiOH plus H2, which is hydrogen gas.

Let's have a little more practise at these equations and see how you can get on.

Here's our general equation.

The alkali metal reacts with water to form an alkali metal hydroxide and hydrogen gas.

We have the reactants again on the left hand side and the products on the right.

Let's take an example of sodium again.

Sodium reacts with water to form sodium hydroxide and hydrogen.

What I'd like you to do, is have a go at writing the word equations for the reactions of potassium, rubidium, and caesium again, but this time with water.

You'll need to pause the video now and to come press play when you're ready to check your answers.

Let's see how you got on with these equations then.

The first one is potassium plus water reacts to form potassium hydroxide and hydrogen.

The three that we've looked at, so far, lithium, sodium and potassium are possibly examples that you may have seen in school.

Next was caesium, it's rubidium, sorry.

Rubidium reacts with water to form rubidium hydroxide and hydrogen.

And lastly, we have cesium and water, which react to form caesium hydroxide and hydrogen.

Well done if you got all of those right.

Hopefully starting to begin spot the pattern and how to answer these, complete these equations.

Let's see what trend we see then as these Group 1 metals react with water.

Lithium is the least reactive at the top of Group 1.

When it's added to water it fizzes and moves slowly around the water's surface giving off bubbles of hydrogen gas.

It's quite exciting, but by far not the best one.

Then we have sodium.

Sodium is much more reactive.

When added to water it melts and it whizzes quickly across the surface of the water.

If you're lucky, the hydrogen produced will catch fire and it produces an orangey coloured flame.

And then lastly, by far my favourite and the most dramatic that we're allowed to do in school, is potassium.

This is the most reactive and it will melt and move across the surface of the water incredibly quickly, whizzing around producing hydrogen, which often ignites and gives a lilac flame.

Unfortunately, due health and safety, we're not allowed to show caesium or rubidium in school.

It would be far too dangerous.

But there are lots of clips that you can see online.

Let's do a quick knowledge check to see how much you can recall.

The first three elements of Group 1 are lithium, sodium and potassium, but which one is the most reactive? The correct answer is potassium.

It's the lowest down the group and therefore the most reactive.

Next question.

Which gas is produced when the Group 1 elements react with water? This one will really test to see how well you've been listening.

Is it A, carbon dioxide.

B, hydrogen, or C, oxygen? The correct answer is B, hydrogen.

Well done to all of you who got that.

Now it's time for you to have a little practise task.

In this first part of the practise task we'd like you to complete the sentences by filling in the gaps.

When you think you've completed it to the very best of your ability, press play and we'll go through the answers together.

Ready to take a look and see how you got on? So, sentence one.

The Group 1 metals are called the alkali metals.

They have low densities and melting points.

Lithium is the least reactive in the group.

They react with water to give off hydrogen and make an alkali solution.

Good job on getting those right.

Now for the second part of your task.

I'd like you to answer the two questions.

The first one asking you to describe the trend in reactivity of the alkali metals as you go down the group.

And the next part, asking you to complete the word equations for the reaction of sodium with oxygen and potassium with water.

Again, pause the video now and take your time over these and press play when you're ready to hear the answer.

Okay, let's see how you got on.

First part then, "Describe the trend in reactivity of the alkali metals as you go down the group." As you go down the Group 1 metals from lithium towards caesium, they become more reactive with both water and oxygen.

Well done if you have something like that.

Doesn't have to be word for word but you will need to make sure it has those general points.

And then task C, asks you to complete the word equations.

We've helped you a bit here by putting in spaces for you to write the answers.

So sodium reacts with oxygen to form sodium oxide.

Okay.

Notice the changing is the ending of oxygen in order to show it's now part of a compound.

Then we have potassium reacts with water to form potassium hydroxide and that gas that's been given off, hydrogen.

Well done if you manage to get those correct.

If you need to go back and watch any parts of the lesson again, you can do at any point.

So let's take a look at what we've learned today.

The Group 1 metals are also known as the alkali metals.

The alkali metals have some unique properties.

They have unusually low melting and boiling points.

They have very low densities and are soft.

They are very reactive with oxygen and water and the metals become more reactive as you go down the group.

I hope you've enjoyed our lesson today as much as I have.

We'd like to thank you for using Oak National Academy and we hope to see you again soon.

Bye for now.