Welcome to today's video, IGCSE chemistry
paper 6, specimen paper 2020.
Question 1.
The diagram shows the apparatus used to prepare
a dry sample of a gas.
The gas is more dense than air.
Part (a).
Complete the boxes to name the apparatus.
The left one here is the separating funnel,
it is used to put only a specific type of
substance, let's say when you only want to
put water from oil & water mixture.
This long thin passageway is the delivery
tube, it is used to deliver gases produced
from the reaction inside the flask.
And the delivery tube is connected to the
gas jar, something that is used to collect
gas.
Part (b).
Identify one mistake in the apparatus.
Going back to the main question, it says the
gas is more dense than air.
This means that the gas collected will sort
of sink when released in air.
So the gas jar should be upside down, so that
the gas can be collected downwards.
Part (c).
Suggest a reason why the gas is passed through
concentrated sulfuric acid.
One of the uses of concentrated sulfuric acid
is as a dehydrating agent.
The gas is passed through this so that water
or other impurities can be removed and only
the wanted gas is collected.
Question 2.
A student investigated the rate of reaction
between excess magnesium and two different
dilute acids, X and Y.
Two experiments were carried out.
Experiment 1.
The apparatus was set up as shown in the diagram.
We have conical flask here where reaction
occurs and gas is collected here in the measuring
cylinder.
Using a measuring cylinder, 50cm^3 of dilute
acid X was poured into the conical flask.
0.5g of magnesium ribbon was added to the
conical flask and the bung added.
The timer was started and the volume of gas
collected in the measuring cylinder was measured
every 30 seconds for three minutes.
Part (a).
Use the measuring cylinder diagrams to record
the volumes of gas collected.
For the first column, it's at 0 so it's just
0.
Next, it's at 13.
Then 22.
This question is simple just read the readings
from the cylinder.
At 90 seconds, it's at 30.
Then 36.
Then 43.
Then 49.
Experiment 1 was repeated using 50cm^3 of
dilute acid Y.
Part (b).
Use the measuring cylinder diagrams to record
the volumes of gas collected.
Same thing just look at the readings.
0, 5, 10, 13, 17, 20 and 23.
Part (c).
Plot the results for both experiments on the
grid below.
For each set of results, draw a smooth line
graph.
Indicate clearly which line represents Experiment
1 and which line represents Experiment 2.
The first thing we should do is to label and
scale x and y axes.
x-axis should be the time in seconds as this
is the independent variable.
y-axis should be the total volume of gas produced
in cm^3 since this is the dependent variable.
Time is from 0 to 180 seconds and we have
6 boxes here so 1 box stands for 30 seconds.
For the volume, it should be at least from
0 to 49 and we have 6 boxes here so let's
make one box to stand for 10cm^3, having a
range of 0 to 60cm^3.
Now it's time to plot them, make sure you
don't make any mistakes here.
Okay this is how it looks like and after we
connect the dots, it's like this.
Finally, we need to show which graph stands
for which experiment.
This is the complete answer.
Part (d).
State which experiment had the faster rate
of reaction and suggest why the rate was faster
in this experiment.
We can see from the graph that experiment
1 had the faster rate of reaction and this
can be due to acid X being a stronger acid,
or a more concentrated acid.
Part (e).
From your graph, deduce the time required
to collect 25cm^3 of gas in Experiment 1.
Show clearly on the graph how you worked out
your answer.
We first need to see where the graph is at
25cm^3.
It shows that it's 72 seconds, so write down
72 seconds and leave evidence on the graph.
Part (f).
The rate of this reaction can be calculated
using this formula, volume divided by time
taken.
For the experiment with the higher rate, calculate
the rate of reaction for the first 30 seconds
of the reaction.
Deduce the units.
The question said experiment with the higher
rate so it's about Experiment 1.
Going back to our table, for the first 30
seconds, 13 cm^3 of gas was produced.
So it's 13 divided by 30 which is 0.43.
The unit should be cm^3 divided by s, written
in this way.
Part (g).
Give one advantage and one disadvantage of
using a measuring cyliner to add the acids
to the flask.
The advantage is that flasks are easy to use
and the procedure is quick.
The disadvantage is that the measurement will
be much more inaccurate when compared to burette
or pipette.
Part (h).
Suggest and explain one improvement to this
experiment.
There are lots of possible answers to this.
Well like I just mentioned, we can use a burette
to increase the accuracy of the measurement
taken.
We can repeat the experiments and take average
or just take more frequent readings to draw
a more accurate graph.
Also, instead of using magnesium strips, we
can calculate the mass of magnesium and put
them in since the strips may differ in mass
from one another.
Question 3.
Concentrated aqueous sodium chloride was broken
down by electricity using the apparatus shown.
This is another example of electrolysis.
Part (a).
Suggest a suitable method from which to make
the electrodes.
Electrodes are often made from platinum or
graphite or carbon so you can write one of
them.
Part (b).
Gas A is chlorine.
Give a test for chlorine.
Test for gases should be memorised.
So since chlorine acts as a bleach, it will
bleach the damp blue litmus paper.
It won't turn it in red, but in white.
Part (c).
Gas B pops when tested with a lighted splint.
What is gas B?
Which gas pops with a lighted splint?
It's hydrogen.
Question 4.
Solid E was analysed.
E was an aluminium salt.
Some of the observations are shown below.
The appearance, it is a white crystalline
solid and when it was heated, there were colourless
drops of liquid at the top of the tube.
Part (a).
A little of solid E was dissolved in distilled
water.
The solution was divided into four test-tubes
and the following tests were carried out.
Complete the observations for tests 2 and
3.
Test 2.
Drops of aqueous sodium hydroxide were added
to the first test-tube.
So the question said that it's an aluminium
salt.
When you add just few drops of aqueous sodium
hydroxide to it, white precipitate will be
formed.
You need to memorise this test and the result.
Then next, when you add excess sodium hyroxide,
the white precipitate will be dissolved.
Test 3.
Drops of aqueous ammonia solution were added
to the second test-tube.
Excess ammonia solution was then added.
When you first add drops of ammonia solution
to aluminium, again, white precipitate will
be formed.
But this time, even when excess ammonia solution
is added, the precipitate will not dissolve
and there will be no change.
Okay then two further tests were carried out.
In test 4, dilute hydrochloric acid was added,
followed by barium nitrate solution.
There was no reaction.
In test 5, aqueous sodium hydroxide and aluminium
foil were added and was warmed.
There was effervescence, pungent gas given
off and gas turned damp litmus paper blue.
Part (b).
What does test 1 tell you about solid E?
So back to test 1 where we had colourless
drops liquid formed at the top of the tube.
This is simple.
It's because it contained water and it's actually
water droplets formed.
Part (c).
Identify the gas given off in test 5.
Test 5 is where we had a pungent gas.
If they mention the word pungent, there's
only one answer for it.
It's ammonia.
Be careful, it's not ammonium but just ammonia.
Part (d).
What conclusions can you draw abut solid E?
First of all you can say that it is hydrated
salt, just like I mentioned in Part (b).
Then, from test 5, when ammonia gas is produced
from adding sodium hydroxide and aluminium
foil, we can conclude that it contains nitrate.
Therefore, that can be our answer.
You have one more option to write.
From test 4, they added hydrochloric acid
and barium nitrate solution.
This can mean that it does not contain sulfate
because if it did, then white precipitate
will be formed but here, there was no reaction.
Part (e).
Test 5 states that the mixture should be warmed
carefully.
In terms of safety, explain why it is neccessary
to warm carefully.
Well, we are adding sodium hydroxide and they
are known for being hazardous so that's one
reason.
Also, this reaction is highly exothermic and
it can cause the mixture to spit its content.
Question 5.
E-numbers identify chemicals which are added
to foods.
Part (a).
E210 is benzoic acid.
How could you show that a solution of benzoic
acid is a weak acid?
Benzoic acid sounds like some complex molecule
but just focus on it being a weak acid.
You usually test it using a Universal indicator
or pH paper.
So you will either get pH ranging from 4 to
6 or just orange from the pH paper.
Part (b).
E110 is Sunset Yellow.
Outline a method you could use to show the
presence of E110 in a food colouring.
You may draw a diagram to help answer the
question.
Alright the names sound complicated but in
this question you just need to find how to
identify a pure substance from a mixture.
Well the only way to do this is to use chromatography
so just basically describe its process.
First, we gotta get a chromatography paper
and put drops of food colouring on it at the
base line drawn with pencil.
Then place it in a container with solvent
which will dissolve the food colouring.
Next, wait for the substances to travel across
the paper and you will see different spots
produced from the initial drop.
Finally, identify the substance by calculating
Rf value or by comparing the distance it travelled
with the known sample.
You may or may not draw a diagram but I would
recommend you to do so as you might get some
points that you missed in your description.
Okay that's the end of this video.
Thank you so much for watching, I hope this
helped with your revision.
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