GCSE OCR Gateway Chemistry C2.1d–f Elements, Mixtures and Compounds

New OCR Gateway specification from September 2016 Higher tier: grades 9 to 4: 
In this and subsequent posts I’m simply going to explain and illustrate each learning objective as they come up in the topics in the new GCSE specification. 

I’m giving you my notes from each lesson.

You can really get ahead of your class if you follow this blog and all the posts that will appear here about the new GCSEs over the coming months. 

This rejigging of the specification is just that: there is nothing really new here it has all been with us for the past half century at least. 

That written in italics is for the higher tier paper only.

C2 Elements, Mixtures and Compounds

C2.1d deduce the empirical formula of a compound from the relative numbers of atoms present or from a model or diagram and vice versa

So what is an empirical formula?

Empirical formula is the simplest formula of a compound.

H₂O and H₂SO₄ and CuSO₄ are the simplest formulas.

But H₂O₂ and C₂H₄ and C₆H₆ are not the simplest formulas.

For H₂O₂ the simplest formula is HO and for C₂H₄ and C₆H₆ their simplest formulas are CH₂ and CH.

So the empirical formula is the formula with the simplest ratio of elements.

Now we’ve just worked out the empirical formula from the compound’s actual formula, the relative number of atoms present.

Let’s now work out an empirical formula from a model like this:

If you count the number of sodium’s in blue on the face of this model and the number of chlorides in green you’ll see they are the same eight of each particle.

So the sodium to chloride ratio is

Sodium   :    Chloride

   8            :          8

but the simplest ratio is

   1            :          1

and so the simplest formula is NaCl

There are experiments you can carry out to determine an empirical formula and you can find one here on You Tube

Also you can get the script for such an experiment here on BBC Bitesize Chemistry

C2.1e explain that many useful materials are formulations of mixtures e.g. alloys

Here is a list of some typical alloys and their composition and uses that I have collected together point being they show how the alloy is a formulation of different metals to give the property needed that fits the use.

Name of Alloy	Composition	Uses
Bronze	90% copper, 10% tin	coinage, bells,
Brass	67% copper, 33% zinc	household items
Solder	67% lead, 33% tin	circuits boards
Nitinol	55% nickel, 45% titanium	memory metal uses, spectacle frames
Nichrome	60% nickel, 16% chrome, 24% iron	high performance electrical wiring,
Sterling Silver	92.5% silver, 7.5% copper	jewellery, coins
Dental Amalgam	70% silver, 18% tin, 10% copper, 2% mercury	dental work, fillings
Magnelium	5% magnesium, 95% aluminium	aircraft airframes
Duralumin	95% aluminium, 4% copper, 0.5% magnesium, 0.5% manganese	aircraft parts

The same idea is used to produce some typical plastics blending two or more polymers together e.g. to make a yarn like polyester cotton. 

C2.1f  describe, explain and exemplify the processes of filtration, crystallisation, simple distillation, and fractional distillation

Here I’ve just given you my notes on each separation technique with a summary table to help you remember how each one is used.

1. Filtration

Filtration separates a mixture of insoluble solid particles in a liquid like muddy water. 

It works because the filter paper holes are too small to let the insoluble solid particles through but small enough to let the liquid molecules through.

Diagram of filtration

Diagram of filter paper

You can see how the paper works in the diagram above

The insoluble solid is the residue and the purified liquid from filtration is the filtrate. 

You would use filtration in a salt preparation e.g. the preparation of copper sulphate from copper oxide and sulphuric acid.

Add the copper oxide to the acid until no more dissolves on heating which usually leaves a residue of insoluble copper oxide.

Filtering the mixture removes the excess copper oxide.

You can watch the preparation of copper sulphate here on Youtube

2. Crystallisation

Crystallisation is carried when an impure solution is heated very gently so that enough water evaporates to leave a hot saturated solution.

A simple test to see if the solution is saturated is to put a cold glass rod in the hot solution and see if the solution crystallises on the end of the rod.

Leaving the hot saturated solution to cool results in pure crystals of the solute because solubility decreases with temperature as you can see in the graph below:

Any trace of impurity is left dissolved in the solution.

In this way, pure crystals are separated from any impurities the solution contained.

Simple Distillation

Simple distillation is often used to remove a solvent from a solution in order to leave the solute behind.

The diagram below shows the simple distillation of salt water.

Heating the salt water it boils and water vapour passes down the condenser where it turns back, condenses into pure water. 

It’s important that the thermometer bulb is placed outside the entrance to the condenser so that it measures the temperature of the vapour given off.

Simple distillation can also be used to separate a volatile liquid from a less volatile liquid provided the boiling points are about 25 or 30^oC apart. 

A good example is the distillation of an alcohol water mixture. 

Alcohol has a boiling point of 78^oC and water 100^oC. 

Simple distillation of an alcohol water mixture removes about 96% of the alcohol but the separation is not perfect.

Fractional Distillation

The apparatus below is often described as fractional distillation. 

Actually it is only a more efficient method of separating two liquids where one is more volatile than the other.

The tall column with a high internal surface area helps to separate the liquids from each other.

It becomes fractional distillation when the mixture separated contains more than two parts or fractions and these fractions have widely separated boiling points say 30^oC apart.

A good example is the fractional distillation of crude oil because crude oil contains several hydrocarbons with widely different boiling points.

However the apparatus used to separate crude is to use a fractionating column with trays of bubble caps as in the diagram below: