GCSE OCR Gateway Chemistry C2.1a-c 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

C2.1 Purity and separating mixtures

C2.1a explain what is meant by the purity of a substance, distinguishing between the scientific and everyday use of the term ‘pure’.

What you have to be able to do here is to explain that in science a pure substance contains particles of the same type.

So an element contains only one type of atom. 

Iron contains only iron atoms—I know it sounds so simple but that's what we mean by a pure element.

A pure compound, that's a tad more difficult to explain, bit we can that a pure compound contains more than one type of atom but these particles are in a simple ratio.

So for example in pure water, the molecule contains two hydrogen atoms bound to an oxygen atom.  In the pure compound there are two hydrogen atoms for every oxygen atom and hence the formula is H₂O.

Or take pure anhydrous copper sulphate: CuSO₄ .  Here we’ve got a copper atom and a sulphur atom for every four oxygen atoms.

In everyday use pure just refers to pure stuff like butter its pure because all the packet contains is butter. 

Or you can think of orange juice being pure juice but it contains a mixture of things like water sugar etc. as you can see from the Tropicana label below:

C2.1b use melting point data to distinguish pure from impure substances

Now one of the most effective ways to show an element or compound is pure is to measure its melting point.

The effective of an impurity in a pure element or compound is to lower its melting point. 

This is why rock salt is added to icy roads in winter.  The salt mixes with the ice and lowers its melting point to below that of the air temperature and below freezing (0^oC) so the ice seems to “melt” and go slushy.

Ice treated with rock salt doesn’t usually freeze until it’s around –17^oC.

Measurement of melting point.

You can find out about measuring a melting/freezing point here.

And you can watch the same experiment here on YouTube.

C2.1c calculate relative formula masses of species separately and in a balanced chemical equation

Before you can calculate the Relative Formula Mass you need to know some definitions: the definition of relative atomic mass, relative molecular mass and relative formula mass.

These three definitions hang on one key idea: atoms are just too small and light to weigh absolutely. 

Like I said in the previous post atoms are mainly empty space!!

Their mass would be so small that the term would be unmanageable.  Say 1.7 * 10^–27g and that number is incredibly light and awkward to use.

So to get round this problem instead we just compare the mass of an atom with a standard mass of a standard atom.

The standard atom is the isotope of Carbon–12 ¹²C.

The mass of this atom is defined as 12.000000 atomic mass units.

Any other atom is compared to 1/12^th of the mass of an atom of carbon–12.  This is the atom’s relative atomic mass or RAM or A_r.

Thing is, atoms of the same element have different numbers of neutrons so the relative atomic mass of an element is the weighted average mass of its atom’s isotopes. 

The picture below shows how you can work out the RAM of an element like chlorine:

If you have got how to define relative atomic mass then relative molecular mass is simply the atomic masses of all the elements in the molecule added together like this:

H2O   H     1 *2  =     2

                                      +

            O    16 *1  =  16

RMM                 =      18

If the compound is made of ions and not molecules then it has a relative formula mass like most oxides and salts.

The relative formula mass of sodium chloride NaCl is

Na  –  23    +    Cl  –  35.5    =    58.5

You should try as many examples as you can to build up your confidence especially the really long and difficult ones like this formula for iron ammonium sulphate:

(NH₄)₂Fe(SO₄)₂.6H₂O