#c2.3

electrical conductivity - metals can conduct electricity - non-metals cannot conduct electricity

It is possible to calculate:

The percentage of an element that is in a compound given the relative mass of the element in the formula and the relative formula mass of the compound

The empirical formula of a compound given the masses or percentages of the elements in the compound

The masses of reactants and products in a reaction given the balanced symbol equation of the reaction.

No atoms are gained or lost during a chemical reaction. However, it is not always possible to obtain the calculated amount of a product in a reaction because:

If the reaction is reversible, it might not go to completion

Some of the product might be lost when it is separated from the reaction mixture

Some of the reactants might react in different ways from what was predicted.

The amount of a product obtained from a chemical reaction is called the yield. When the actual yield is compared with the theoretical yield (as calculated using the masses or formulae of the reactants), it is called the percentage yield.

In some reactions, the products can react to form the original reactants. These are called reversible reactions. They are represented:

A + B ⇌ C + D

It is possible to detect and identify elements and compounds using instrumental methods, which are accurate, sensitive and rapid, and are particularly useful when there is only a small amount of a sample available.

Chemical analysis can be used to identify additives and paper chromatography can be used to detect and identify artificial colours in foods.

An example of an instrumental method used to identify elements and compounds is gas chromatography used in conjunction with mass spectroscopy (GC-MS).

In gas chromatography, different travel, carried by a gas, at different speeds through a column packed with a solid material, so that they become separated.

By measuring the time a substance takes to travel through the column, it is possible to identify the substance. The number of peaks on the output of the gas chromatograph shows the number of compounds present.

By linking the output from the gas chromatography column to a mass spectrometer, it can be used to identify substances leaving the end of the column. The position of the peaks on the output indicates the retention time of the substances.

The mass spectrometer can identify substances very quickly and accurately, detect very small quantities of substances and provide the relative molecular mass of each of the substances separated in the column.

Atoms' relative masses can be used to calculate how much of a reactant to use and how much we can make of a product when we are carrying out a reaction. This is because no atoms are gained or lost in chemical reactions; they can move about and change, but they are not created or destroyed. (The same is true of energy.)

There are various ways to analyse substances.

You should be able to use your skills, knowledge and understanding to:

Evaluate sustainable development issues relating to industrial processes, including materials, product yield and energy requirements of the reactions.

Practical work to develop your skills and understanding:

Investigating food colours using paper chromatography

Calculating the empirical formulae of copper oxide and magnesium oxide

Calculating yields, e.g. magnesium burning to produce magnesium oxide; wire wool burning to produce iron oxide

Building on the ideas of instrumental precision, e.g. for collecting gases; using boiling tubes, gas jars or syringes

Investigating the hydration/dehydration of copper sulphate

Heating ammonium chloride

Adding acid and alkali to bromine water or potassium chromate solution

Carrying out the "blue bottle" reaction