Chemistry GCSE Revision: Chemical Analysis

Chemical analysis is an integral and crucial part of chemistry. Great discoveries and advancements wouldn’t have been possible without analytical chemistry, which determines the chemical composition and molecular structure of all substances. Continue reading to revise topics in chemical analysis, including the various analytical techniques it uses.

Analysing and Identifying Substances

Chemical analysis can either be qualitative or quantitative. Analysis is the opposite of synthesis, but it would be very difficult to understand the exact mechanisms involved in synthesising materials or products without analysing the process.

There are synthetic processes that humans have been carrying out for centuries without understanding the fundamental chemical mechanisms behind them. For instance, for thousands of years, humans have been synthesising ethanol or ethyl alcohol from grapes or other fruits – but the exact chemical mechanisms of how this works have only recently become known. As a result of this analysis, the process is more efficient and precise enough to be done on an industrial scale. This is just one example of the importance of chemical analysis. 

Chemistry student analysing test tubes in science lab

Breaking-up Substances

The English word ‘analysis’ has its origin in Ancient Greek ἀνάλυσις, which literally mean ‘breaking-up’ or ‘untying’. This is rather fitting since chemical analysis is literally the breaking-up of a complex or unknown substance into its simpler constituents in order to understand its composition, molecular structure, and properties. This process has several applications, such as identifying pollutants, studying the toxicity of natural or synthetic chemicals in food, and designing pharmaceutical products.

Various techniques

Chemical analysis involves different methods and techniques, requiring various types of laboratory equipment and specialisations. There are several analytical chemistry tests that you will come across in your studies, but in general they can be categorised into two main groups: qualitative tests and quantitative tests. 

  1. Qualitative tests

Qualitative tests involve observing any colour or physical changes in the substance being analysed when subjected to temperature changes and chemicals. They determine the chemical composition of a substance, but not how much of a given component is present in the substance. 

Flame tests

Flame tests are used to determine what type of metal is contained in salts. Salts are compounds that arise from the reaction of acids and bases. They dissolve in water as ions, and always contain either metal cations or ammonium-derived cations.

Here are the five most common metal cations and the colours that they produce when subjected to the flame test:

Ion presentFlame test colour
Lithium, Li+Crimson
Sodium, Na+Yellow
Potassium, K+Lilac
Calcium, Ca2+Orange-red
Copper, Cu2+Green

Some examples of salts that have metal cations in them are the following:

  • Sodium chloride: NaCl
  • Magnesium carbonate: MgCO3
  • Iron(III) Hydrogen phosphate: Fe2(HPO4)3
  • Potassium dichromate: K2Cr2O7
  • Calcium chloride: CaCl2
  • Sodium bisulfate: NaHSO4
  • Copper sulphate: CuSO4

Metal hydroxide precipitate tests

Sodium hydroxide (NaOH) is a common chemical used in analysing the metal cation constituents of substances. As a strong base, it’s a very active proton acceptor, and readily reacts with substances that have metals in them, forming precipitates. The colour of the precipitates depends on the type of metal. 

Precipitate tests work by adding a drop of diluted NaOH solution to a solution of an analyte, and then observing the precipitate colour in order to determine whether the analyte has metal cations and if so, what they are. Here are some examples:

Metal ionPrecipitate colour
Aluminium, Al3+White
Calcium, Ca2+White
Magnesium, Mg2+White
Copper(II), Cu2+Blue
Iron(II), Fe2+Green
Iron(III), Fe3+Brown

Ammonia tests

Similar to the sodium hydroxide tests, you can also use ammonia to test for metal precipitates. Complexes of metals are formed when a substance reacts with ammonia as a base. Here are some examples of such reactions:

Acid SolutionBasic SolutionSolution with Excess NH3Color of Complex
Ni2+(aq)Ni(OH)2(s)Ni(NH3)62+(aq)Violet
Cu2+(aq)Cu(OH)2(s)Cu(NH3)42+(aq)Blue
Zn2+(aq)Zn(OH)2(s)Zn(NH3)62+(aq)Colourless
Ag+(aq)Ag2O(s)Ag(NH3)2+(aq)Colourless
Cd2+(aq)Cd(OH)2(s)Cd(NH3)42+(aq)Colourless

Acid tests

Similar to the sodium hydroxide and ammonia tests, which use alkaline or basic substances, acids can also be used to test for precipitates. However, instead of testing for metal cations, acid tests are used to detect anions. 

Acid tests involve the formation of complexes that have varying colours depending on the substance being tested. Hydrochloric acid, nitric acid, and sulfuric acid are commonly used as test agents. For instance, a few drops of nitric acid and a few drops of silver nitrate solution is used to detect halides.

Halide ionPrecipitate colour
Chloride, ClWhite
Bromide, BrCream
Iodide, IYellow
Copper flame test chemical analysis
Flame tests are an example of qualitative chemical analysis. They’re used to determine which metal is contained in a salt.
  1. Quantitative tests

These are analytical methods that involve measurements of quantities, such as mass, molarity, and volume, in order to determine how much of a given component is present in a substance. The two most common categories of quantitative analyses are gravimetric analysis and volumetric analysis.

Gravimetric analysis

As the name implies, this method of analysis involves using gravity as reference, or, more precisely, the effect of gravity on mass, which is expressed in weight. The weight of a substance is measured before and after it’s transformed. One common example is the weight difference before and after water is removed from a hydrate through heating. 

For instance, cobalt(II) chloride hexahydrate can be transformed into anhydrous cobalt chloride by applying heat:

CoCl2·6H2O + heat => CoCl2

The process is reversible when water is added to the anhydrous cobalt chloride.

Volumetric analysis

Otherwise known as titration, this method is a wholly separate topic that you should revise thoroughly for your GCSE in order to understand it properly. You’ve probably performed it several times already in your school’s laboratory, but in a nutshell it involves two reactants, typically an acid and a base, either of which could be the analyte or the titrant.

The analyte is the substance of known type, but unknown concentration. The titrant is a substance of known type and concentration that is gradually added to the analyte until a point of equivalence or neutrality is reached. The volume is measured before and after the end point is reached, which is determined using a pH metre or a chemical indicator.

Analysing Substances

Analysing substances requires skill, theoretical knowledge, and the right instruments. When you’re in secondary school, you’ll be required to perform several analytical experiments in the laboratory, such as titration and precipitation analysis. You must be meticulous in your observations and measurements in order to accurately answer the lab questions. Your practical laboratory knowledge can also help you solve many GCSE problems in chemistry, so it’s a good foundation to have.

In this subject, you’ll study the classical basic techniques of chemical analysis. If you take more advanced courses in chemistry at college or university, you’ll also learn about instrumental methods such as spectroscopy, mass spectrometry, and thermal analysis. These are usually not covered in-depth in secondary school laboratory chemistry, but you may be given an overview of how they work.

Further reading: Chemistry GCSE Revision: Chemical Analysis

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