Forensic science and fingerprints
Forensic science and fingerprints

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

Forensic science and fingerprints

Redox reactions

The second kind of chemical reaction that is used extensively in forensic science, and analytical science in general, is the redox reaction. Redox is a contraction of 'reduction and oxidation' and refers to reactions in which one molecule or ion is oxidised and the other reduced. One simple definition of oxidation is the addition of oxygen to a substance. There are lots of examples of an oxidation reaction where oxygen in the atmosphere reacts with something to form a new substance. For example, burning coal, which is carbon, causes it to combine with oxygen to form carbon dioxide (Equation 1) and burning methane gas (CH4) in air produces carbon dioxide and water (Equation 2). In our bodies, glucose (C6H12O6) reacts with the oxygen that we breathe in to produce the carbon dioxide that we breathe out and water (Equation 3).

The definition of oxidation in terms of addition of oxygen is not appropriate for all oxidation reactions, and oxidation is better understood in terms of the loss of electrons. Rusting of iron metal (Fe) is another familiar example of an oxidation reaction. Here, each iron atom loses three electrons to form an iron(III) (ferric) ion as shown in Equation 5, where the symbol eˉ is used to denote an electron and 3eˉ means three electrons. The symbol Fe3+ means that the iron nucleus has three more protons that it has electrons.

Equation label: (Equation 5)

Question 14

If an oxidation is loss of electrons, what do you think a reduction might be?

Answer

A reduction is where something gains electrons, such as that shown in Equation 6 where an aluminium ion is reduced to metallic aluminium. (This is the energy-intensive process used to make aluminium metal.)

Equation label: (Equation 6)

Another redox reaction, in which the copper atom is oxidised by the silver ion which is correspondingly reduced, is shown in Equation 7.

Equation label: (Equation 7)

What this equation shows in chemical language is that if you dip, say, a copper coin into a solution containing silver ions then the copper atoms are oxidised by losing two electrons and the silver ions are simultaneously reduced with each one gaining an electron. The result is that the copper coin becomes silver plated. This is the principle behind Sheffield plating in which copper items such as candlesticks are coated in silver. A similar principle is used in a technique called 'physical developer', where oily or greasy fingerprints may be visualised by the deposition of metallic silver in a redox reaction (Equation 8).

Equation label: (Equation 8)

Question 15

Is the reaction of the silver ion to form silver metal in Equation 8 an oxidation or a reduction?

Answer

The addition of an electron to the silver ion is a reduction, because the silver has gained an electron.

Skip Your course resources
S187_1

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371