Chemical Changes



One of the first things we do in chemistry is look at acids and alkalis. When dissolved in water (as a solution with the symbol (aq)), acids make H+ ions and alkalis make OH- ions. On the pH scale, we see that acids are between 1 and 6, alkalis are between 8 and 14 leaving pure water in the middle, a neutral 7. Universal indicator changes colour depending on the pH as seen. below.


When acids and alkalis react with chemicals or each other, they follow a simple pattern that can easily be predicted. Firstly, when acid meets metal.
Acid + Metal--> a Salt + Hydrogen


This is the case every time as long as the metal is reactive enough (won't work with gold or platinum), here is an example:
2HCl(aq) + Zn(s) --> ZnCl2(aq) + H2(g)

When any acid reacts with any alkali, it is a neutralisation reaction, below is the general formula and an example:

Acid + Alkali --> a Salt + Water

H2SO4(aq) + MgO(s) --> MgSO4(aq) + H2O(l)

When making a salt, bases or alkalis are slowly added to an acid until a neutral pH tells us that all of the acid has been used up (becomes neutral). You either can then filter the solution (if salt is insoluble) or boil off the water to leave the solid crystals of salt behind.

In these reactions, the main activity goes on between the acid and alkali ions:

H+ + OH- --> H2O

The other ions behave as expected. You can select the salt by choosing a base with the desired metal and an acid with the desired non metal ions, e.g. nitric acid makes nitrates, hydrochloric acid makes chlorides etc. If the salt that is produced is not soluble, it can be filtered as a precipitate.

In earlier sections, we discussed that ionic solids do not conduct electricity, however, they can if molten or in solution. A solution may undergo electrolysis providing you have inert electrodes (usually graphite). During electrolysis, positive ions are attracted to the negative electrode (cathode) where they take on electrons and return to their atomic state (no longer ions) and negative ions flow to the anode or positive electrode to deposit their electrons and return to their atomic state. Gaining electrons is called reduction and losing electrons is called oxidation. OILRIG -Oxidation Is Loss - Reduction Is Gain.

Depending on reactivity, the ions that make up water (H+ & OH-) may be electrolysed to produce hydrogen gas or oxygen gas.

Aluminium is obtained from the ore Bauxite and as Al is more reactive than carbon, electrolysis is the method of extraction. Cryolite is added to lower the melting temperature then large currents are passed through it to oxidise and reduce the ions. Oxygen is produced at the positive electrode (oxidation) and Aluminium at the negative (reduction). Look at these half equations to show movement of electrons:

Al3+(l) + 3e- --> Al(l)

2O2-(l) --> 2O2(g) + 4e-

When electrolysing brine or salt water, there are Na+, Cl-, H+ & OH- ions in solution. During the process, hydrogen gas is given off at the negative electrode and chlorine at the positive making the solution more alkaline all the time. There are loads of uses for the products, hydrogen for margarine and fuels, chlorine for pools and bleach and the sodium hydroxide left in solution for soap and paper manufacture.

This process can be used without an inert electrode, instead an object that conducts electricity and that you want to get covered in the metal that would otherwise be deposited on graphite. This is called electroplating and it is how objects get nickel or zinc layers for appearance or protection.

Key words and terms for this topic: neutral, acid, alkali, pH, base, state symbols, universal indicator, salt neurtralisation, precipitate, electrolysis, electrolyte, inert, reduction, oxidation, brine, electroplating.

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