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Audio Script for Video on Strengths of Acids and Alkalis
(5 Years Time playing, title fades on screen "Strengths of Acids and Alkalis)
Music fades out.
Narrator: Welcome to Strengths of Acids and Alkalis. In this video I will be going over the basics of what
you need to know on this subject for Unit Three of GCSE Chemistry with AQA.
Firstly, you need to know what acids and alkalis actually are.
Acids, such as vinegar, (holds up picture of vinegar, with ethanoic acid written underneath) are
acidic because they contain hydrogen ions. This means that when they are in aqueous solution (added to
water), they dissociate to produce hydrogen ions (holds up H+ sign) This H+ ion is simply a proton which
hydrates in water (or chemically bonds to the water, if you will).
Alkalis on the other hand, such as sodium bicarbonate (holds up a picture of baking powder,
labelled sodium bicarb) are alkaline because they contain hydroxide ions (holds up OH- sign). Once
again, this means that when they are in aqueous solution they dissociate to produce hydroxide ions,
giving them their alkaline characteristics. Alkalis can also be determined by the "hydroxide" prefix that
nearly always follows, i.e. sodium hydroxide, potassium hydroxide...
In other words, we can define an acid as a proton DONOR as it gives a proton (the hydrogen ion)
when reacted with an appropriate base. However, alkalis can be defined as proton ACCEPTORS as they
require another proton (for example, when reacted with an acid they accept the hydrogen ion.)
Both acids and alkalis can be dangerous if their pH is too strong either way (for example if an
acid's pH is 1 or an alkali's pH is 14, then they would be very dangerous.) However they can be very
useful and many household objects that we use on a regular basis are acidic or alkaline, including foods
such as orange juice!
"Strengths of Acid and Alkalis" fades back on screen
Acids and alkalis can be classified by the extent of their ionisation in water. This means how many
ions dissociate when in the presence of water.
A strong acid or alkali is one that is completely ionised in water, meaning that all the compound
dissociates into ions. Some very strong acids are hydrochloric, sulphuric and nitric acids and some
examples of strong alkalis are sodium and potassium hydroxide.
A weak acid or alkali, however, is one that is only partially ionised in water, meaning that not all
of the compound dissociates into ions. Ethanoic acid (or vinegar), citric acid and carbonic acid are all
examples of weak acids, while ammonia solution is an example of a weak acid.
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However, rather confusingly, you can have different acids of the same concentration that have
different pHs. This is because it simply depends on the extent of their ionisation in water. By this token, a
strong concentration of an acid will register a much lower pH than a weak concentration of the same acid.
Acids can also be distinguished by their rate of reaction with metals to produce metal salts and
hydrogen. Strong acids react more vigorously with metals than weak acids.…read more