Acids, Bases and Salts
A strong acid is 100% ionised in water.

Acids, Bases and Salts

In this first quiz on GCSE Chemistry we take a look at one vital part of the subject - acids, alkalis, bases and salts. In it we find out the difference between alkalis and bases, how acids and alkalis react with one another, how neutralisation can produce salts and the relevance of pH values.

Acids and alkalis are incredibly important for manufacturing, and for everyday life. Some examples of things made using acids and alkalis are soaps and detergents, fertilisers, car batteries and medicines - you will have probably learnt a lot more examples during your lessons, as well as the properties of acids and alkalis. The most important of the reactions of acids and alkalis is neutralisation. Mixing the correct amounts of acid and alkali will result in a solution that is neutral because the two substances react together to produce water and a salt.

During the 20th century, people knew about air pollution but little was done about it until the final decades. During the 1970s, it was noticed that trees were dying and some lakes were becoming too acidic to support life. It was realised that acid rain was the cause. The first response to this was to try to neutralise the water in lakes by using huge amounts of calcium carbonate (remember, acids react with carbonates as well as with alkalis). Rain is normally very slighly acidic as it contains dissolved carbon dioxide from the air. Acid rain is rain that is much more acidic than normal and it occurs where fossil fuels are being burnt in large quantities. Governments introduced laws that forced the polluters to look at ways of reducing the amounts of acidic gases in the fumes that they produced. The problem is not as bad as it was then, but it is still with us.

There are several different ways of defining what an acid or an alkali is (alkalis and bases are NOT exactly the same thing. See Q2 for more details). The simplest and probably the first that you learnt is the pH. A pH value of 7 is neutral, anything lower than this is acidic and anything more than pH7 is alkaline. Values that are further away from neutral indicate stronger acids and alkalis.

Be careful not to confuse the terms strong and weak with dilute and concentrated. You might think that a strong acid or alkali would always be more harmful than weak acids or alkalis. That is not necessarily the case. Take for example hydrochloric acid. This is a strong acid yet it is found inside your stomach! How can that be? Well, it's a matter of concentration, the hydrochloric acid inside your stomach is extremely diluted. Your stomach acid is so dilute that it is easily contained by a thin layer of mucus secreted by cells in the walls of your stomach.

Finding out whether something is acidic or alkaline can be done in different ways. The substance known as 'litmus' is used simply to find out if a solution is acidic or alkaline, it cannot be used to find the strength of an acid or alkali. Litmus is naturally blue and the dye comes from lichens, which are becoming more and more scarce. It turns red in the presence of an acid. Some other plant colours can be used in the same way - you may have done some experiments with red cabbage extract at KS3. Universal indicator (UI) paper is more useful as it can give you a pH reading. The pH scale runs from 0 - 14 but some UI paper only measures from 0 - 12. For the most accurate readings of pH, using a meter is is the best.

Have a go at this quiz and see if your understanding of acids, alkalis, bases and salts is up to scratch

What is an acid?
A substance that will neutralise an acid
A substance that produces H+ ions when added to water
A soluble base
A chemical which changes colour when added to acids and alkalis
The H+ ion is simply an H atom with a missing electron i.e. a proton
What is an alkali?
A substance that will neutralise an acid
A substance that produces H+ ions when added to water
A soluble base
A chemical which changes colour when added to acids and alkalis
A base will neutralise an acid, but is only called an alkali when it is dissolved in water
A substance with a pH of 6 is...
a weak acid
a strong acid
a weak alkali
pH values of less than 7 are acidic, 7 is neutral and values greater than 7 are alkaline
Pick the correct equation for the reaction between hydrochloric acid and iron oxide.
Hydrochloric acid + solid iron (III) oxide → iron (III) chloride solution + water
Hydrochloric acid + iron (III) oxide → iron (II) chloride solution + water
Hydrochloric acid + iron (III) oxide → iron (III) chlorate solution + water
Sulfuric acid + iron (III) oxide → iron (III) chloride solution + water
This is an example of an acid-base reaction. An alkali is a base that dissolves in water
We can react an acid and an alkali together to produce a salt. The correct name for this type of chemical reaction is...
thermal decomposition
It is called neutralisation because the products of the reaction are neutral
When an acid and an alkali react, a salt and water are formed. Pick the correct salt produced when ammonium hydroxide reacts with nitric acid.
Ammonium chloride
Ammonium nitrate
Ammonium sulfate
Ammonium phosphate
Ammonium nitrate is commonly used as a fertiliser, although it is also explosive when mixed with other chemicals. It was used in the terrorist bomb that killed 169 people in Oklahoma City in 1995
Salts can also be made by reacting metals with acids. What gas is given off when a metal reacts with an acid?
Carbon dioxide
Hydrogen can be tested for using the 'squeaky pop' test
Precipitation reactions occur when an insoluble salt is formed. Precipitation reactions can be used in industry to...
remove some substances from waste water
split large molecules into smaller ones
remove unwanted impurities from metals
remove metals from their ores
By raising the pH of the water, insoluble metal hydroxides precipitate out of solution, which produces a sludge that can be easily removed
A strong acid...
turns universal indicator paper green
has a high pH number
is partially ionised in water
is 100% ionised in water
The amount of ionisation is measured by pH
Pick the correct combination of definitions.
Acid - proton acceptor
Alkali - proton donor
Acid - proton donor
Alkali - proton acceptor
Acid - proton giver
Alkali - proton donor
Acid - proton acceptor
Alkali - proton acceptor
An acid is a proton donor because it forms hydrogen ions and, as mentioned in the first question, a hydrogen ion is in fact simply a proton
You can find more about this topic by visiting BBC Bitesize - Acids, alkalis and salts

Author:  Kate Gardiner

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