Bases and alkalis
Indicators and the pH scale
Bases are substances that react with acids and neutralise them. They are usually metal
oxides, metal hydroxides, metal carbonates or metal hydrogen carbonates. Many
bases are insoluble - they do not dissolve in water.
When an acid is dissolved in water we get an acidic solution, and alkalis make alkaline
solutions. If a solution is neither acidic nor alkaline we call it neutral. Pure water is
neutral.
If a base does dissolve in water, we call it an alkali.
Indicators
Copper oxide is a base because it will react with acids and neutralise them, but
it is not an alkali because it does not dissolve in water.
Indicators are substances that change colour when they are added to acidic or alkaline
solutions. You can prepare homemade indicators from red cabbage or beetroot juice these will help you see if a solution is acidic or alkaline.
Sodium hydroxide is a base because it will react with acids and neutralise them.
It's also an alkali because it dissolves in water.
Litmus and universal indicator are two indicators that are commonly used in the
laboratory.
All alkalis are bases, but only soluble bases are alkalis
some strong bases and alkalis at school, such as sodium hydroxide
solution. Like acids, their bottles are labelled with the warning symbol
for 'irritant'. This means that they will make your skin red or blistered
unless you wash off any spills with plenty of water.
Alkalis feel soapy when they get on your skin, so it is easy to tell when you have
had an accident and must wash your hands.
Concentrated alkalis are corrosive. They can attack metals and destroy skin if
spilled. They are just as dangerous as concentrated acids, but many people do
not realise this.
Red Litmus
Blue Litmus
Acidic solution
Stays red
Turns red
Neutral solution
Stays red
Stays blue
Alkaline solution
Turns blue
Stays blue
Notice how we say 'stays red'. This is better than saying 'nothing' or 'stayed the
same', because it tells us the colour we actually see.
Bases in the home
Bases react with oils and fats, so they are often used in strong household
cleaners. Drain cleaners and oven cleaners usually contain sodium hydroxide for
example.And ammonia is also commonly used in cleaners. Ammonia can be
recognised by its choking smell.
It is wise to wear gloves when using these substances, otherwise they will react
with your skin and burn it.
Weak bases and alkalis are found in toothpaste, antacid tablets (to help cure an
upset stomach) and baking powder.
Universal indicator and the pH scale
Universal indicator is a mixture of several different indicators. Unlike litmus,
universal indicator can show us exactly how strongly acidic or alkaline a solution
is. This is measured using the pH scale. The pH scale runs from pH 0 to pH 14.
Universal indicator has many different colour changes, from red for strong acids
to dark purple for strong bases. In the middle, neutral pH 7 is indicated by green.
Using neutralisation
Farmers use lime (calcium oxide) to neutralise acid soils.
Your stomach contains hydrochloric acid, and too much of this causes indigestion.
Antacid tablets contain bases such as magnesium hydroxide and magnesium
carbonate to neutralise the extra acid.
Bee stings are acidic. They can be neutralised using baking powder, which contains
sodium hydrogen carbonate.
Reactions of acids with metals
These are the important points about the pH scale:
 neutral solutions are pH 7 exactly
 acidic solutions have pH values less than 7
 alkaline solutions have pH values more than 7
 the closer to pH 0 you go, the more strongly acidic a solution is the
closer to pH 14 you go, the more strongly alkaline a solution is
Acids react with most metals and a salt is produced. But unlike the reaction between acids
and bases we don't get any water. Instead we get hydrogen gas.
This is the general word equation for the reaction:
metal + acid → salt + hydrogen
Salts
The salt produced depends upon the metal and the acid. Here are two examples:
Reactions of acids with bases
zinc + sulphuric acid → zinc sulphate + hydrogen
A chemical reaction happens if you mix together an acid and a
base. The reaction is called neutralisation, and a neutral
solution is made if you add just the right amount of acid and base
together
magnesium + hydrochloric acid → magnesium chloride + hydrogen
Metal oxides and metal hydroxides
Metal oxides and metal hydroxides are two types of bases. For example
copper oxide and sodium hydroxide.
Here are general word equations for what happens in their neutralisation
reactions with acids.
metal oxide [Alkali] + acid → a salt + water
metal hydroxide [alkali] + acid → a salt + water
It doesn't matter which metal or acid is used, if there is a reaction we always get hydrogen
gas as well as the salt.
The test for hydrogen
There is a simple laboratory test to see if a gas is hydrogen. A lighted wooden splint goes
pop if it is put into a test tube of hydrogen. This is because the flame ignites the hydrogen,
which burns explosively to make a loud sound.
Acids and hydrogen
All acids contain hydrogen atoms. Apart from hydrochloric acid, this is not clear
from their names, but you can tell they contain hydrogen from their chemical
formulae. Remember that the chemical symbol for hydrogen is H.
Name of acid
Chemical formula of acid
hydrochloric acid
HCl
nitric acid
HNO3
sulphuric acid
H2SO4
carbonic acid
H2CO3
phosphoric acid
H3PO4
Example
Where does the name potassium nitrate come from?
Formulas of Common Bases
Sodium Hydroxide - NaOH
Potassium Hydroxide - KOH
Ammonium Hydroxide - NH4OH
Calcium Hydroxide - Ca(OH)2
Magnesium Hydroxide - Mg(OH)2
Aluminum Hydroxide - Al(OH)3
Ferrous Hydroxide or Iron (II) Hydroxide - Fe(OH)2
Ferric Hydroxide or Iron (III) Hydroxide - Fe(OH)3
Zinc Hydroxide - Zn(OH)2
These are the rules for the second part of the name of a salt:
Acid used
Second part of salt's name
hydrochloric acid
chloride
sulphuric acid
sulphate
nitric acid
nitrate
Naming salts
A salt is always made when an acid is neutralised by a base. But the exact salt
made depends upon which acid and base were used.
The name of a salt has two parts:
the first part comes from the metal in the base used
the second part comes from the acid that was used
copper oxide + sulphuric acid → copper sulphate + water
copper carbonate + sulphuric acid → copper sulphate + water + carbon
dioxide
sodium hydroxide + hydrochloric acid → sodium chloride + water
sodium hydrogen carbonate + hydrochloric acid → sodium chloride + water +
carbon dioxide