Notes:
What are bases ?
Bases are chemical substances that dissolve in water to produce hydroxide ions.
Bases that dissolve in water are also called alkalis.
An hydroxide ion (OH-) is an oxygen atom (O) and a hydrogen atom (H) that are
joined together.
O
H
The hydroxide ion has a single negative (-) charge. This means that there is one more
electron compared with the number of protons in the oxygen (O) and hydrogen (H)
atoms combined.
All metal hydroxides (e.g. NaOH, Mg(OH)2, Ca(OH)2 etc.) are bases.
Some other compounds are also bases, such as metal oxides (e.g. MgO, CaO) and
sodium hydrogen carbonate (NaHCO3).
When a base dissolves in water, it splits to produce an hydroxide ion (OH -) and
another ion, depending on what the base is.
e.g.
NaOH(s)  Na+ (aq) + OH-(aq)
MgOH(s)  Mg2+ (aq) + 2OH-(aq)
Examples of Bases and their Uses:
Bases have two main uses in the home:
 neutralising acids (i.e. cancelling the effect of acids)
e.g. Toothpaste is a mild base that neutralises the effect of the acid produced by
plaque bacteria in the teeth that could otherwise damage tooth enamel and cause
tooth decay.
 cleaning grease and dirt
e.g. oven cleaner contains potassium hydroxide (KOH) which dissolves grease; cloudy
ammonia (NH4OH) removes dirt from floors.
Base
sodium hydroxide
(caustic soda)
magnesium hydroxide
(milk of magnesia)
aluminium hydroxide
calcium hydroxide
(slaked lime
or lime water)
Formula
NaOH
Use
making soap, drain cleaner, oven cleaner
Mg(OH)2
potassium hydroxide
(caustic potash)
ammonium hydroxide
KOH
used as a medicine to neutralise excess
stomach acid
relieving upset stomachs (indigestion)
used in the preparation of leather, making
paper from wood pulp, making plaster for
ceilings, cement, bleaching powder, testing
for carbon dioxide
oven cleaners
sodium hydrogen carbonate
(sodium bicarbonate or
baking powder)
NaHCO3
Al(OH)3
Ca(OH)2
NH4OH
cleaning copper, bronze and brass, dissolving
dirt and grease, fertilisers, explosives
Baking powder.
Dilute and Concentrated Bases:
A base can be dilute or concentrated.
 A dilute base is one where there is a small amount of base dissolved in a large
amount of water.
 A concentrated base is one where there is a large amount of base dissolved in a
relatively small amount of water.
Weak and Strong Bases:
Like acids, bases may be weak or strong. This is NOT the same as dilute or
concentrated.
 A weak base is one that only splits up into its ions a small amount when it is
dissolved in water, releasing only a few hydroxide ions.
 A strong base is one that splits up into its ions almost completely, releasing a
large number of hydroxide ions.
e.g.
sodium hydrogen carbonate (NaHCO3) is a weak base
sodium hydroxide (NaOH) and potassium hydroxide (KOH) are strong bases
SAFETY WARNING:
Be extremely careful when using bases in the laboratory. ALWAYS wear SAFETY
GOGGLES when handling bases.
Bases, like acids, can do a lot of damage to people, clothes and other materials. If
you ever spill a base on your self, remain clam and immediately wash with lots of
water. Keep washing for several minutes and get someone to tell the teacher what
has happened.


Concentrated bases are more dangerous than dilute acids.
Strong bases are more dangerous than weak acids.
Properties of Bases:
 Bases can neutralise acids (cancel out the effects of acids)
 Bases taste bitter
 Bases feel soapy in solution
 Some bases are corrosive
 Bases turn litmus to blue
 Bases have a pH greater than 7
Questions:
1. What is a base?
2. . What is the formula for a hydroxide ion?
3. A student spills some base on her sleeve. What are two things she should she do?


4. Complete the table below:
Base
sodium hydroxide
Formula
Mg(OH)2
Al(OH)3
calcium hydroxide
potassium hydroxide
5. Write the names of the bases that are found in each of the substances or uses
below (choose from the following list of bases): sodium hydroxide, potassium
hydroxide, ammonium hydroxide, magnesium hydroxide, aluminium hydroxide,
calcium hydroxide (there may be more than one answer in some cases – give all the
correct answers).
Substance or Use
oven cleaner
floor cleaner
indigestion medicine
fertiliser
explosives
cement & plaster
making paper
making soap
testing for carbon dioxide
making leather
Base(s)
6. Complete the following sentences:


A concentrated base has a
amount of base dissolved in a relatively
amount of water. A dilute base has a
amount of base dissolved in a
relatively
amount of water.
A strong base releases a
number of
ions when it dissolves in water.
A weak base releases a
number of
ions when it dissolves in water.
7. What is one safety procedure you must always follow when using bases in the
science laboratory?
8. Although bases feel soapy, why shouldn’t you feel them in the laboratory unless
instructed to do so?