Section 4 a: acids, alkalis and salts
Indicators and pH
4.1 describe the use of the indicators litmus, phenolphthalein and methyl orange to distinguish
between acidic and alkaline solutions
4.2 understand how the pH scale, from 0–14, can be used to classify solutions as strongly acidic,
weakly acidic, neutral, weakly alkaline or strongly alkaline
4.3 describe the use of universal indicator to measure the approximate pH value of a solution
Acid-base indicators are chemicals which change to a different colour when the pH of a solution changes.
As a result they have a different colour in an acidic solution then in an alkali.
The different indicators and their colours in acids and alkalis that you need to know are litmus,
phenolphthalein and methyl orange.
indicator
colour in acid
colour in alkali
litmus
red
blue
phenolphthalein
colourless
pink
methyl orange
red
yellow
The best indicator is the universal indicator, which is a mixture of indicators, as it tells us the pH of a
solution, which apart from telling us whether the solution is acidic or alkali or not, also tells how acidic or
how alkaline the solution is. The pH, which is shown by the colour of the universal indicator, is a measure
of how acidic or how alkaline a solution is; it is an approximate measure of the amount of hydrogen ions
there are in a solution; the lower the pH number, the higher the amount of hydrogen ions in the solution.
pH number
1
2
3
strong acidic
(many H+ ions)
4
5
6
7
weakly acidic neutral
8
9
10
11
weakly alkaline
12
13
14
strongly alkaline
Acids and alkalis
4.4 define acids as sources of hydrogen ions, H+, and alkalis as sources of hydroxide ions, OH¯
acids
Acids are substances which dissociate in water
and release hydrogen ions, H+.
Examples: HCl (aq), H2SO4 (aq), HNO3 (aq),
H2CO3 (aq)
alkalis
Alkalis are substances which dissolve in water to
release hydroxide ions, OH-.
Examples: NaOH (aq), KOH (aq), NH4OH (aq)
A reaction between an acid and an alkali is called a neutralization reaction as it involves the hydrogen
ions, H+, of the acid being changed into water molecules by reacting with hydroxide ions, OH-, of an alkali
as shown in the equation below.
H+ (aq) +
OH- (aq)
 H2O (l)
Section 4 a acids, alkalis and salts
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Reactions of acids
4.5 predict the products of reactions between dilute hydrochloric, nitric and sulfuric acids; and
metals, metal oxides and metal carbonates (excluding the reactions between nitric acid and
metals)
Acids carry out the following reactions.
metal 
acid +
salt
+
hydrogen
During this type of reaction the metal displaces the hydrogen ion from the acid which is then liberated
as hydrogen gas molecules; the metal combines with the non metal of the acid to form a salt. The
metal ionises.
Example: HCl (aq)
+
 MgCl2 (aq)
Mg (s)
+
H2 (g)
Complete the following equations:
1.
HCl (aq)
+
Ca (s)

2.
H2SO4 (aq)
+
Ca (s)

3.
HCl (aq)
+

Fe (s)
4. H2SO4 (aq)
+
Zn (s)
5. HCl (aq)
+
Al (s)


metal oxide 
acid +
salt
+
water
During this type of reaction the metal displaces the hydrogen ion which reacts with the oxide ion to
form water; the metal combines with the non metal of the acid to form a salt – the metal ionises.
Example:
2HCl (aq)
+
MgO (s)
 MgCl2 (aq)
+
H2O (l)
Complete the following equations:
1. HCl (aq)
+
CuO (s)

2. H2SO4 (aq)
+
CaO (s)

3. HCl (aq)
+
4. H2SO4 (aq)
5. HNO3 (aq)
ZnO (s)
+

Fe2O3 (s)
+
acid +
Al2O3 (s)


metal hydroxide (alkali) 
salt
+
water
During this type of reaction the metal displaces the hydrogen ion which reacts with the hydroxide ion
to form water; the metal ion combines with the non metal ion of the acid to form a salt.
Example:
HCl (aq)
+
NaOH (aq)
 NaCl (aq)
+
H2O (l)
Complete the following equations:
Section 4 a acids, alkalis and salts
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1. HNO3 (aq)
+
2. H2SO4 (aq)
3. HCl (aq)
4. H2SO4 (aq)
KOH (aq)

NH4OH (aq)

+
+
+
5. HNO3 (aq)

NaOH (aq)

NH4OH (aq)
+

NaOH (aq)
acid + metal carbonate  salt + water + carbon dioxide
During this type of reaction the metal displaces the hydrogen ion which reacts with the carbonate ion
to form water and carbon dioxide; the metal combines with the non metal of the acid to form a salt.
Example:
2HCl (aq)
+
CaCO3 (s)
 CaCl2 (aq)
+
H2O (l) + CO2 (g)
Complete the following equations:
1. HNO3 (aq)
+
2. H2SO4 (aq)
3. HCl (aq)
4. HNO3 (aq)
+
+

CaCO3 (s)
CaCO3 (s)


Na2CO3 (aq)
+
MgCO3 (s)

Hydrogen chloride – a halogen compound (this was also studied in topic 2b)
2.11 understand the difference between hydrogen chloride gas and hydrochloric acid
2.12 explain, in terms of dissociation, why hydrogen chloride is acidic in water but not in methylbenzene
Hydrogen chloride gas, HCl (g), is a covalent compound which has a low melting and boiling point, and
does not conduct electricity .
However when hydrogen chloride gas is dissolved in water and forms hydrogen and chloride ions
(hydrogen chloride dissociates) it becomes hydrochloric acid; it becomes a very corrosive chemical with a
very low pH which reacts easily with metals, bases and metal carbonates. It is the hydrogen ions which
give the hydrogen chloride solution its acidic character.
The equation for the dissociation of hydrogen chloride:
HCl (g)
+
H2O (l)

H3O+ (aq)
+
Cl- (aq)
We can write a simplified version of the above equation:
HCl (aq)

H+ (aq)
+
Cl- (aq)
When hydrogen chloride is added to methylbenzene the dissociation process does not occur as a result
hydrogen chloride does not behave like an acid i.e. does not change the colour of an universal indicator
solution or blue litmus and does not conduct electricity.
Preparation of salts
Section 4 a acids, alkalis and salts
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4.6 recall the general rules for predicting the solubility of salts in water
4.7 describe how to prepare soluble salts from acids
4.8 describe how to prepare insoluble salts using precipitation reactions
4.9 describe how to carry out acid-alkali titrations
Salts are ionic compounds which consist of a positive ion (metal ion or ammonium ion, NH4+) and a
negative ion (non-metal ion like chloride ion, Cl-, or non-metal ion radical like sulphate or nitrate).
Most salts are soluble in water except a few as shown in the table below.
soluble
insoluble
All nitrates
Salts
All chlorides except….
….silver chloride, lead chloride
All sulphates except…..
….barium sulphate, lead sulphate, calcium
sulphate
sodium and potassium carbonates
….all other carbonates
All sodium, potassium and
ammonium salts
Section 4 a acids, alkalis and salts
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How do we now make samples of pure dry crystals of a salt. The flowchart below will help you to identify the correct method.
no
yes
Is salt
soluble?
Precipitation
 Make 2 solutions each one containing 1
ion of the salt to be made (use soluble
salts!!!)
 Add the solutions
 Filter off precipitate which is the insoluble
salt
 Wash the residue with distilled water
 Dry with filter paper or on warm gauze
Obtaining dry salt from solution




Concentrate solution until it becomes
saturated which is done by driving off
most of the water by heating it. Solution is
saturated when crystals form on a glass
rod dipped in the solution.
Stop heating and allow solution to cool
and salt to crystalize
Crystals can be washed with distilled
water
Dry crystals with filter paper or place on
warm gauze
base
soluble?
no
Titration
 Add acid via burette to alkali`+ indicator until
neutral (or until temperature stops rising)
 Measure volume of acid added; throw away
solution as it contains the indicator;
 Add same volume of acid to same amount of
alkali without indicator
 Obtain dry salt from solution
Section 4 a acids, alkalis and salts
yes
Add excess base
 Add excess base to acid until
no more dissolves (warm acid if
necessary)
 Filter off excess base
 Obtain salt from solution (see
to the left)
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For each of the following salts, select suitable chemicals and decide which method you will use to make
the salt.
salt you are going to make
starting chemicals you will use
method
magnesium sulfate
sodium nitrate
calcium chloride
barium chloride
ammonium nitrate
copper sulphate
Complete the table below about the making of insoluble salts.
salt
starting chemicals
1. copper carbonate
2. silver chloride
3. lead sulphate
4. lead chloride
5. barium sulphate
For each of the above examples write a balanced symbol equation.
Section 4 a acids, alkalis and salts
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