Acids, Bases & Salts
Name: _______________________________ (
1.
)
Class: _______
Date: __________
Acids
 Definition
An acid is a substance which produces hydrogen ions as the only positive ions when it is dissolved in
water.
Example: HCl molecules dissolve in water to give hydrogen ion and chloride ion.
 Examples
Strong acids
Common / Mineral Name
Hydrochloric acid
Nitric acid
Sulfuric acid
Chemical Name
Hydrogen chloride
Hydrogen nitrate
Hydrogen sulfate
Chemical Formula
HCl
HNO3
H2SO4
Weak acids
Name of Acid
Acetic acid (ethanoic acid)
Citric acid
Tartaric acid
Source
vinegar
lemon
grape
 Strength of an acid
The strength of an acid depends on its degree of dissociation / ionisation in water to form
hydrogen ions.
Strong acids
A strong acid is one that ionises / dissociates completely in water to produce hydrogen ions (H+).
There are no molecules left. The solution contains a high concentration of ions: good electrical
conductor.
Pure HCl (g)
HCl (aq)
: covalent
→ H+ (aq) + Cl- (aq)
Pure HNO3 (l)
HNO3 (aq)
: covalent
→ H+ (aq) + NO3- (aq)
Pure H2SO4 (l)
H2SO4 (aq)
: covalent
→ 2H+ (aq) + SO42- (aq)
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Weak acids
A weak acid is one that ionises / dissociates incompletely / partially in water to produce few
hydrogen ions (H+). Most of the acid molecules remain as molecules. The solution contains a low
concentration of ions: poor electrical conductor.
CH3COOH (aq) ⇌ CH3COO- (aq) + H+ (aq)
 Importance of water for acidity
Pure acids exist as molecules instead of ions. Pure acids do not behave as acids as the
properties of acids are due to the presence of hydrogen ions. When acids are mixed with water,
ionisation of acids occurs, and hydrogen ions are produced. Therefore, acids can only behave as
acids when they are dissolved in water.
 Properties of Acids
An acid, when dissolved in water, forms a colourless solution.
following properties:
1.
2.
3.
4.
Solutions of acids have the
sour taste.
change the colour of indicators (e.g. turn blue litmus paper red).
contain hydrogen ions and conduct electricity.
react with metals, carbonates and bases.
 Reactions with metals
Acids react with metals to produce a salt and hydrogen
Acid + metal → salt + hydrogen
Example:
2HCl (aq) + Zn (s) → ZnCl2 (aq) + H2 (g)
Observations:
(a) Effervescence observed. Colourless, odourless gas evolved which
extinguished a lighted wooden splint with a ‘pop’ sound.
(b) Grey solid reduced in size (when in excess). Grey solid dissolves to form
a colourless solution
(c) Heat liberated.
Zinc chloride is called a salt. Salts of nitric acid are called nitrates. Salts of sulfuric acid are called
sulfates, and those of hydrochloric acid are called chlorides. A salt is generally made up a cation
(that comes from a base) and an anion (that comes from an acid). A salt is the product of the
neutralisation of an acid and a base
Acid
Examples of Salts
Sulfuric acid
Zinc sulfate
Sodium sulfate
Nitric acid
Potassium nitrate
Copper (II) nitrate
Hydrochloric acid
Magnesium chloride
Sodium chloride
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Formula
ZnSO4
Na2SO4
KNO3
Cu(NO3)2
MgCl2
NaCl
Exceptions
a) Metals which are less reactive than hydrogen is not reactive enough to react with acids.
Reactivity Series:
Potassium
Sodium
Calcium
Magnesium
Aluminium
Zinc
Iron
Tin
Lead
Hydrogen
Copper
Silver
Gold
Most Reactive
Least reactive
 Reactions with carbonates
Acids react with carbonate to produce a salt, carbon dioxide and water.
Acid + carbonate → salt + carbon dioxide + water
Example:
Na2CO3 (s) + H2SO4 (aq) → Na2SO4 (aq) + CO2 (g) + H2O (l)
Observations:
(a) Effervescence occurs. Colourless, odourless gas evolved which reacts
with calcium hydroxide to form a white precipitate.
(b) White solid dissolves to form a colourless solution.
 Reactions with bases
Acids react with bases (metal oxides and metal hydroxides) to produce a salt and water.
Acid + base → salt + water
Example:
MgO (s) + H2SO4 (aq) → MgSO4 (aq) + H2O (l)
Observation:
(a) White solid dissolves to form a colourless solution.
Example:
2KOH (aq) + H2SO4 (aq) → K2SO4 (aq) + 2H2O (l)
Observation:
(a) Heat is evolved. No visible change.
 Uses of acids
Sulfuric acid:
in making detergents and fertilisers
in car batteries as battery acid
Ethanoic acid:
to preserve food
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 Colour change of indicators
Indicators
Observation(s)
Litmus paper
An acid turns blue litmus paper red.
Methyl Orange
Used to identify strong acids.
Colour change at pH 4
Strong acid turns methyl orange red.
Alkali turn methyl orange yellow.
Phenolphthalein
Used to identify strong alkalis.
Colour change at pH 9
Strong alkali turns phenolphthalein pink.
Phenolphthalein remains colourless in acid.
Universal Indicator
In neutral solutions, universal indicator remains green.
Strong acid: red
Strong alkali: violet
 pH
pH values depend on the concentration of acid/base and degree of dissociation. The use of pH in
measuring the strength of an acid is limited since its value changes with concentration.
The higher the concentration of the hydrogen ion, the lower the pH value. For instance, a solution
with a pH value of 1 has a higher concentration of hydrogen ions as compared to a solution with a
pH value of 2.
2.
Bases and Alkalis
 Definition of bases
Bases are the oxides or hydroxides of metal that reacts with an acid to form a salt and water only.
 Definition of alkali
An alkali is a metal hydroxide which is soluble in water and produces hydroxide ions (OH -) in water.
All alkalis are bases but not all bases are alkalis.
 Examples
Insoluble base
Copper (II) oxide, aluminium oxide, zinc oxide, zinc hydroxide
Soluble base / alkali
Sodium oxide, potassium oxide, sodium hydroxide, potassium hydroxide, calcium hydroxide
(slightly soluble), ammonium hydroxide
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 Types of Oxides
1.
Acidic oxide : carbon dioxide, sulphur dioxide, nitrogen dioxide etc.
2.
Basic oxide : calcium oxide, iron(III) oxide, magnesium oxide etc.
3.
Neutral oxide : carbon monoxide, nitric oxide, water etc.
4.
Amphoteric oxide : zinc oxide, aluminium oxide, lead(II) oxide, lead(IV) oxide
Acidic oxide
An acidic oxide is a non-metallic oxide that neutralises a base to form salt and water only. It reacts
with water slightly to form an acid solution.
Neutralization:
CO2 (g) + 2KOH (aq)  K2CO3 (aq) + H2O (l)
Reaction with water:
CO2 + H2O  H2CO3
SO2 + H2O  H2SO3
Basic oxide
A basic oxide is a metallic oxide that neutralises an acid to form salt and water only. If it is soluble
in water, it forms an alkali.
Neutralization :
MgO (s) + H2SO4 (aq)  MgSO4 (aq) + H2O (l)
Reaction with water:
Na2O + H2O  2NaOH
CaO + H2O  Ca(OH)2
Neutral oxide
Neutral oxides are non-metallic oxides that show neither acidic nor basic properties. They are
insoluble in water and do not react with water.
E.g. H2O, CO, NO
Amphoteric oxide
Amphoteric oxides are metallic oxides that react with both acid and base to form salt and water.
They have both acidic and basic property.
E.g. ZnO, Al2O3, PbO, PbO2
Reaction with acid:
ZnO (s) + 2HCl (aq)  ZnCl2 (aq) + H2O (l)
Reaction with alkali:
ZnO (s) + 2NaOH (aq)  Na2ZnO2 (aq) + H2O (l)
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 Strength of an alkali
The strength of an alkali depends on its degree of dissociation / ionisation in water to form
hydroxide ions. The properties of alkalis are due to the hydroxides ions.
Strong alkali
A strong alkali is one that ionises / dissociates completely in water to produce hydroxide ions
(OH-). There are no molecules left. The common strong alkalis are sodium hydroxide and
potassium hydroxide.
→ Na+ (aq) + OH- (aq)
→ K+ (aq) + OH- (aq)
NaOH (aq)
KOH (aq)
Weak alkali
A weak alkali is one that ionises / dissociates incompletely / partially in water to produce few
hydroxide ions (OH-). Most of the alkali molecules remain as molecules.
 Properties of bases
1. Alkalis have a soapy feel and a bitter taste.
2. Alkalis can change the colour of indicators (e.g. turn red litmus paper blue).
3. Bases can react with acids to form salt and water only – Neutralisation.
Base + acid → salt + water
Example:
NaOH (aq) + HCl (aq) → NaCl (aq) + H2O (l)
Note: Neutralisation – The formation of molecules of water from hydrogen ion of an acid and
hydroxide ion of an alkali. H+ (aq) + OH- (aq) → H2O (l).
4. Bases can react with ammonium salts to form salt, ammonia and water.
Base + ammonium salt → salt + ammonia + water
Example:
NaOH (aq) + NH4Cl (s) → NaCl (aq) + NH3 (g) + H2O (l)
Observation:
(a)
A colourless, pungent gas is evolved.
Gas turns damp red litmus paper blue.
5. Alkalis can react with a solution of one metal salt to give metal hydroxide and another metal
salt
Example:
CuSO4 (aq) + 2NaOH (aq) → Cu(OH)2 (s) + Na2SO4 (aq)
 Uses of Bases and Alkalis
Magnesium hydroxide:
in toothpaste to neutralise the acids on our teeth produced by
bacteria.
Ammonia solution:
used for cleaning glass windows
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