ACIDS, BASES AND SALTS

- Acids react with carbonates and hydrogencarbonates (bicarbonates)

The Characteristics of Acids and Bases
Carbon dioxide is to be formed. To test this, the gas produced is bubbled into
Common Acids
limewater which forms a white precipitate.
Acids in daily life:
Carbonates:
 Ethanoic acid – found in vinegar and tomato juice MgCO3(s) + 2HCl(aq) → MgCl2(aq) + CO2(g) + H2O (l)
 Citric acid – found in citrus foods like lemons, oranges and grapefruit Bicarbontes:
 Lactic acid – found in sour milk and yoghurt, and in muscle respiration NaHCO3(s or aq) + HCl(aq) → NaCl2(aq) + CO2(g) + H2O (l)
 Tartaric acid – found in grapes
 Tannic acid – found in tea and ant’s body - Acids react with metal oxides and hydroxides
 Formic acid – found in bee stings Metal oxides & hydroxides react slowly with warm dilute acid to form salt+water
 Hydrochloric acid – found in stomach juices Cu(OH)2(s) + H2SO4(aq) → CuSO4(aq) + 2H2O (l)

Laboratory acids: 3 common laboratory acids Storage of Acids

 Hydrochloric acid (HCl) Acids are stored in claypots, glass or plastic containers as sand, glass and plastic do
 Sulphuric acid (H2SO4) not react with acids. If it’s stored in metal container, metal would react with acids
 Nitric acid (HNO3)
Uses of Acids
Dilute acids – solution containing small amount of acid dissolved in water
Concentration acids – solution containing large amount of acid dissolved in water  Sulphuric Acid - Used in car batteries
- Manufacture of ammonium sulphate for fertilisers
Properties of Dilute Acids - Manufacture of detergents, paints, dyes, artificial fibres &
- Acids have a sour taste plastics

- Acids are hazardous  Hydrochloric acid can remove rust (iron(III) oxide) which dissolves in acids
acids are irritants (they cause skin to redden and blister)
 Acids are used in preservation of foods (e.g. ethanoic acid)
- Acids change the colour of indicators
Acids turn common indicator litmus – blue litmus to red 7.2 Acids and Hydrogen Ions
The Need for Water in Acids
- Acids react with metals Acids are covalent compounds and do not behave as acids in the absence of water
Acids react with metals to produce hydrogen gas. The gas is tested with a as water reacts with acids to produce H+ ions, responsible for its acidic properties.
burning splint which shows hydrogen burns with a ‘pop’ sound. e.g. Citric acid crystals doesn’t react with metals and doesn’t change colours of
2Na(s) + 2HCl(aq) → 2NaCl(aq) + H2(g) indicators; citric acid in water reacts with metals and change turns litmus red.
Hydrogen Ions Weak Acids - acids that partially ionise in water. The remaining molecules remain
Hydrogen gas is formed by acids as H+(aq) ions are present in acid solutions unchanged as acids. Their reactions are reversible. E.g. CH3COOH, H2CO3, H3PO4
- This means when a solid/gas acid dissolved in water, they produce H+ ions in it H3PO4(aq) ⇌ 3H+(aq) + PO42-(aq)
Chemical eqation: HCl(s) water HCl(aq) Weak acids react slowly with metals than strong acids – hydrogen gas bubbles are
Ionic Equation: HCl(s) water H+(aq) + Cl-(aq) produced slowly.
*Note that for ionic equation only aqueous solutions are ionised*
- However when dissolved in organic solutions, they don’t show acidic properties Comparing Strong and Weak Acids with Concentrated and Dilute Acids
When metals react with acids, only the hydrogen ions react with metals, e.g.: CONCENTRATION STRENGTH
Chemical equation: 2Na(s) + 2HCl(aq) → 2NaCl(aq) + H2(g) Is the amount of solute (acids or alkalis) Is how much ions can be disassociated
Ionic equation: 2Na(s) + 2H+(aq) → 2Na+(aq) + H2(g) dissolved in 1 dm3 of a solution into from acid or alkali
It can be diluted by adding more water
to solution or concentrated by adding The strength cannot be changed
Basicity of an acid is maximum number of H+ ions produced by a molecule of acid
more solute to solution
Some Acids With Their Basicity
Acids Reaction with water Basicity
Comparing 10 mol/dm3 and 0.1 mol/dm3 of hydrochloric acids and 10 mol/dm3 and
Hydrochloric acid HCl(aq) → H+(aq) + Cl-(aq) monobasic
0.1 mol/dm3 of ethanoic acids
Nitric acid HNO3(aq) → H+(aq) + NO3-(aq) monobasic
Ethanoic acid CH3COOH(aq) ⇌ H+(aq) + CH3COO-(aq) monobasic - 10 mol/dm3 of ethanoic acid solution is a concentrated solution of weak acid
Sulphuric acid +
H2SO4(aq) → 2H (aq) + SO4 (aq)2-
dibasic - 0.1 mol/dm3 of ethanoic acid solution is a dilute solution of weak acid
The fizz of drinks - 10 mol/dm3 of hydrochloric acid solution is a concentrated solution of strong acid
Soft drink tablets contains solid acid (e.g. citric acid, C6H8O7) & sodium bicarbonate - 0.1 mol/dm3 of hydrochloric acid solution is a dilute solution of strong acid
- When tablet is added to water, citric acid ionises and the H+ produced reacts with
sodium bicarbonate to produce carbon dioxide gas, making them fizz Bases and Alkalis
Bases are oxides or hydroxides of metals
Strong and Weak Acids Alkalis are bases which are soluble in water
Strong Acid - acid that completely ionises in water. Their reactions are irreversible.
E.g. H2SO4, HNO3, HCl Laboratory Alkalis
H2SO4(aq) → 2H+(aq) + SO42-(aq) - Sodium Hydroxide, NaOH
In above H2SO4 has completely been ionized in water, forming 3 kinds of particles: - Aqueous Ammonia, NH4OH
- H+ ions - Calcium Hydroxide, Ca(OH)2
- SO42- ions
- H2O molecules All alkalis produces hydroxide ions (OH-) when dissolved in water. Hydroxide ions
Strong acids react more vigorously with metals than weak acids – hydrogen gas give the properties of alkalis. They don’t behave as acids in absence of water.
bubbles are produced rapidly Alkalis are therefore substances that produce hydroxide ions, OH-(aq), in water.
Properties of Alkalis  Floor and oven cleaners contain NaOH (strong alkalis)
- Alkalis have a slippery feel  Ammonia (mild alkalis) is used in liquids to remove dirt and grease from glass

- Alkalis are hazardous Indicators and pH

 Dilute alkalis are irritants Indicators are substances that has different colours in acidic and alkaline solutions
 Concentrated alkalis are corrosive and burn skin (caustic(i.e. burning) alkalis) Common indicators:
 Litmus
- Alkalis change the colour of indicators Methyl orange
Alkalis turn common indicator litmus – red litmus to blue Thymolphtalein
The table shows the change of colours made by some indicators
- Alkalis react with acids Indicator Colour in acids colour changes at pH Colour in alkalis
The reaction is called neutralisation Phenolphtalein Colourless 9 Pink
Methyl orange Red 4 Yellow
- Alkalis react with ammonium compounds Litmus Red 7 Blue
Thymolphtalein Colourless 4 Blue
They react with heated solid ammonium compounds to produce ammonia gas
Bromothymol blue Yellow 7 Blue
(NH4)2SO4(s) + Ca(OH)2(aq) → CaSO4(aq) + 2NH3(g) + 2H2O(l)
The pH Scale
A measure of acidity or alkalinity of a solution is known as pH
- Alkalis react with solutions of metal ions
 pH 7 is neutral – in pure water
Barium sulphate, BaSO4(aq), contains Ba2+(aq) ions
 solutions of less than pH 7 are acidic. The solutions contain hydrogen ions. The
Ca(OH)2(aq) + BaSO4(aq) → Ba(OH)2(s) + CaSO4(aq)
smaller pH, the more acidic the solution is and more hydrogen ions it contains.
The solid formed is precipitate – the reaction is called precipitate reaction
 solutions of more than pH 7 are alkaline. The solution contains hydroxide ions.
The biger pH, the more alkaline the solution and more hydroxide ions it contains.
Strong and Weak Alkalis
Strong Alkalis - base that completely ionises in water to form OH-(aq) ions. Their
reactions are irreversible. E.g. NaOH, KOH, Ca(OH)2
Ca(OH)2(s) → Ca2+(aq) + 2OH-(aq)
Weak Alkalis - base that partially ionise in water. The remaining molecules remain
unchanged as base. Their reactions are reversible. E.g. NH3
NH3(g) + H2O(l) ⇌ NH4+(aq) + OH-(aq)
Uses of Alkalis
 Alkalis neutralise acids in teeth (toothpaste) and stomach (indigestion)
 Soap and detergents contain weak alkalis to dissolve grease
Measuring pH of a Solution Reaction Between Metals and Acids
1. Universal indicators For example, reaction of sodium with hydrochloric acid
It can be in paper or solution form. Universal paper can be dipped into a solution 2Na(s) + 2HCl(aq) → 2NaCl(aq) + H2(g)
then pH found is matched with the colour chart. It gives approximate pH value. Its ionic equation is written as:
2Na(s) + 2H+(aq) + 2Cl-(aq) → 2Na+(aq) + 2Cl-(aq) + H2(g)
2. pH meter Since 2 Cl-(aq) ions don’t change, they’re not involved in reaction. As ionic equation
A hand-held pH probe is dipped into solution and meter will show the pH digitally is used to show changes in reactions, we omit Cl-(aq) ions. So we’re left with:
or by a scale. Measures pH water in lakes, water, and streams accurately 2Na(s) + 2H+(aq) → 2Na+(aq) + H2(g)

3. pH sensor and computer Reaction Between Soluble Ionic Compounds and Acids
A probe is dipped into solution and will be sent to computer through interface e.g. Reaction of sodium hydrogencarbonate with hydrochloric acid
used to measure pH of solution. The pH reading is displayed on computer screen. NaHCO3(aq) + HCl(aq) → NaCl(aq) + CO2(g) + H2O (l)
Its ionic equation is:
pH Around Us Na+(aq) + H+(aq) + CO32-(aq) + H+(aq) + Cl-(aq) → Na+(aq) + Cl-(aq) + CO2(g) + H2O(l)
- Substances in body involved in good digestion have different pH values Since Na+(aq) and Cl-(aq) ions don’t change, we omit them, leaving:
- Blood to heart and lungs contains CO2 making blood slightly acidic H+(aq) + CO32-(aq) + H+(aq) → CO2(g) + H2O(l)
- Acids are used in food preservations (ethanoic acid to preserve vegetables; CO32-(aq) + 2H+(aq) → CO2(g) + H2O(l)
benzoic acid used in fruit juices, jams and oyster sauce)
- pH affects plant growth – some plants grow in acidic soil; some need alkaline soil Reaction Between Insoluble Ionic Compounds and Acids
- When hair is cleaned with shampoo which is alkali to dissolve grease, hair can be e.g. Reaction between iron(II) oxide and sulphuric acid
damaged unless it’s rinsed or acid conditioner is used to neutalise excess alkali FeO(s) + H2SO4(aq) → FeSO4(aq) + H2O(g)
Its ionic equation is:
Ionic Equations FeO(s) + 2H+(aq) + SO42-(aq) → Fe2+(aq) + SO42-(aq) + H2O(g)
Ionic equation is equation involving ions in aqueous solution, showing formation Note: FeO is written in full as it’s solid (although it’s an ionic compound)
and changes of ions during the reaction Since SO42-(aq) ions don’t change, we omit SO42- ions, leaving:
FeO(s) + 2H+(aq) → Fe2+(aq) + H2O(g)
Rule to make ionic equations: E.g. Reaction between calcium carbonate and hydrochloric acid
- Only formulae of ions that change is included; ions don’t change = omitted CaCO3(s) + 2HCl(aq) → CaCl2(aq) + CO2(g) + H2O(l)
- Only aqueous solutions are written as ions; liquids, solids and gases written in full Its ionic equation is:
CaCO3(s) + 2H+(aq) + 2Cl-(aq) → Ca2+(aq) + 2Cl-(aq) + CO2(g) + H2O(l)
Since 2 Cl-(aq) ions don’t change, we omit Cl- ions, leaving:
CaCO3(s) + 2H+(aq) → Ca2+(aq) + CO2(g) + H2O(l)
Reactions Producing Precipitate OXIDES
E.g. Reaction between calcium hydroxide and barium sulphate Acidic Oxide Basic Oxide Amphoteric Oxide Neutral Oxide
Ca(OH)2(aq) + BaSO4(aq) → Ba(OH)2(s) + CaSO4(aq) Oxides of non-metals, Oxides of metals, Oxides of transition Oxides that don’t
Its ionic equation is written as: usually gases which usually solid which metals, usually solid, react with either
Ca2+(aq) + 2OH-(aq) + Ba2+(aq) + SO42-(aq) → Ba(OH)2(s) + Ca2+(aq) + SO42-(aq) reacts with water to reacts with water which reacts with acids/alkalis, hence
produce acids, e.g. to produce alkalis, acids/alkalis to form do not form salts,
Since Ca2+(aq) and SO42-(aq) ions don’t change, we omit them, leaving:
CO2, NO3, P4O10, SO2 e.g. CaO, K2O, BaO salt and water, e.g. e.g. H2O, CO, NO
Ba2+(aq) + 2OH-(aq) → Ba(OH)2(s) Al2O3, FeO, PbO

Displacement Reactions 7.3 Preparation of Salts

E.g. Reactions between magnesium with zinc sulphate Soluble and Insoluble Salts
Mg(s) + ZnSO4(aq) → MgSO4(aq) + Zn(s) Soluble Insoluble
Its ionic equation is written as: All Nitrates -
Mg(s) + Zn2+(aq) + SO42-(aq) → Mg2+(aq) + SO42-(aq) + Zn(s) All Supates BaSO4, CaSO4, PbSO4
Since SO42-(aq) ions don’t change, we omit them, leaving: All Chlorides PbCl, AgCl
Mg(s) + Zn2+(aq) → Mg2+(aq) + Zn(s) Potassium, Sodium, Ammonium salts -
K2CO3, Na2CO3, NH4CO3 All Carbonates
K2O, Na2O All Oxides
Neutralization is the reaction between acid and base to form salt and water only.
Preparation of Insoluble Salts
From ionic equation, we know that the reaction only involves H+ ions from acids
Insoluble salts, e.g. BaSO4, CaSO4, PbSO4, PbCl, AgCl and most carbonates, can be
with OH- ions from alkali to form water .
prepared by reacting compound containing the wanted cation with another
E.g. NaOH + H2SO4 forms Na2SO4 + H2O
compound containing the wanted anion. This is precipitation reaction.
H2SO4(aq) + NaOH(aq)  Na2SO4(aq) + H2O(g)
E.g. Preparation of BaSO4
Ionic equation is:
First BaCl, since it contains wanted barium ion, is reacted with H2SO4, since it
H (aq) + OH-(aq)→ H2O(g)
+
contains wanted sulphate ion, to produce solid BaSO4 & aqueous KCl. BaSO4 then
Plants don’t grow well in acidic soil. Quicklime (calcium hydroxide) is added to
separated from KCl by filtration, leaving filtrate KCl & BaSO4 left on filter paper.
neutralise the acidity of soil according to equation:
Salt is washed with water to completely remove KCl & filter paper is squeezed with
Acid(aq) + Ca(OH)2(aq)  Ca(acid anion)(aq) + H2O(g)
another filter paper to dry BaSO4.

Reaction between Base and Ammonium Salts

E.g. Reaction between NaOH and NH4OH
NaOH(aq) + NH4Cl(aq)  NaCl(aq) + NH3(g) + H2O(g)
Ionic equation:
NH4+(aq) + OH-(aq) → NH3(g) + H2O(g)
Preparation of Soluble Salts Only metals like zinc and magnesium, which moderately react with dilute acids,
By Neutralization are used.
25.0cm3 acid, as standard solution, is placed in conical flask using pipette. Add few E.g. Reacting Zn with H2SO4 to prepare ZnSO4
drops of indicator & titrate with alkali from burette until indicator changes colour, Zn(s) + H2SO4(aq)  ZnSO4(aq) + H2(g)
showing all acid has just reacted. Volume of alkali added is measured. Prepare new Zn is added to H2SO4 until in excess to ensure no more H2SO4 is present. Then
25.0cm3 acid again with pipette & add same volume of alkali as before to prevent the mixture is filtered off to separate Zn from ZnSO4. The filtrate (ZnSO4) is then
excess alkali/acid because both reactant & product are aqueous. Next, the product placed in evaporating dish to evaporate most of water then it’s cooled after ZnSO4
is evaporated to dryness to obtain the salt. crystals are formed. The crystals then filtered and squeezed between filter papers
to dry.
By Reacting Insoluble Base with Acid
E.g. Reacting MgO with Acids
MgO(s) + H2SO4(aq)  MgSO4(aq) + H2O(l)
The same method as reaction of acid with metal is used, so refer to diagram and
above explanation, substituting reactants and products.
By Reacting Carbonate with Acid
E.g. Reacting CaCO3 with Acids
K2CO3(s) + H2SO4(aq)  K2SO4(aq) + CO2(g) + H2O(l)
The same process is used as reaction of acid with metal, just that carbon dioxide is
produced. Carbon dioxide can be tested by bubbling it into limewater which will
By Reacting Metal with Acid turn limewater colourless to milky.
Hydrated & Anhydrous Salt

When salts are being prepared, some water can be retained within the structure of
the salt during the crystallisation process. This affects the crystal's shape and colour.
Salts that contain water within their structure are called hydrated salts. Anhydrous
salts are those that contain no water in their structure. A common example is
copper(II) sulfate which crystallises forming the salt hydrated copper(II) sulfate, which
is blue. When it is heated, the water from its structure is removed, forming anhydrous
copper(II) sulfate, which is white.The hydrated salt has been dehydrated to form the
anhydrous salt.

This reaction can be reversed by adding water to anhydrous copper(II) sulfate:

hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water
 Water of Crystallization

Water molecules included in the structure of some salts during the crystallisation process are known
as water of crystallisation. A compound that contains water of crystallisation is called a hydrated
compound. When writing the chemical formula of hydrated compounds, the water of crystallisation is
separated from the main formula by a dot.
E.g. hydrated copper(II) sulfate is CuSO4·5H2O
Hydrated cobalt(II) chloride is CoCl2·6H2O
The formula shows the number of moles of water contained within one mole of the hydrated salt
E.g. hydrated copper(II) sulfate, CuSO4·5H2O, contains 5 moles of water in 1 mole of hydrated salt
A compound which doesn’t contain water of crystallisation is called an anhydrous compound
E.g. anhydrous copper(II) sulfate is CuSO4
Anhydrous cobalt(II) chloride is CoCl2

The conversion of anhydrous compounds to hydrated compounds is reversible by heating the

hydrated salt:
Anhydrous to hydrated salt: CuSO4 + 5H2O → CuSO4·5H2O
Hydrated to anhydrous salt (by heating):
CuSO4·5H2O → CuSO4 + 5H2O