Acids and Alkalis
Year 11
CONTENTS
• Acidity and alkalinity
• Indicators
• pH
• Acids
• General methods for making salts
• Making salts from metal oxides
• Making salts from metal carbonates
• Making salts from metals
• Making salts from alkalis – by titration
• Making insoluble salts – by precipitation
• Questions
Acids
• Sour
• Turn litmus red
• pH less than 7
• Dissolve carbonate rocks
• Corrode metals
• Conduct electricity
Acids
• Acids are substances that give H+ ions
when added to water.
• Hydrogen ion is equal to a proton.
• So acid is a proton donor
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Common Acids are:
Hydrochloric acid HCl
Sulphuric acid H2SO4
Nitric acid HNO3
Phosphoric acid H3PO4
Ethanoic acid CH3COOH
Carbonic acid H2CO3
Bases
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A substance which neutralises an acid is called a Base
• If a base dissolves in water, it is also called an
alkali
• So alkalis are bases that dissolve in water.
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Sodium hydroxide NaOH base and alkali
Potassium hydroxide KOH base and alkali
Ammonia solution NH4OH base and alkali
Copper(II) oxide CuO is a base
Magensium oxide MgO is a base
• Metal oxides and hydroxides are generally bases
• Ammonium hydroxide is a base (alkali)
Alkalis
• Bitter
• Turn litmus blue
• pH greater than 7
• Slippery
• caustic
Alkalis
• An alkali is a substance that
gives OH- ions when added to
water
• All alkalis dissolve in water.
Neutralisation reactions
• Acids and alkalis neutralise each other to form a
salt and water.
• Example:
Hydrochloric acid + Sodium hydroxide  Sodium
chloride + Water
HCl + NaOH  NaCl + H2O
• Reactions of acids and alkalis are called neutralisation
reactions
• Neutralisation reaction is expressed by:
H+ + OH-  H2O
Neutralization examples
• Indigestion is caused by too much
amounts of hydrochloric acid in the
stomach.
• It is cured by indigestion tablets.
• Indigestion tablets contain
magnesium oxide that neutralises
the excess HCl.
Neutralization examples
• Fields with acidic soil can be
improved (by neutralisation) by
adding lime.
• The lime contains calcium
hydroxide Ca(OH)2 which is an
alkali
Neutralization examples
• Lakes affected by acid rain can be
neutralized by adding lime.
Strong vs. weak
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Strong = complete dissociation
• Example, when HCl is dissolved in water, all of it becomes
H+ + Cl• HCl, H2SO4, HNO3 are strong acids
HCl  H+ + ClH2SO4  2H+ + SO4 2-
• Weak = incomplete dissociation
• Acetic acid (vinegar) also called Ethanoic acid is a weak
acid.
• CH3COOH
CH3COO- + H+
• CH3COOH, small amounts dissociate into H+ and CH3COO• CH3-COOH, H2CO3, HF are weak acids (Ethanoic acid, Carbonic
acid, Hydroiodic acid
• More dissociation  stronger acid
Strong and weak bases
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Strong bases ionises completely
NaOH  Na+ + OHNaOH, KOH, Ba(OH)2, Ca(OH)2
Weak bases ionises partially
Mg(OH)2, CuO, Al2O3, NH4OH
NH4OH
NH4+ + OHStrong acids have a small pH(1-2)
strong bases have high pH(13-14)
Neutral solutions have pH 7
Smaller the pH, stronger the acid
Larger the pH, stronger the base
Indicators
• Indicators are substances that change
colour when placed in an acid or an alkali.
• There are four important indicators:
• Methyl orange
• Phenolphthalein
• Litmus paper
• Universal indicator.
• Universal indicator (pH indicator) is the best
as it shows the pH of the solution so that we
can know how strong is the acid or base
• Colour
HCl
Methyl orange
Phenolphthalein
Litmus paper
Universal indicator
Red
Colourless
Red
Red
Pink
Blue
Purple/violet
NaOH Orange/yellow
• <— 0 1 2 3 4 5 6
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strongly
acidic
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HCl
HYDROCHLORIC
ACID
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7
weakly neutral
acidic
CH3COOH
ETHANOIC
ACID
H2O
8 9 10 11 12 13 14 —>
weakly
alkaline
strongly
alkaline
NH3
AMMONIA
NaOH
SODIUM
HYDROXIDE
Acidic solutions (acidic)
Alkaline solutions (basic)
Neutral solutions
pH < 7
pH > 7
pH = 7
Measuring pH
• Indicator Add a few drops of universal (full-range) indicator to
the solution and compare the colour against a chart.
• The colour depends on how strong is the acid or base
• pH meters Electrical/electronic devices which measure the
conductivity of the solution.
• You place the probe in any solution and read off the pH value
on a scale or digital read-out
REACTIONS OF ACIDS
• Neutralisation
Acid reacts with a base to form salt and water.
Base can be a metal oxide, metal carbonate or metal hydroxide.
Ammonium hydroxide is also a base.
• Calcium oxide + Nitric acid  Calcium nitrate + water
CaO + HNO3  Ca(NO3)2 + H2O
• Copper(II) oxide + Hydrochloric acid  Copper(II)chloride + water
CuO + HCl  CuCl2 + H2O
• Sodium hydroxide + Sulphuric acid  Sodium sulphate + water
2NaOH + H2SO4  Na2SO4 + 2H2O
• Ammonium hydroxide + hydrochloric acid 
Ammonium chloride + water
NH4OH + HCl  NH4Cl + H2O
• Acid reacts with metals to form salt and hydrogen gas
• Most metals react with dilute acids to form salt and hydrogen
• Copper, Silver, Gold etc do not react with acids because
these metals are not reactive.
• Magnesium + sulfuric acid  Magnesium sulfate + Hydrogen
Mg + H2SO4  MgSO4 + H2
• Zinc + Hydrochloric acid  Zinc chloride + Hydrogen
Zn + HCl  ZnCl2 + H2
Acids react with carbonate to form salt, carbon dioxide and water
• Sodium carbonate + Hydrochloric acid 
Sodium chloride + Carbon dioxide + water
Na2CO3 + 2HCl  NaCl + CO2 + H2O
• Copper(II) carbonate + sulphuric acid 
Copper(II) sulphate + Carbon dioxide + water
CuCO3 + H2SO4  CuSO4 + CO2 + H2O
Salts
• Salts formed from an acid and a base.
• First part of a salt is from the base (+). First part of a salt i
s normally a metal ion or ammonium ion (NH4+)
• Second part from the acid (-)
• For example, Sodium chloride is a salt.
• Sodium part comes from a base like sodium hydroxide.
• Chloride part comes from an acid (hydrochloric acid)
• Second part of the salt depends on the acid.
• Hydrochloric acid HCl - chloride Cl (-)
• Sulphuric acid H2SO4 - sulphate SO4 (2-)
• Nitric acid HNO3 - nitrate NO3 (-)
• Carbonic acid H2CO3 - carbonate CO3 (2-)
• Phosphoric acd H3PO4 - phosphate PO4 (3-)
How to make salts?
• Acid react with metal
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Example: Making Zinc sulphate.
Take sulphuric acid in a beaker. (bec. the salt is sulphate)
Add excess zinc powder and warm the mixture.
Wait until no more bubbles come out.
Cool the mixture and filter to get rid of excess Zinc.
Take the filtrate in an evaporating dish, heat and
evaporate until crystallisation point.
• Cool it in room temperature; filter to get the crystals of Zinc
sulphate. Dry the crystals using filter paper or in an oven.
From an acid and an alkali (both soluble)
• Example: When hydrochloric acid and potassium hydroxide
solution are mixed, we can make Potassium chloride salt.
• KOH (aq) + HCl (aq)  KCl (aq) + H2O (l)
• First conduct a titration to know what volume of acid and base
exactly required for neutralisation. It needs an indicator.
• Then mix acid and base in larger volume in the same ratio you
found in the titration for complete neutralisation.
This time no indicator must be used so that salt will be pure.
• Heat and evaporate the resulting solution until crystallisation point.
• Cool it and filter to get the crystals of potassium chloride.
• Dry the crystals using filter papers or in an oven.
Double decomposition or Precipitation.
( make an insoluble salt)
• This method is useful to make a salt which is insoluble in
water.
• For example to make calcium carbonate salt which is
insoluble. Mix solutions of sodium carbonate and calcium
chloride; both soluble and the calcium carbonate precipitates.
Na2CO3 (aq) + CaCl2 (aq)  CaCO3 (s) + 2NaCl (aq)
• When two soluble salt solutions when mixed, the insoluble
salt is precipitated out.
• The mixture is then filtered and the solid salt is left for
drying.
Soluble and insoluble
• Following statements help you to identify if a chemical can dissolve
in water or not.
• All compounds of SODIUM, POTASSIUM and AMMONIUM are
soluble in water.
• All NITRATE compounds are soluble.
• All CHLORIDES are soluble except Silver chloride and
Lead(II) chloride.
• All SULPHATES are soluble except Barium sulphate and
Lead(II) sulphate.
• All CARBONATES are insoluble except sodium carbonate,
potassium carbonate and ammonium carbonate.
• All metal oxides and metal hydroxides are insoluble in water except
sodium, potassium and calcium oxide/hydroxide.
Soluble or insoluble???
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Sodium carbonate Na2CO3
Soluble
Copper(II) sulphate CuSO4
Soluble
Lead(II) chloride PbCl2
Insoluble
Silver sulphate Ag2SO4
Soluble
Barium chloride BaCl2
Soluble
Ammonium sulphate (NH4)2SO4
Soluble
Iron(III) oxide Fe2O3
Insoluble
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Copper(II) hydroxide Cu(OH)2
Insoluble
Silver chloride AgCl
Insoluble
Barium sulphate BaSO4
Insoluble
Lead(II) nitrate Pb(NO3)2
Soluble
Sulphuric acid H2SO4
Soluble
Potassium chloride KCl
Soluble
Calcium sulphate CaSO4
Soluble
Magnesium carbonate MgCO3
Insoluble
• THE END