Acids, Alkalis and Indicators

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Acids, Alkalis and Indicators

• Prepared by Mdm RY Leow
• The Chinese High School

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Acids

• What causes the sour taste in these beverages?

The presence of citric acid.
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Some Common Acids

• Organic acids are found in living things.
• Which of the acids listed above are organic
  acids?
• Hydrochloric acid is an mineral acid (inorganic
  acid) and is commonly found in science
  laboratories.

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Acids

• 1 All solutions of acids contain hydrogen
  ions,H
• 2 Examples of acids commonly found in the
  laboratories.
• Hydrochloric acid (HCl)
• Sulphuric acid (H2SO4)
• Nitric acid (HNO3)
• Ethanoic acid (CH3COOH)

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Physical properties of acidic solutions

• Concentrated acids are corrosive.

Note All acids only show acidic properties in
the presence of water!
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Physical properties of acidic solutions

• Acids taste sour.
• Acids turn blue litmus paper red but have
  noeffect on red litmus paper.
• They have pH less than 7.
• They are good electrical conductors.
• All acids can only show their acidic properties
  in the presence of water.

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Chemical properties of acids

• 1. Dilute acids react with some metals (K, Na,
  Ca, Mg, Zn, Fe) to produce hydrogen gas.

Dilute acids do not react with copper, silver,
gold
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Chemical properties of acids

• 2. An acid reacts with a carbonate to produce

a salt, water and carbon dioxide.
Carbon dioxide gas is bubbled into limewater
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Chemical properties of acids
3. An acid reacts with a base to
form a salt and water only.
The reaction between an acid and a base to form a
salt and water only is called neutralisation
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Some Uses of Acids

• Hydrochloric acid
• Sulphuric acid
• Nitric acid
• Ethanoic acid
• Citric acid

• Cleaning metallic surfaces before they are
  painted or coated with other metals
• Used as an electrolyte in car batteries making
  fertilisers, and plastics
• Making fertilisers and explosives
• Preserving food and cooking
• Making health salts

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Alkalis

• Many household products such as toothpaste, soap
  and detergents contain alkalis.

Click the picture for uses of alkalis.
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Alkalis

• 1 All solutions of alkalis contain hydroxide
  ions, OH -
• 2 Examples of common alkalis found in the
  laboratories are
• Sodium hydroxide solution NaOH
• Potassium hydroxide solution KOH
• Calcium hydroxide solution Ca(OH)2
• Ammonia solution NH3.H2O

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Physical Properties of Alkaline solutions
All alkalis can only show their alkaline
properties in the presence of water.

• Strong alkalis are corrosive liquids.

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Physical Properties of alkaline solution

• Alkalis taste bitter.
• They feel soapy.
• They turn red litmus paper blue but have no
  effect on blue litmus paper.
• 4. Alkalis are good electrical conductors.
• 5. They have pH greater than 7.

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Chemical Properties of Alkalis

• 1. An alkali reacts with an ammonium salt to
  produce

e.g. NaOH NH4Cl

1. An alkali neutralises an acid to form

e.g. KOH HCl
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Alkalis
Alkalis Common names
Uses Sodium hydroxide caustic soda
Making soap, washing powder, drain
cleaner Potassium hydroxide caustic potash
Making dyes for fabric and
paint removers Calcium hydroxide slaked lime
Reducing acidity of soil and
making mortar Aqueous ammonia Making
fertilisers and (Ammonia solution) household
cleaning liquids
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Indicators

• Indicators are dyes which change colour with the
  acidity or alkalinity of a solution.
• Some common indicators are (litmus, methyl
  orange, phenolphthalein, universal indicator)

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pH scale

• The pH scale is a range of numbers from 0 to 14,
  which indicates how acidic or alkaline a
  substance is.

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pH scale

• Other indicator such as pH paper can be used
• A pH meter is an electronic device that can
  be use to measure the acidity or the alkalinity
  of solutions very effectively.

(a) pH meter being used to measure the pH of
water in swimming pool
(b) pH meter is used to measure pH of the soil
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Neutralisation

• 1 Neutralisation occurs when an acid neutralises
  a base or an alkali to form salt and water.
• (An indicator can be use to find the pH 7 of the
  solution)
• acid base salt water
• H2SO4 MgO MgSO4 H2O
• acid alkali salt water
• HNO3 KOH KNO3 H2O

• Acid and alkali are reactants.
• salt and water are products.

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Neutralisation

• 2 Example of everyday uses of neutralisation
• Shampoo (a mild alkali) and conditioner (a mild
  acid)
• Toothpaste (alkaline) neutralises acids produced
  by bacteria
• wasp sting (alkaline) is treated with vinegar to
  relieve pain
• Excess acid in the soil is neutralised by adding
  slaked lime (calcium hydroxide)

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Salts

• Salts are metal compounds that can be made from
  acids.

All acids contain hydrogen. When the hydrogen
in an acid is replaced by a metal, we get a
salt.
nitric acid
nitrates
hydrochloric acid
chlorides
sulphuric acid
sulphates
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Preparation of Salts

• 1 The four methods of preparing salts are
• (a) precipitation of an insoluble salt (mixing)
• HCl AgNO3 AgCl HNO3
• (b) neutralisation of an acid with a base
  (excess insoluble oxide or hydroxide)
• CuO H2SO4 CuSO4 H2O
• (c) neutralisation of an acid with an alkali
  (soluble hydroxide)
• HCl KOH KCl H2O
• (d) reaction of an acid with excess insoluble
  carbonate
• CuCO3 2HCl CuCl2 H2O
  CO2

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Flowchart to decide which method to use
Start
(a) PRECIPITATION METHOD Mix solutions of two
soluble salts to precipitate an insoluble
salt. eg NaCl AgNO3 AgCl NaNO3
No
Is the salt soluble?
Yes
React (b) acid with metal oxide or (c) acid with
alkali or(d) acid with metal carbonate
Are all reactants soluble in water?
(c) Mix acid with alkali to produce salt and
water eg HCl NaOH NaCl H2O
Yes
No
(b)Mix acid and excess metal oxide or (d) excess
metal carbonate to produce salt? eg CuO H2SO4
CuSO4 H2O MgCO3 H2SO4
MgSO4 H2OCO2
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Salts
Soluble Insoluble All sodium, potassium and
ammonium salts All nitrates All chlorides
except lead(II) chloride and silver
chloride. All sulphates except lead(II)
sulphate, barium sulphate and
calcium sulphate sodium carbonate, All
other carbonates potassium carbonate and ammonium
carbonate