Nature of Bases and Alkalis

03 April 2023
Properties of Bases and Alkalis
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Slide 1: Tekstslide
PhysicsSecondary EducationAge 12,13

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03 April 2023
Properties of Bases and Alkalis

Slide 1 - Tekstslide

Content objectives
  • I can describe acids in terms of their effect on (a) litmus (b) thymolphthalein (c) methyl orange.
  • I can describe the characteristic properties of bases in terms of their reactions with (a) acids; (b) ammonium salts.
  • I can describe alkalis in terms of their effect on (a) litmus (b) thymolphthalein (c) methyl orange.
  • I can state that bases are oxides or hydroxides of metals and that alkalis are soluble bases.
  • I can state that aqueous solutions of acids contain H+ ions and aqueous solutions of alkalis contain OHions.

Slide 2 - Tekstslide

RECAP: Properties of Acids
  • pH values of below 7
  • sour taste (when edible) 
  • corrosive
  • can neutralise a base
  • form a salt and water
  • acids + water = forms hydrogen ions (H+)
  • presence of H+ ions = acidic solution

Slide 3 - Tekstslide

  • Two colour indicators are used to distinguish between acids and alkalis
  • Many plants contain substances that can act as indicators and the most common one is litmus which is extracted from lichens
  • Synthetic indicators are organic compounds that are sensitive to changes in acidity and appear in different colours in acids and alkalis

Slide 4 - Tekstslide

Synthetic Indicators
  • thymolphthalein 
  • methyl orange 
  • these two are frequently used in acid-alkali titrations

Slide 5 - Tekstslide

Synthetic Indicators
  • used to show the endpoint in titrations as they have a very sharp
  • very sharp change of colour when an acid has been neutralised by alkali and vice-versa
  • Litmus is not suitable for titrations as the colour change is not sharp and it goes through a purple transition colour in neutral solutions making it difficult to determine an endpoint.

Slide 6 - Tekstslide

Synthetic Indicators
  • Litmus is very useful as an indicator paper and comes in red and blue versions, for dipping into solutions or testing gases

Slide 7 - Tekstslide

Slide 8 - Tekstslide

03 April 2023
Properties of Bases and Alkalis

Slide 9 - Tekstslide

Properties of Bases and Alkalis
  • pH values of above 7
  • A water-soluble base is referred to as an alkali.
  • bitter taste (almost all are NOT edible) 
  • In basic (alkaline) conditions: red litmus paper turns blue methyl orange indicator turns yellow; thymolphthalein indicator turns blue

Slide 10 - Tekstslide

Properties of Bases and Alkalis
  • Bases can neutralise acid, forming salt and water.
  • Bases are usually oxides or hydroxides of metals.
  • When alkalis are added to water, they form negative hydroxide ions (OH)
  • presence of OH- ions = alkali solution

Slide 11 - Tekstslide

NaOH (s) → Na+ (aq) + OH- (aq)
Example of alkali

Slide 12 - Tekstslide

Typical reactions of bases
Bases + acids
  • neutralisation reaction occurs
  • Acid + Base → Salt + Water

Slide 13 - Tekstslide

Slide 14 - Tekstslide

Typical reactions of bases
Alkalis and ammonium salts
  • Ammonium salts undergo decomposition when warmed with an alkali.
  • Even though ammonia is itself a weak base, it is very volatile and can easily be displaced from the salt by another alkali.
  • A salt, water and ammonia ar e produced.

Slide 15 - Tekstslide

NH4Cl + NaOH →NaCl + H2O + NH3

This reaction is used as a chemical test to confirm the presence of the ammonium ion (NH4+).


Slide 16 - Tekstslide

Test for Ammonium Ion (NH4+)
  • Alkali is added to the substance with gentle warming followed by the test for ammonia gas using damp red litmus paper.
  • The damp litmus paper will turn from red to blue if ammonia is present.

Slide 17 - Tekstslide