Nature of Bases and Alkalis

• Physics
• Secondary Education
• Age 12,13

03 April 2023

Properties of Bases and Alkalis

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 OH^– ions.

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

Indicators

• 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

Synthetic Indicators

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

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.

Synthetic Indicators

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

03 April 2023

Properties of Bases and Alkalis

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

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

NaOH (s) → Na+ (aq) + OH- (aq)

Example of alkali

Typical reactions of bases

Bases + acids

• neutralisation reaction occurs
• Acid + Base → Salt + Water

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.

NH4Cl + NaOH →NaCl + H2O + NH3

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

Example 

Test for Ammonium Ion (NH4+)

Procedure

• 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.