The Ions in Acids & Alkalis

• Physics
• Secondary Education
• Age 12,13

04 April 2023

The Ions in Acids & Alkalis

Content objectives

• I can describe the characteristic properties of bases in terms of their reactions with (a) acids; (b) ammonium salts.
• 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.
• I can describe how to compare hydrogen ion concentration, neutrality, relative acidity, and relative alkalinity in terms of colour and pH using universal indicator paper.

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

Acids and Bases

• acids + water= hydrogen ions (H⁺) - this makes it acidic
• alkalis (soluble base) + water = hydroxide ions (OH^–) - this makes it alkaline

What is a neutralisation reaction?

• when an acid reacts with an alkali
• the H+ ions react with the OH– ions to produce water

Example

Net ionic equation

• The net ionic equation of acid-alkali neutralisations, and what leads to a neutral solution, since water has a pH of 7, is:

H+ + OH– ⟶ H2O

03 April 2023

The Ions in Acids & Alkalis

How do I know the degree of acidity and alkalinity?

How do I know the degree of acidity and alkalinity?

• The pH scale is a numerical scale that is used to show how acidic or alkaline a solution is, in other words, it is a measure of the number of ions present in the solution.

Hydrogen Ion Concentration & pH

• The more hydrogen ions, the stronger the acid, but the lower the pH.
• The more hydroxide ions in a solution, the higher the pH.
• pH is a measure of the concentration of H+ ions in solution, but they have an inverse relationship.

Logarithmic

• each change of 1 on the scale represents a change in concentration by a factor of 10

Example 1

• an acid with a pH of 3 has ten times the concentration of H+ ions than an acid of pH 4

Example 2

• An acid with a pH of 2 has 10 x 10 = 100 times the concentration of H+ ions than an acid with a pH of 4

03 April 2023

The Ions in Acids & Alkalis

The Universal indicator

• Universal indicator is a mixture of different indicators which is used to measure the pH.
• A drop is added to the solution and the colour is matched with a colour chart which indicates the pH which matches specific colours.

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.