- Describe the meanings of the terms acid and alkali in terms of the ions they produce in aqueous solution and their effects on Universal Indicator.
- Describe qualitatively the difference between strong and weak acids in terms of the extent of ionisation
- Describe how to test hydrogen ion concentration and hence relative acidity using Universal Indicator and the pH scale.
Acid is a substance that dissociates in water to produce hydrogen (H+) ions – the only positive ion produced. Note that not all acids contain hydrogen and not all substances that contain hydrogen are acids (e.g. NH3, CH4). To dissociate means that a molecule is split into its constituent particles, atoms or ions, reversibly.
Due to the presence of the H+ ions, acid has a sour taste, turns blue litmus red and has a pH of less than 7.
The strength of an acid is dependent on the extent the acid dissociate in water and hence the amount of H+ produced in the water.
When a strong acid is added into water, all of its molecules will dissociate in water. On the other hand, for a weak acid, only some of the acid molecules, when added into water, will dissociate into its sub-components.
An example of the strong and weak acids that we often talk about are shown below.
However, a strong acid may not necessary have a high concentration in water and conversely a concentrated acid may not be a strong acid.
A concentrated acid contain a large number of hydrogen ions in a fix volume while a dilute acid contains a small number of hydrogen ions in a fixed volume.
A base is a substance which can neutralize an acid, but may or may not be soluble in water.
Specifically, an alkali is a type of base, which is soluble in water. When dissolved in water, an alkali dissociates to release the hydroxide ion (OH-) – the only negative ion produced. It has a bitter taste, turns red litmus blue and has a pH of more than 7.
Similar to acids, the strength of an alkali refers to the extent that the alkali molecules dissociate in water. Hence, a strong alkali is one which have all of its molecules dissociated in water while a weak alkali is one which only have some of its molecules dissociated in water.
An example of the strong and weak alkalis that we often talk about are shown below.
The strength of an acid or base can also be determined using a pH scale.
A pH scale is a measure of the hydrogen ion concentration which spans from 0 to 14 with the middle point (pH 7) being neutral (neither acidic or basic). This is represented be a change or difference in colour observed when an acid or alkali is added to an pH indicator. A pH indicator is a weak acid that exist as natural dyes which indicate the concentration of H+ ions in a solution via a 2 phase-color change.
In order to make use of the different colours and pH properties of different pH indicators, a universal indicator is created. A universal indicator is a pH indicator composed of a solution of several compounds that exhibits several smooth colour changes over a pH value range from 1 to 14, which allows us to determine the identity (acid or base) and strength of a solution.
Any solution with a pH number greater than 7 is considered to be a base and any others with a pH number less than 7 is considered an acid. 0 is the strongest acid, containing the highest amount of H+ ions in solution, and 14 is the strongest base, with the highest amount of OH- in a solution. Weaker acids or bases have a pH value which is greater than 0 or smaller than 14, respectively, but not equal to 7.