Document page
chemistry igcse acapprinted 91 · PDF 91
145 pagesrevision 46934
printed 91 · PDF 91
Page image rendered from the stored file at 110 dpi; it sharpens as you zoom. Outlines are placed from each region’s stored position on the page (612 × 792 pt).
readable
Source of this text: embedded text layer. The better of the embedded text and OCR is kept.
91
Ammonium Salts :
Ammonium salts decompose when gently heated with an alkali. Despite ammonia being a weak
base, its high volatility allows it to be readily displaced from the ammonium salt by another alkali,
resulting in the formation of a salt, water, and ammonia gas.
For example:
(NH₄)₂SO₄ + 2KOH → K₂SO₄ + 2H₂O + 2NH₃
This reaction is commonly used to test for ammonium ions (NH₄⁺). To perform the test, alkali is
added to the sample, followed by gentle heating. Ammonia gas, if released, can be detected by
holding damp red litmus paper near the reaction; the paper will turn blue, indicating the presence
of ammonia gas.
Neutralization Reactions
A neutralization reaction occurs when an acid and a base react together. Acids release hydrogen
ions (H⁺) in solution, while bases (or alkalis) release hydroxide ions (OH⁻). When combined, these
ions react to form water, which has a pH of 7, resulting in a neutral solution. This is the general
ionic equation for any acid-base neutralization:
H⁺ (aq) + OH⁻ (aq) → H₂O (l)
Non-Neutralization Reactions with Acids:
Not every reaction involving an acid is a neutralization. For instance, when a metal reacts with an
acid, a salt is produced without forming water. Such reactions do not meet the strict definition of
neutralization, as they don’t involve H⁺ and OH⁻ ions producing water.
The pH Scale and Hydrogen Ion Concentration
Understanding the pH Scale:
The pH scale, ranging from 1 to 14, indicates whether a solution is acidic, alkaline, or neutral.
Acids have pH values below 7, with stronger acids near 0, while alkalis have pH values above 7,
with strong alkalis near 14. The strength of an acid or alkali can be classified as follows:
Strong acids: pH 0-2
Weak acids: pH 3-6
Neutral solutions: pH 7
Weak alkalis: pH 8-11
Strong alkalis: pH 12-14
Logarithmic Nature of the pH Scale:
The pH scale is logarithmic, so each change of 1 in pH represents a tenfold change in H⁺