Study Chemistry notes on Acids, Bases and Salts for O-Level. Useful for revision, homework, and exam preparation in Zimbabwe.
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Chemistry
Acids, Bases and Salts
Notes
Focus Area
- Distinguish acids and alkalis by chemical and physical tests
- Distinguish between strong and weak acids and alkalis
- Derive the respective ionic equations for chemical reactions
- Distinguish between the 4 types of oxides and their properties
- Understand alkalinity and acidity of a solution through the colour change of a pH indicator solution
- Describe the correct procedures involved to produce a soluble or insoluble salt or highlight inappropriate means of the salt production
Acid
1. An acid is a substance that forms hydrogen $\left(\mathrm{H}^{+}\right)$ions as the only positive ion when dissolved in water.
2. There are two types of acids: mineral acid (or inorganic acid) and organic acid.
a. Mineral acids are acids that are derived from one or more mineral elements or inorganic matter and can normally be prepared in the laboratory. Phosphoric acid $\left(\mathrm{H}_{3} \mathrm{PO}_{4}\right)$, sulfuric acid $\left(\mathrm{H}_{2} \mathrm{SO}_{4}\right)$, hydrochloric acid (HCl), carbonic acid $\left(\mathrm{H}_{2} \mathrm{CO}_{3}\right)$ and nitric acid $\left(\mathrm{HNO}_{3}\right)$ are examples of mineral acids.
b. Organic acids are obtained from plants and animals and are mainly naturally occurring organic compounds with acidic properties. They include malic acid $\left(\mathrm{C}_{4} \mathrm{H}_{6} \mathrm{O}_{5}\right)$ which is found in apples, ethanoic acid $\left(\mathrm{C}_{2} \mathrm{H}_{4} \mathrm{O}_{2}\right)$ that can be found in vinegar and citric acid $\left(\mathrm{C}_{6} \mathrm{H}_{8} \mathrm{O}_{7}\right)$ that is present in citrus fruits like lime and lemon.
3. Strength of an acid refers to the extent to which an acid molecule dissociates or ionises in an aqueous solution to form ions.
a. Strong acids are acids that are completely dissociated in water to produce a high concentration of hydrogen ions. Hence, strong acids ionise completely. Strong acids have a pH in the region of about 1. Sulfuric acid, nitric acid and hydrochloric acid are examples of strong acids.
\[
\mathrm{HCl}(\mathrm{aq}) \rightarrow \mathrm{H}^{+}(\mathrm{aq})+\mathrm{Cl}^{-}(\mathrm{aq})
\]
(dilute hydrochloric acid contains only hydrogen and chloride ions; no HCl molecules are present)
\[
\mathrm{H}_{3} \mathrm{PO}_{4}(\mathrm{aq}) \rightarrow 3 \mathrm{H}^{+}(\mathrm{aq})+\mathrm{PO}_{4}{ }^{3-}(\mathrm{aq})
\]
b. Weak acids are acids that are partially dissociated in …