7. Acids, Bases and Salts (Part 1) (1/3) (Cambridge IGCSE Chemistry 0620 for 2023, 2024 & 2025)

Acids are substances releasing hydrogen ions in water, resulting in a pH less than 7. Bases release hydroxide ions in water, leading to a pH greater than 7. A pH of 7 is neutral. The presence of hydrogen ions makes a solution acidic, while hydroxide ions make it alkaline.

Acids react with metals to produce a salt and hydrogen gas (e.g., hydrochloric acid with zinc). They react with bases in a neutralization reaction to form a salt and water (e.g., sulfuric acid with potassium hydroxide). Acids also react with carbonates to yield a salt, water, and carbon dioxide (e.g., nitric acid with calcium carbonate).

Common indicators like litmus, thymolphthalein, and methyl orange change color in the presence of acids or alkalies. Acids turn blue litmus red, keep thymolphthalein colorless, and turn methyl orange red. Alkalies turn red litmus blue, thymolphthalein blue, and methyl orange yellow.

Bases are typically metal oxides or hydroxides. Alkalies are bases that are soluble in water and produce hydroxide ions. All alkalies are bases, but not all bases are alkalies, as only those that dissolve in water are considered alkalies.

Bases react with acids to form a salt and water (e.g., sodium hydroxide with hydrochloric acid). They also react with ammonium salts to produce a salt, water, and ammonia gas (e.g., potassium hydroxide with ammonium sulfate).

In terms of proton transfer, acids donate protons (H+ ions), and bases accept protons. A hydrogen atom losing its electron becomes an H+ ion, which is essentially a proton. For example, HCl donates a proton to NH3.

Strong acids (e.g., hydrochloric acid) completely dissociate into ions in aqueous solutions, indicated by a single arrow in their chemical equations. Weak acids (e.g., ethanoic acid) only partially dissociate, shown by a double arrow.

Universal indicator changes color across the pH spectrum: red/orange/yellow for acids, green for neutral, and blue/purple for alkalies. A lower pH signifies a higher concentration of hydrogen ions and stronger acidity. A higher pH indicates a higher concentration of hydroxide ions and stronger alkalinity.

The neutralization reaction between an acid and an alkali produces water and a salt. The acid provides hydrogen ions, the alkali provides hydroxide ions, and they combine to form neutral water. This reaction effectively cancels out the acidic and basic properties of the solution.