Acids, Bases and Salts

Key Definitions

An acid is a substance that can donate a proton (H+) to another substance during a chemical reaction.
Strong acids, such as hydrochloric acid, fully ionise in solution, releasing all of their H+ ions into the solution.
Weak acids, such as ethanoic acid, partially ionise in solution, releasing some of their H+ ions into the solution.
Bases are substances that can accept a proton (H+) from another substance during a chemical reaction.
Alkalis are bases that are soluble in water.
Neutralisation is the reaction of an acid and a base forming a salt and water. The H+ ions from the acid combine with the OH- ions from the base to form water.
Salts are ionic compounds produced when an acid is neutralised by a base.

Properties of Acids and Bases

An acid turns blue litmus paper red and reacts with metals to form a salt and hydrogen gas.
A base turns red litmus paper blue and may react with acids to form a salt and water.
Alkalis also turn red litmus paper blue and react with acids to form a salt and water.

Methods to Make Salts

From an acid and base (neutralisation reaction): Acid and base react together to produce a salt and water.
From an acid and a metal: Metal reacts with acid to produce a salt and hydrogen gas.
From an acid and a carbonate: Carbonate reacts with acid to produce a salt, water and carbon dioxide.

Acid-Base Titrations

Titration is a method used to determine the concentration of an unknown solution (often an acid or base) by reacting it with a solution of known concentration.
It involves the accurate measurement of volumes.
Indicators such as litmus, phenolphthalein, or methyl orange, change colour to show when the reaction is complete, i.e., when all the acid has reacted with the alkali.

Universal Indicator and pH Scale

The pH scale measures how acidic or alkaline a substance is. It ranges from 0 (very acidic) to 14 (very alkaline).
Universal indicator is a mixture of dyes which changes colour depending on the pH of the solution it is added to.
Each pH value has a different colour associated with it. For example, neutral solutions (pH 7) turn the universal indicator green.