The pH Scale & Neutralisation

Ion Concentration: The pH value is inversely related to the concentration of hydrogen ions. Every decrease of 1 on the pH scale represents a ten-fold increase in the concentration of $H^+$ ions in the solution.

Water Equilibrium: Pure water is neutral because it contains equal concentrations of $H^+$ and $OH^-$ ions. When an acid is added, the balance shifts toward $H^+$; when a base is added, it shifts toward $OH^-$.

Logarithmic Nature: Because the scale is logarithmic, a solution with pH 3 is 100 times more acidic than a solution with pH 5. This allows a wide range of ion concentrations to be expressed using simple numbers.

General Equation: When an acid reacts with a base, they undergo a neutralisation reaction to produce a salt and water. The general word equation is: $\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water}$

The Ionic Equation: At the molecular level, neutralisation is the reaction between hydrogen ions and hydroxide ions to form water molecules. This is represented by the net ionic equation: $H^+(aq) + OH^-(aq) \rightarrow H_2O(l)$

Salt Formation: The 'salt' produced consists of the positive ion from the base and the negative ion from the acid. For example, reacting hydrochloric acid with sodium hydroxide produces sodium chloride.

Strong vs. Weak: Acid strength refers to the degree of ionisation (dissociation) in water, not the total amount of acid present. Strong acids like sulfuric acid ionise completely, while weak acids like citric acid only ionise partially.

Base vs. Alkali: All alkalis are bases, but not all bases are alkalis. Bases like copper(II) oxide are insoluble in water, whereas alkalis like sodium hydroxide dissolve to form alkaline solutions.

Indicators: These are chemical substances that change color depending on the pH of the solution. Universal Indicator is a mixture of several indicators that displays a rainbow of colors to estimate pH across the full 0-14 range.

pH Probes and Meters: For precise scientific work, digital pH meters are used. These devices measure the electrical potential of the solution and provide a numerical pH value that is much more accurate than visual color matching.

Titration Indicators: In neutralisation titrations, single-color-change indicators (like phenolphthalein) are preferred over Universal Indicator because they provide a sharp, distinct 'end-point' when neutralisation is complete.

Naming Salts: Always derive the first part of the salt's name from the metal in the base and the second part from the acid. Hydrochloric acid makes chlorides, sulfuric acid makes sulfates, and nitric acid makes nitrates.

State Symbols: In ionic equations, always include state symbols. Ions in solution are $(aq)$, while the water produced in neutralisation is $(l)$. This is a frequent requirement for full marks in chemistry exams.

Predicting pH: If a question asks for the pH after a reaction, check if the reactants were in exact stoichiometric proportions. If there is excess acid, the final pH will be below 7; if there is excess alkali, it will be above 7.