The pH scale is a numerical representation of the concentration of hydrogen ions in a solution, typically ranging from 0 to 14. A pH of 7 is considered neutral (like pure water), values below 7 are acidic, and values above 7 are alkaline.
The scale is logarithmic, meaning each whole number change on the pH scale represents a ten-fold change in ion concentration. For example, a solution with pH 3 is ten times more acidic than a solution with pH 4, and 100 times more acidic than pH 5.
Measurement of pH can be achieved using Universal Indicator, which provides a spectrum of colors, or more precisely using a pH meter, which measures the electrical voltage generated by the ions in the solution.
Neutralization is the chemical reaction between an acid and a base that results in the formation of a salt and water. The general word equation is: .
At the molecular level, the hydrogen ions () from the acid react with the hydroxide ions () from the alkali to form water molecules. The ionic equation for this process is: .
The specific salt produced depends on the acid used: Hydrochloric acid forms chlorides, Sulfuric acid forms sulfates, and Nitric acid forms nitrates. The positive ion of the salt comes from the base (e.g., Sodium from Sodium Hydroxide).
The strength of an acid refers to its degree of dissociation (ionization) in water. Strong acids (like ) dissociate completely into ions, whereas Weak acids (like ethanoic acid) only partially dissociate, creating an equilibrium between the molecules and the ions.
Because strong acids release all their available ions, they have a much higher concentration of hydrogen ions and a lower pH compared to weak acids of the same molar concentration.
Weak acid dissociation is a reversible process, often represented by an equilibrium arrow (). The equilibrium typically lies far to the left, meaning most of the acid remains as intact molecules.
| Feature | Strength | Concentration |
|---|---|---|
| Definition | Degree of ionization in water | Amount of acid/base per unit volume |
| Terms | Strong vs. Weak | Concentrated vs. Dilute |
| Nature | Fixed property of the chemical | Variable based on how much water is added |
| Example | is always a strong acid | can be 0.1 mol/dm³ (dilute) or 10 mol/dm³ (concentrated) |
Indicator Selection: Always remember that Universal Indicator is unsuitable for titrations because it changes color gradually. For titrations, use a synthetic indicator like phenolphthalein or methyl orange which has a 'sharp' end-point.
Salt Naming: To determine the name of a salt, combine the metal name from the base with the suffix derived from the acid (e.g., Potassium Hydroxide + Sulfuric Acid Potassium Sulfate).
pH Calculations: If a question mentions a pH change from 4 to 2, recognize that the concentration has increased by 100 times (), not just doubled.
State Symbols: In neutralization equations, water is always for liquid, while the acid, alkali, and resulting salt are usually for aqueous.
