Chemical changes Revision Notes

pH Scale and What It Shows

The pH scale runs from 0 to 14. A pH of 7 indicates a neutral solution, like pure water. Acids have pH values less than 7, and bases (alkalis) have values greater than 7.

Classifying Solutions by pH

You can classify any solution by its pH. If the value is below 7, the solution is acidic. If it's above 7, it's alkaline. Exactly 7 means the solution is neutral.

Acids and Metals

When an acid reacts with a metal more reactive than hydrogen, a salt and hydrogen gas are produced. The general equation is: Acid + Metal → Salt + Hydrogen. For example, sulfuric acid + zinc → zinc sulfate + hydrogen. Metals less reactive than hydrogen (like copper, mercury, silver, gold, platinum) generally do not react with dilute acids to produce hydrogen.

Acids and Metal Oxides/Hydroxides

Metal oxides and metal hydroxides are bases. When an acid reacts with a base, a neutralisation reaction occurs, producing a salt and water. The general equation is: Acid + Base → Salt + Water. For example, nitric acid + sodium hydroxide → sodium nitrate + water. This type of reaction is exothermic, releasing heat.

Acids and Metal Carbonates

When an acid reacts with a metal carbonate, a salt, water, and carbon dioxide gas are produced. The general equation is: Acid + Metal Carbonate → Salt + Water + Carbon Dioxide. For example, hydrochloric acid + calcium carbonate → calcium chloride + water + carbon dioxide. The carbon dioxide gas causes effervescence.

Acids as Sources of Hydrogen Ions

Acids are substances that, when dissolved in a solution (typically water), produce hydrogen ions (H⁺). These H⁺ ions are responsible for the acidic nature of the solution. For instance, hydrochloric acid (HCl) dissociates in water to form H⁺ and Cl⁻ ions. The presence of these hydrogen ions is what gives acids their characteristic properties, such as turning blue litmus paper red and reacting with carbonates to produce carbon dioxide.

Alkalis as Sources of Hydroxide Ions

Alkalis are soluble bases that, when dissolved in a solution (typically water), produce hydroxide ions (OH⁻). These OH⁻ ions are responsible for the alkaline nature of the solution. For example, sodium hydroxide (NaOH) dissociates in water to form Na⁺ and OH⁻ ions. Alkalis are typically corrosive and feel slippery to the touch. They turn red litmus paper blue and neutralise acids by reacting with the H⁺ ions.

The Role of Water in Dissociation

Both acids and alkalis require water (or another suitable solvent) to dissociate and release their respective ions. Without a solvent, a substance like hydrogen chloride gas behaves as a covalent molecule, not an acid. Similarly, solid sodium hydroxide does not exhibit alkaline properties until it is dissolved in water, allowing the hydroxide ions to become mobile and reactive.

Why Titration is Necessary for Soluble Reactants

When preparing a soluble salt from a soluble acid and a soluble base (alkali), both reactants dissolve completely in water without forming a precipitate. This means there's no visible change to indicate when the reaction is complete and neutralisation has occurred. A titration is performed to accurately determine the exact volume of acid needed to neutralise a known volume and concentration of alkali, or vice versa, ensuring a pure salt solution is produced without excess reactants.

Ensuring Purity of the Salt

The goal of using titration in this context is to ensure that the final solution contains only the salt and water after the reaction, with no excess acid or alkali. In a typical titration, an indicator is added to signal the endpoint of the reaction. For preparing a pure salt, a preliminary titration is carried out with an indicator to find the correct proportions. A second, larger scale reaction is then performed using these exact volumes but without the indicator, to avoid contaminating the pure salt with the indicator chemicals. This way, the solution can then be evaporated to crystallise the salt.