Chemical changes Revision Notes

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.

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.

Litmus Indicator

Litmus is a well-known indicator used to distinguish acids from alkalis. In acidic solutions, litmus turns red. In alkaline solutions, it turns blue. For most practical school purposes, it is not used to distinguish neutral solutions, as the litmus colour is more subtle near neutrality.

Methyl Orange Indicator

Methyl orange is used in titrations and gives clear colour changes: it is red in acidic solutions and yellow in alkaline solutions. It also appears yellow at neutral pH. It does not turn orange in a genuinely neutral (pH 7) solution; orange is only seen within its pH transition region between strong acid and neutral.

Phenolphthalein Indicator

Phenolphthalein is colourless in acidic and neutral solutions, and it becomes pink in the presence of an alkali. Its sudden colour change makes it a useful indicator when testing for alkaline conditions, especially with bases.

Summary Table

Litmus: red in acid, blue in alkali; Methyl orange: red in acid, yellow in alkali or neutral; Phenolphthalein: colourless in acid and neutral, pink in alkali. Remember these changes when identifying unknown solutions.