<h3>Acids and bases</h3><p><strong>Acids</strong> are substances that taste sour, turn blue litmus paper red, and react with metals and carbonates. Examples include the citric acid in lemons and the acetic acid in vinegar.</p><p><strong>Bases</strong> are substances that turn red litmus paper blue and feel slippery. A base that dissolves in water is called an <strong>alkali</strong>, such as sodium hydroxide.</p><h3>The pH scale and indicators</h3><p>The <strong>pH scale</strong> runs from 0 to 14:</p><ul><li><strong>pH below 7</strong> — acidic.</li><li><strong>pH equal to 7</strong> — neutral (like pure water).</li><li><strong>pH above 7</strong> — basic / alkaline.</li></ul><p><strong>Indicators</strong> like litmus and universal indicator change colour to reveal the pH of a solution.</p><h3>Neutralisation and salts</h3><p>When an acid reacts with a base, the two cancel out in a reaction called <strong>neutralisation</strong>, producing a <strong>salt</strong> and water:</p><p><code>Acid + Base → Salt + Water</code></p><div class="callout"><strong>Kenyan context:</strong> Farmers in tea-growing areas with acidic soils add agricultural lime (a base) to neutralise the acidity so crops grow better. At home, an antacid tablet neutralises excess acid in the stomach — the same chemistry that helps a farmer's soil settles an upset stomach.</div>