Summary of Neutralization Reactions and pH

## Introduction Neutralisation is a common chemical reaction that happens when an **acid** meets a **base** (also called an alkali when it dissolves in water). The acid and base react to make a **salt** and **water**, and this reaction changes the pH of the mixture. Neutralisation helps in everyday life, such as treating heartburn, protecting teeth, and cleaning rust. > **Definition:** Neutralisation is a chemical reaction where an acid and a base (alkali) react to form a salt and water. ## What are acids and bases? - **Acid**: A substance that tastes sour and has a low pH (like lemon juice). Acids can be strong or weak. - **Base (alkali)**: A substance that can neutralise an acid. Alkalis dissolve in water and have a higher pH (like baking soda solution). > **Definition:** An alkali is a base that dissolves in water. ### pH and neutralisation - pH is a number that tells how acidic or alkaline a solution is. - pH scale goes from 0 to 14: low numbers (0–6) are acidic, 7 is neutral (pure water), and high numbers (8–14) are alkaline. - During neutralisation the pH moves toward 7: an acid (low pH) becomes less acidic; an alkali (high pH) becomes less alkaline. ## Word equation for neutralisation - General word equation: Acid + Base → Salt + Water Examples from daily life: 1. Stomach indigestion remedy (antacid): - Word equation: magnesium hydroxide + hydrochloric acid → magnesium chloride + water - In symbols using chemical notation (mhchem): $$\ce{Mg(OH)2 + 2HCl -> MgCl2 + 2H2O}$$ 2. Toothpaste neutralising mouth acid: - Toothpaste may contain magnesium hydroxide or calcium hydroxide to help neutralise acids produced by food. - Word idea: calcium hydroxide + acid in mouth → salt + water 3. Rust removal: - Sulfuric acid reacts with iron oxide (rust) to make iron sulfate and water. - Word equation: sulfuric acid + iron oxide → iron sulfate + water - Chemical form (mhchem): $$\ce{H2SO4 + Fe2O3 -> Fe2(SO4)3 + H2O}$$ > **Definition:** A salt is a compound formed when the hydrogen in an acid is replaced by a metal or another positive ion during neutralisation. ## Everyday examples and why they matter - Indigestion tablets (antacids) contain mild bases like magnesium hydroxide. They neutralise some stomach acid to ease heartburn. They are kept only slightly alkaline (about pH 9) so they do not stop the stomach from working. - Toothpaste ingredients such as magnesium hydroxide or calcium hydroxide reduce mouth acidity and help prevent tooth decay. - Baking soda (sodium bicarbonate) has been used as a home remedy to neutralise bee stings because bee venom contains acids; vinegar (a weak acid) was used for wasp stings when people thought they were alkaline. - Factories spray calcium hydroxide into acidic waste gases to neutralise them and reduce acid rain. Fun fact: A single honey bee sting contains acids such as methanoic acid and can cause a lot of pain, but most of the pain comes from many other substances in the sting, not just the acid. ## Safety and common-sense rules - Don’t use very strong alkalis like sodium hydroxide (pH 13) to neutralise stomach acid — they are too corrosive and can harm tissues. - Wear gloves and eye protection when using strong acids or bases, for example when removing rust with acid. ## Simple experiment idea: Which indigestion tablet works best? 1. Prepare small samples of stomach-acid-like solution (dilute hydrochloric acid) in identical cups. 2. Measure the starting pH of each cup (or use universal indicator paper). 3. Drop one type of indigestion tablet into each cup and stir gently. 4. After the tablet dissolves, measure the final pH. 5. Compare how much the pH changed and how long it took. The tablet that raises pH fastest and most is the most effective at neutralising in this test. ## Comparison table | Thing | Type | Why used in neutralisation | |---|---:|---| | Milk of Magnesia | Mild alkali (magnesium hydroxide) | Neutralises excess stomach acid safely | | Bakin