Acids And Alkalis

Acids and Alkalis Things you must know about this topic!

1.Acids Definition : Acids are substances that produces hydrogen ions as the only positive ions in water. Properties ▪Acid + Metal = Salt + Hydrogen gas ➔Eg. Mg(s) + 2HCl(aq) = MgCl (aq) + H (g) ➔Hydrogen Test ◦Dissolve Magnesium Ribbon in dilute Hydrochloric Acid. Lit a burning splint and place it at the top of the tube. Test to see if there is a pop sound. The sound indicates the presence of the gas Hydrogen. ▪Acid + Carbonate = Salt + Carbon Dioxide + Water ➔E.g CaCO (s) + 2HCl(aq) = CaCl (aq) + CO (g) + H O(l) ➔Carbon Dioxide Test ◦Dissolve Calcium Carbonate in Hydrochloric Acid. Bubble the gas produces through limewater. Check if a limewater turns milky. A white precipitate should form in the limewater should Carbon Dioxide be produced. ▪Acid + Metal Oxide/Hydroxide = Salt + Water ➔E.g CuO(s) + H SO (aq) = CuSO (aq) + H 0(l) ▪Acids change the colours of indicators ➔Acids turn blue litmus paper red. ➔They have a pH<7 ◦The lower the pH, the more acidic the solution. Uses ▪Strong acids used to make useful products such as fertilisers, detergents and paints. ▪Rust removal ▪Preservation of food – Green vegetables are preserved in vinegar, which is a solution of ethanoic acid. Note: Hydrogen ions are produced when acids dissolve in water. Acids only behave as acids

1.Bases Metal oxides and hydroxides are bases. Properties ▪Acid + base = salt + water - Neutralisation ➔Acid are neutralised in this reaction and its acidic properties have been destroyed. ◦E.g Hcl(aq) + Na OH(aq) = NaCl(aq) + H O(l) Most bases are insoluble in water. Bases that can be dissolved in water are known as alkalis. ▪Hydroxide ions has the formula OH . It is produced when alkalis dissolve in water. ➔When acids react with alkalis, the hydrogen ions and hydroxic ions combine together to form water. ◦H (aq) + OH (aq) = H O(l)- Neutralisation Reaction ▪Alkali + Ammonium salt = Ammonia + water + salt ➔E.g NH Cl(s) + NaOH(aq) = NaCl(aq) + NH (g) + H O(l) ➔Ammonia test ◦Dissolve ammonium chloride and sodium hydroxide together. A pungent smell will be released should ammonia gas be produced. The salt will also turn red litmus paper blue. ▪Alkali + Salt (Metal A) = Metal Hydroxide + Salt (Metal B) ➔E.g 2NaOH(aq) + CuSO (aq) = Cu(OH) (s) + Na SO (aq)

➔Metal hydroxide test ◦Dissolve Copper(II) Sulphate in Sodium Hydoxide. A coloured precipitate (blue in this case) will appear if the metal hydroxide is produced. ▪Alkalis change the colour of indicators. ➔Alkalis turn red litmus paper blue and they have a pH>7. The higher the pH, the more alkaline the solution. Uses ▪Neutralise acids. ➔ E.g Toothpaste (magnesium Hydroxide) The alkaline in toothpaste neutralises acids on our teeth produced by bacteria when they feed on sugars in our food. ➔E.g Medicine. Excess acids in our stomach causes gastric. The medicine for treating this includes alkali and carbonates which neutralises some acid. ▪Dissolve dirt and grease. ➔E.g Soaps and detergents (sodium hydroxide) FYI, pH and agriculture. Most food crops will not grow well if the soil is too acidic or too alkaline. Soils become acidic due to excessive use of acidic fertiliser and acid rain. Farmers neutralise excess acidity by adding calcium hydroxide and this is often known as liming the soil. Exercises Hydrochoric acid + Zinc metal = Sulphuric acid + Magnesium metal = Potassium carbonate + Nitric acid = Sulphuric acid + Magnesium carbonate = Zinc oxide + Sulphuric acid = Zinc hydroxide + Nitric acid = Sodium hydroxide + Ammonium Nitrate = Sodium hydroxide + Copper sulphate = Calcium hydroxide + Iron chloride =