Acids, Alkalis and Indicators Prepared by Mdm RY Leow The Chinese High School

Acids What causes the sour taste in these beverages? The presence of citric acid.

Some Common Acids Organic acids are found in living things. Which of the acids listed above are organic acids? Hydrochloric acid is an mineral acid (inorganic acid) and is commonly found in science laboratories.

Acids 1 All solutions of acids contain hydrogen ions,H + 2 Examples of acids commonly found in the laboratories. Hydrochloric acid (HCl) Sulphuric acid (H 2 SO 4 ) Nitric acid (HNO 3 ) Ethanoic acid (CH 3 COOH)

Physical properties of acidic solutions Concentrated acids are corrosive. Note: All acids only show acidic properties in the presence of water!

Physical properties of acidic solutions 1.Acids taste sour. 2.Acids turn blue litmus paper red but have no effect on red litmus paper. 3.They have pH less than 7. 4.They are good electrical conductors. *All acids can only show their acidic properties in the presence of water.

Chemical properties of acids 1.Dilute acids react with some metals ( K, Na, Ca, Mg, Zn, Fe ) to produce hydrogen gas. metal + acid salt + hydrogen *Dilute acids do not react with copper, silver, gold eg Mg + 2HCl MgCl 2 + H 2 Mg + H 2 SO 4 MgSO 4 + H 2

Chemical properties of acids 2. An acid reacts with a carbonate to produce 2HCl + CaCO 3 CO 2 + CaCl 2 + H 2 O Carbon dioxide gas is bubbled into limewater White precipitate is formed acid + carbonate carbon dioxide gas + salt + water a salt, water and carbon dioxide.

Chemical properties of acids 3. An acid reacts with a base to form a salt and water only. The reaction between an acid and a base to form a salt and water only is called neutralisation acid + base salt + water HCl + NaOH NaCl + H 2 O HNO 3 + KOH KNO 3 + H 2 O H 2 SO 4 + Ca(OH) 2 CaSO 4 + 2H 2 O H 2 SO 4 + CuO CuSO 4 + H 2 O

Some Uses of Acids Hydrochloric acid Sulphuric acid Nitric acid Ethanoic acid Citric acid Cleaning metallic surfaces before they are painted or coated with other metals Used as an electrolyte in car batteries making fertilisers, and plastics Making fertilisers and explosives Preserving food and cooking Making health salts

Alkalis Many household products such as toothpaste, soap and detergents contain alkalis. Click the picture for uses of alkalis.

Alkalis 1All solutions of alkalis contain hydroxide ions, OH - 2Examples of common alkalis found in the laboratories are Sodium hydroxide solution NaOH Potassium hydroxide solution KOH Calcium hydroxide solution Ca(OH) 2 Ammonia solution NH 3. H 2 O

Physical Properties of Alkaline solutions Strong alkalis are corrosive liquids. *All alkalis can only show their alkaline properties in the presence of water.

Physical Properties of alkaline solution 1.Alkalis taste bitter. 2.They feel soapy. 3.They turn red litmus paper blue but have no effect on blue litmus paper. 4. Alkalis are good electrical conductors. 5. They have pH greater than 7.

NaCl + H 2 O + NH 3 Chemical Properties of Alkalis 1. An alkali reacts with an ammonium salt to produce 2.An alkali neutralises an acid to form a salt and water. alkali + acid salt + water ammonia gas, salt and water. alkali + ammonium salt salt + water + ammonia gas KCl + H 2 O e.g. NaOH + NH 4 Cl e.g. KOH + HCl

Alkalis Alkalis Common names Uses Sodium hydroxide caustic soda Making soap, washing powder, drain cleaner Potassium hydroxide caustic potash Making dyes for fabric and paint removers Calcium hydroxide slaked lime Reducing acidity of soil and making mortar Aqueous ammonia Making fertilisers and (Ammonia solution) household cleaning liquids

Indicators Indicators are dyes which change colour with the acidity or alkalinity of a solution. Some common indicators are (litmus, methyl orange, phenolphthalein, universal indicator)

pH scale The pH scale is a range of numbers from 0 to 14, which indicates how acidic or alkaline a substance is.

pH scale Other indicator such as pH paper can be used A pH meter is an electronic device that can be use to measure the acidity or the alkalinity of solutions very effectively. (a) pH meter being used to measure the pH of water in swimming pool (b) pH meter is used to measure pH of the soil

Neutralisation 1Neutralisation occurs when an acid neutralises a base or an alkali to form salt and water. (An indicator can be use to find the pH 7 of the solution) *acid + base salt + water H 2 SO 4 + MgO MgSO 4 + H 2 O *acid + alkali salt + water HNO 3 + KOH KNO 3 + H 2 O Acid and alkali are reactants. salt and water are products.

Neutralisation 2Example of everyday uses of neutralisation Shampoo (a mild alkali) and conditioner (a mild acid) Toothpaste (alkaline) neutralises acids produced by bacteria wasp sting (alkaline) is treated with vinegar to relieve pain Excess acid in the soil is neutralised by adding slaked lime (calcium hydroxide)

Salts Salts are metal compounds that can be made from acids. * All acids contain hydrogen. *When the hydrogen in an acid is replaced by a metal, we get a salt. nitric acid hydrochloric acid sulphuric acid nitrates chlorides sulphates

Preparation of Salts 1The four methods of preparing salts are: (a) precipitation of an insoluble salt (mixing) HCl + AgNO 3 AgCl + HNO 3 (b) neutralisation of an acid with a base (excess insoluble oxide or hydroxide) CuO + H 2 SO 4 CuSO 4 + H 2 O (c) neutralisation of an acid with an alkali (soluble hydroxide) HCl + KOH KCl + H 2 O (d) reaction of an acid with excess insoluble carbonate CuCO 3 + 2HCl CuCl 2 + H 2 O + CO 2

Flowchart to decide which method to use Start Is the salt soluble? (a) PRECIPITATION METHOD Mix solutions of two soluble salts to precipitate an insoluble salt. eg NaCl + AgNO 3 AgCl + NaNO 3 React (b) acid with metal oxide or (c) acid with alkali or(d) acid with metal carbonate Are all reactants soluble in water? (c) Mix acid with alkali to produce salt and water eg HCl + NaOH NaCl + H 2 O (b)Mix acid and excess metal oxide or (d) excess metal carbonate to produce salt? eg CuO + H 2 SO 4 CuSO 4 + H 2 O MgCO 3 + H 2 SO 4 MgSO 4 +H 2 O+CO 2 No Yes No Yes

Salts SolubleInsoluble All sodium, potassium and ammonium salts All nitrates All chlorides exceptlead(II) chloride and silver chloride. All sulphates exceptlead(II) sulphate, barium sulphate and calcium sulphate sodium carbonate, All other carbonates potassium carbonate and ammonium carbonate