Acids, Bases, Salts III• Describe how to test for hydrogen ion concentration and hence relative acidity using Universal indicator paper and the pH scale.• Classify oxides as acidic, basic or amphoteric, based on metallic/non- metallic character.
pH scale• pH is a measure of how acidic or alkaline a solution in water is. The pH scale is numbered from 0 to 14.• pH less than 7 ⇒ acidic• pH greater than 7 ⇒ alkaline• pH of 7 ⇒ neutral
pH scaleHydrochloric acid Sodium Lemon Canned Pure Toothpaste hydroxide juice fizzy drink water blood detergent vinegar 1 2 3 4 5 6 7 8 9 10 11 12 14 Hydrogen ions Hydroxide ions More acidic neutral More alkaline
Testing with indicators• Indicators are compounds that have different colours in acidic and alkaline solutions.• A solution may be tested with indicators to ﬁnd out whether it is acidic or alkaline.Egs. of common indicators :litmus, Universal Indicator, methyl orange, screened methyl orange
Colour changes of some indicatorsLitmus 7 acids turn blue bases turn red litmus red litmus blue
Colour changes of some indicators Methyl orange 4 Screened methyl orange 4Phenolphthalein 9
Colour changes of some indicators1 2 3 4 5 6 7 8 9 10 11 12 13 14 Red Orange Yellow Green Blue Indigo Violet ROY G BIV
Oxides are formed when an element combine with oxygen. OXIDES Element + O2 The element can be a metal or a non-metal.
OXIDES Metal + O2 Non-Metal + O2 most most are areAmphoteric Basic oxides Acidic Neutral oxides oxides oxides• ZnO • CO2 • H2O• lead(II) oxide PbO • SO2 • CO• aluminium oxide Al2O3 • NO2 & NO • N2O
Basic Oxides• react with acids to produce salt and water only; they neutralise acidsEg. zinc oxide + hydrochloric acid ZnO(s) + 2HCl(aq) → ZnCl2(aq) + H2O(l)• are oxides of metals ; are solids at room temperature;• Most are insoluble in water; – basic oxides that dissolve in water are called alkalis. Eg : sodium oxide, potassium oxide dissolve in water to form aq. sodium hydroxide & aq. potassium hydroxide respectively
Acidic Oxides• are oxides of non-metals ; are usually gases at room temperature• react with water to produce acids ;Eg. SO2(g) + H2O(l) → H2SO3(aq) SO3(g) + H2O(l) → H2SO4(aq)• react with alkalis to produce salt and water only.Eg. SO2(g) + 2NaOH(aq) → Na2SO3(aq) + H2O(l)
Amphoteric Oxides• are metal oxides that can behave as both acids and basesEgs: zinc oxide; Aluminium oxide; Lead(II)oxide• react with acids to form salt and water, thereby acting as a base:Eg. ZnO(s) + H2SO4(aq) → ZnSO4(aq) + H2O(l)
Amphoteric Oxides• are metal oxides that can behave as both acids and bases• they also react with an alkali (will dissolve in alkali) to form a salt and water, thereby acting as an acid:Eg. zinc + sodium → sodium zincate oxide hydroxide + water aluminium + sodium → sodium aluminate oxide hydroxide + water lead(II) + sodium → sodium plumbate oxide hydroxide + water
Neutral Oxides• non-metal oxides that - do not react with water to form acids (hence not an acidic oxide), and - do not react with acids to form salts (hence not a basic oxide).• Examples: water (H2O); carbon monoxide (CO); nitric oxide (NO)