Acid and bases Bases

Prior knowledge

What happened the tooth? Our teeth is composed of calcium carbonate. Recall: Acids react with metal carbonates to give salt, water and carbon dioxide.

What will we learn today? What is a base? What are alkalis? Physical properties of alkalis? Chemical properties of alkalis? Characteristics and properties of the Universal indicator.

Bases A base is a substance that reacts with an acid to form salt and water only. General equation ( acid + base) Base + acid  Salt + Water Recall: Metal oxides and metal hydroxides are examples of bases. An alkali is a base that is soluble in water This is the main focus

Examples of bases Bases (metal oxides and hydroxides) Bases that are soluble in water (alkalis) Bases that are insoluble in water Sodium oxide, Na2O Magnesium oxide, MgO Potassium oxide, K2O Magnesium hydroxide, Mg(OH)2 Sodium hydroxide, NaOH Zinc oxide, ZnO Potassium hydroxide, KOH Copper(II) oxide, CuO Ammonia solution, NH3(aq)

Physical Properties of alkalis Bitter taste Soapy feel Turn red litmus paper blue. Turn Universal indicator Blue/violet Alkali

Chemical properties of alkalis (I) Alkalis react with acids to form a salt and water. This reaction is known as neutralisation. Example NaOH(aq) + HCl(aq)  NaCl(aq) + H2O(l) H+(aq) +OH-(aq) H2O(l) Sodium hydroxide Hydrochloric acid Sodium chloride Water

Chemical properties of alkalis (II) Alkalis produce hydroxide ions when dissolved in water. Example NaOH(aq)  Na+(aq) + OH-(aq) Sodium hydroxide Sodium ions Hydroxide ions

Chemical properties of alkalis (III) Alkalis, when heated with ammonium salts, releases ammonia gas. Example NaOH(aq) + NH4Cl(s) NaCl(aq) +H2O(l) +NH3(g) Heat

Chemical properties of alkalis (IV) Alkalis react with some salt solution to form precipitates of metal hydroxides. Example 2NaOH(aq) + FeSO4(aq)  Fe(OH)2(s) +Na2SO4(aq)

Strong Acid vs Weak Acid

Strong acid Strong acid is one that undergoes complete dissociation to form ions in water. In a solution of strong acid, all the acid molecules dissociate to become ions in water. Examples Sulfuric acid, Hydrochloric acid, Nitric acid

Weak acid Weak acid is one that undergoes partial dissociation to form H+ ions in water. In a solution of a weak acid, few acid molecules dissociate to become ions in water. (Most of the acid molecules remain as acid molecules in water.) Examples Ethanoic acid, Citric acid

The pH scale The pH scale is used to illustrate if a solution is acidic, neutral or alkaline. The pH scale ranges from 0 to 14. The lower the pH value of a solution the more acidic it is. The higher the pH value of a solution the higher is its alkalinity.

Understanding the scale Neutral Strong acids e.g. HCl weak alkalis e.g. NH3(aq) 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Acidity increases <<<<<<<<<<<<< Alkalinity increases >>>>>>>>>>>>>>> Thinking point What is the pH range for acidic solutions? What is the pH range for alkaline solutions? What is the pH/ pH range for neutral solutions? What can be a possible value of a strong acid and weak alkali?

Universal indicator An universal indicator measures the pH of a solution. A different pH would result in a different colour change for the universal indicator. yellow Bluish-green 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Red Orange blue Purple Green

A coloured version

What have we learnt today? What is a base? The chemical properties of bases. The physical properties of bases. The neutralisation process. Characteristics and properties of the universal indicators.