Acids And Alkali

Chapter Three

 An acid is a substance that produces hydrogenions when dissolved in water. Examples of acids are hydrochloric acid (HCl),sulphuric acid (H2SO4), nitric acid (HNO3) andethanoic acid (CH3COOH). Sulphuric acid is used in detergents, fertilisers andcar batteries. Ethanoic acid is used in vinegar and makingadhesives. Hydrochloric acid is used in leather processing andcleaning materials.

 Acids have a sour taste. Acids dissolve in water to form solutions thatconducts electricity. Acids turn blue litmus red. Acids have pH value less than 7. Acids react with metals, carbonates and bases.

Metal reactions Acid + Reactive Metal Salt + HydrogenCarbonate reactions Acid + Carbonate Salt + Water + Carbon dioxideBase reactions Acid + Base Salt + Water

 Test for Hydrogen Place a lighted splint at the mouth of a test tube. A pop sound is heard and the splint is extinguished.Hydrogen is thus present. Test for Carbon dioxide Bubble the gas through limewater. If carbon dioxide is present, the limewater will turnchalky and a white precipitate would be formed.

 An alkali is a metal oxide or hydroxide that issoluble in water. Examples : sodium hydroxide (NaOH) potassium hydroxide (KOH) calcium hydroxide [Ca(OH)2] barium hydroxide [Ba(OH)2] aqueous ammonia (NH3).

 Ammonia solution Fertilisers Window cleaning solutions Calcium oxide To neutralise acidic soil Iron, concrete and cement Magnesium hydroxide Toothpaste to neutralise acid on teeth Antacids to relieve indigestion Sodium hydroxide Soaps and detergents Industrial-cleaning detergents

 Alkalis have a bitter taste and soapy feel. Alkalis dissolve in water to produce hydroxideions. Alkalis turn red litmus blue. Alkalis have pH value more than 7. Alkalis react with metals, ammonium salts andacids

Metal reactions Alkali + Salt A Metal hydroxide + Salt BAmmonium salt reactions Alkali + Ammonium Ammonia + Water + SaltBase reactions Alkali + Acid Salt + Water

Taste Solubility Acids have a sour taste. Acids dissolve in waterto form solutions thatconducts electricity. Acids turn blue litmusred. Acids have pH value lessthan 7. Acids react with metals,carbonates and bases. Alkalis have a bittertaste and soapy feel. Alkalis dissolve in waterto produce hydroxideions. Alkalis turn red litmusblue. Alkalis have pH valuemore than 7. Alkalis react withmetals, ammonium saltsand acids.Litmus pH value Reactions

Test for Ammonia Place a piece of moist red litmus paper in thesolution If ammonia is present, the moist red litmus paperwill turn blue.

Indicator Colour in Acids pH value Colour in AlkalisMethyl orange Red 3 – 5 YellowScreened Methyl orange Violet 3 – 5 GreenLitmus paper Red 5 – 8 BlueBromothymol blue Yellow 6 – 8 BluePhenolphtalein Colourless 8 – 10 PinkRed cabbage juice Pink Nil GreenHydrangea flower Blue Nil Red

 When an acid and alkali react together, theyneutralise each other. Their properties areremoved. The pH value of the acid is increased while the pHvalue of the alkali is decreased. The neutralmixture has a pH value of 7. Water is formed together with a new substancecalled a salt.

 A salt is a ionic compound formed when a metallicor ammonium ion neutralises hydrogen ions of anacid. Calcium chloride CaO + 2 HCl CaCl2 + H2O Zinc sulphate ZnO + H2SO4 ZnSO4 + H2O Sodium nitrate NaOH + HNO3 NaNO3 + H2O

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Acids And Alkali

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