Post on 13-May-2015
Chapter Three
An acid is a substance that produces hydrogen ions when dissolved in water.
Examples of acids are hydrochloric acid (HCl), sulphuric acid (H2SO4), nitric acid (HNO3) and ethanoic acid (CH3COOH).
Sulphuric acid is used in detergents, fertilisers and car batteries.
Ethanoic acid is used in vinegar and making adhesives.
Hydrochloric acid is used in leather processing and cleaning materials.
Acids have a sour taste.
Acids dissolve in water to form solutions that conducts 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 turn chalky and a white precipitate would be formed.
An alkali is a metal oxide or hydroxide that is soluble 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 hydroxide ions.
Alkalis turn red litmus blue.
Alkalis have pH value more than 7.
Alkalis react with metals, ammonium salts and acids
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 water to form solutions that conducts electricity.
Acids turn blue litmus red.
Acids have pH value less than 7.
Acids react with metals, carbonates and bases.
Alkalis have a bitter taste and soapy feel.
Alkalis dissolve in water to produce hydroxide ions.
Alkalis turn red litmus blue.
Alkalis have pH value more than 7.
Alkalis react with metals, ammonium salts and acids.
Litmus pH value Reactions
Test for Ammonia Place a piece of moist red litmus paper in the
solution
If ammonia is present, the moist red litmus paper will turn blue.
Indicator Colour in Acids pH value Colour in Alkalis
Methyl orange Red 3 – 5 Yellow
Screened Methyl orange Violet 3 – 5 Green
Litmus paper Red 5 – 8 Blue
Bromothymol blue Yellow 6 – 8 Blue
Phenolphtalein Colourless 8 – 10 Pink
Red cabbage juice Pink Nil Green
Hydrangea flower Blue Nil Red
When an acid and alkali react together, they neutralise each other. Their properties are removed.
The pH value of the acid is increased while the pH value of the alkali is decreased. The neutral mixture has a pH value of 7.
Water is formed together with a new substance called a salt.
A salt is a ionic compound formed when a metallic or ammonium ion neutralises hydrogen ions of an acid.
Calcium chloride
CaO + 2 HCl CaCl2 + H2O
Zinc sulphate
ZnO + H2SO4 ZnSO4 + H2O
Sodium nitrate
NaOH + HNO3 NaNO3 + H2O