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