Acids and Alkalis

Grade 10

Acids• Sour

• Turn litmus red

• pH less than 7

•Dissolve carbonate rocks

•Corrode metals

•Conduct electricity

Acids• Acids are substances that give H+ ions

when added to water.

Alkalis•Bitter

•Turn litmus blue

•pH greater than 7

•Slippery

• caustic

Alkalis•An alkali is a substance that gives OH- ions when added to water

Neutralizations reactions

• Acids and alkalis neutralize each other to form a salt• Example: HCl + NaOH NaCl + H2O

• Reactions of acids and alkalis are called neutralization reactions

Neutralization examples

• Indigestion is caused by too much amounts of hydrochloric acid in the stomach.

• It is cured by indigestion tablets.

• Indigestion tablets contain magnesium oxide that neutralises the excess HCl.

Neutralization examples

• Fields with acidic soil can be improved by adding lime.

• The lime contains calcium hydroxide Ca(OH)2 which is an alkali

Neutralization examples

• Lakes affected by acid rain can be neutralized by adding lime.

Strong vs. weak

• Strong = complete dissociation• Example, when HCl is dissolved in water, all of it

becomes H+ + Cl-

• Weak = incomplete dissociation• Acetic acid (vinegar) is a weak acid

• CH3COOH, small amounts dissociate into H+ and CH3COO-)

• More dissociation = stronger acid

Indicators

• Indicators are substances that change colour when placed in an acid or an alkali.

• There are four important indicators:• Methyl orange• Phenolphthalein• Litmus paper• Universal indicator.

Indicators Practical

• Aim: find the colour change for different indicators

when placed in acids or alkali.

• Materials: HCl, NaOH, phenolphtnalein, litmus paper, universal indicator and methyl

orange.

• Procedure: put 3 cm3 of HCl in a test tube.

Add few drops of methyl orange.

record your observation.Methyl orange

Phenolphthalein

Litmus paper

Universal indicator

HCl

NaOH