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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