Unit – 14
Acids and Alkalis
Acids
• The word acid comes from the
Latin word ACIDUS which means
‘sour’.
• All acids taste sour.
• Malic acid is found in apples
• Tartaric acid is found in grapes
• Yoghurt contains lactic acid
• Citrus fruits contains citric acid
Formic acid
(methanoic
acid)
Common acids in laboratory
•The most commonly used acids are
Acids
1. Ethanoic aciduses
or acetic acid or
Sulfuric acid In car batteries
vinegar
Nitric acid
2. HydrochloricMaking
acid fertilizers and
explosives.
3. Nitric acid and
Ethanoic Preserving
4. Sulfuric acid food and
acid or cooking.
vinegar or
acetic acid
Properties of acids
• All acids have sour taste.
• Acids turn blue litmus paper red. Red
litmus paper remains unchanged.
• Most acids are corrosives.
Alkalis
• Alkalis belongs to a larger class of
chemicals known as Bases
• Alkalis are bases that are soluble in water.
• Alkalis are present in soap, cleaning
products and indigestion tablets.
Common laboratory alkalis
Alkali uses
Sodium hydroxide Making soap
Potassium hydroxide Used in making
alkaline batteries.
Calcium hydroxide Reducing soil acidity
(lime)
Ammonium Making fertilizers
hydroxide. and cleaning liquids.
Properties of Alkalis
• All alkalis have a bitter taste.
• Alkalis turn red litmus paper blue. Blue
litmus paper remains unchanged.
• Alkalis have a soapy feel.
• Alkalis are corrosive.
Acids and alkalis are corrosive. That is
they can burn skin and clothes.
Testing for acids and alkalis.
• We use Indicators to test for acids and
alkalis.
• Indicators are substances that are one
colour in an acid and change to another
colour in an alkali.
• Eg: Red and Blue litmus paper, pH paper
and Universal Indicator.
Litmus indicator
• Litmus is a dye obtained from plants
• Used in paper or solution form.
The acids turns the blue litmus paper to red.
The alkalis turn the red litmus paper to blue.
pH Scale
• The pH value describes how acidic or
alkaline a substance is.
• pH scale is a range of values from 0 to14.
• Acids have a pH value less than 7.
• Alkalis have a pH value greater than7.
• Substance of pH 7 is not an acid or an
alkali.
• They are neutral.
Universal Indicator
• Universal indicator gives different colours
when added to solutions of different pH
values.
pH scale
Strong acid Weak acid Weak alkali Strong alkali
Hydrochloric acid (pH about 1) is a strong acid.
Vinegar(pH about 3) is a weak acid.
The lower the pH value, the stronger the acid.
Sodium hydroxide (pH about 13) is a strong alkali, ammonia
solution(pH about 11) is a weak alkali.
The higher the pH value , the more alkaline it is.
pH and the body
• pH of saliva = 6.2
• pH of blood = 6.5
• pH of stomach = 1.5 Hydrochloric acid
• pH of small
intestine = 8.4
Neutralisation
• The chemical reaction where an acid is
mixed with an alkali to form salt and water
is called neutralisation.
• The word equation is
Acid + alkali salt + water
Hydrochloric acid + sodium hydroxide sodium chloride + water.
