Chemistry Notes Acid Bases and Indicator

Chemistry Notes Acid Bases and Indicator

Chemistry Notes Acid Bases and Indicator

Introduction To Acids, Bases And Indicator

1.In a school laboratory:

(i)An acid may be defined as a substance that tum litmus red.

(ii)A base may be defined as a substance that tum litmus blue.

Litmus is a lichen found mainly in West Africa. It changes its colour depending on Whether the solution it is in, is basic/alkaline or acidic. It is thus able to identify/show whether another substance is an acid, base or neutral.

(iii)An indicator is a substance that shows whether another substance is a base/alkaline,acid or neutral.

2.Common naturally occuring acids include:

1. Name of acid: Citric acid

Occurrence: Found in ripe citrus fruits like passion fruit/oranges/lemon

2.Name of acid:Tartaric acid

Occurrence: Found in grapes/baking powder/health salts

3.Name of acid: Lactic acid

Occurrence: Found in sour milk

4.Name of acid: Ethanoic acid

Occurrence: Found in vinegar

5.Name of acid: Methanoic acid

Occurrence: Present in ants, bees stings

6.Name of acid: Carbonic acid

Occurrence: Used in preservation of fizzy drinks like coke, Lemonade, Fanta

7.Name of acid: Butanoic acid

Occurrence: Present in cheese

8.Name of acid: Tannic acid

Occurrence: Present in tea

3.Most commonly used acids found in a school laboratory are not naturally occurring. They are manufactured. They are called mineral acids.

Common mineral acids include:

Name of mineral acid: Hydrochloric acid (HCl)

Common use: Used to clean/pickling surface of metals

:Is found in the stomach of mammals/human beings

Name of mineral acid: Sulphuric(VI) acid (HZSO4)

Common use: Used as acid in car battery, making battery, making fertilizers

Name of mineral acid: Nitric(V)acid (HNO3)

Common use: Used in making fertilizers and explosives

4.Mineral acids are manufactured to very high concentration. They are corrosive (causes painful wounds on contact with the skin) and attack/reacts With garments/clothes/metals.

In a school laboratory, they are mainly used when added a lot of Water. This is called diluting. Diluting ensures the concentration of the acid is safely low.

5. Bases are opposite of acids. Most bases do not dissolve in water.

Bases which dissolve in water are called alkalis.

Common alkalis include:

Name of alkali: Sodium hydroxide (NaOH)

Common uses: Making soaps and detergents

Name of alkali: Potassium hydroxide(KOH)

Common uses: Making soaps and detergents

Name of alkali: Ammonia solution(NH4OH)

Common uses: Making fertilizers, softening hard Water

Common bases (Which are not alkali) include:

Name of base: Magnesium oxide/hydroxide

Common name: Anti acid to treat indigestion

Name of base: Calcium oxide

Name of base: Making cement and neutralizing soil acidity

6. Indicators are useful in identifying substances which look-alike. An acid-base indicator is a substance used to identify Whether another substance is alkaline or acidic.

An acid-base indicator works by changing to different colours in neutral, acidic and alkaline solutions/dissolved in Water.

Experiment:To prepare simple acid-base indicator

Procedure

(a)Place some flowers petals in a mortar. Crush them using a pestle. Add a little sand to assist in crushing.

Add about 5cm3 of propanone/ethanol and carefully continue grinding.

Add more 5cm3 of propanone/ethanol and continue until there is enough extract in the mortar.

Filter the extract into a clean 100cm3 beaker.

(b)Place 5cm3 of filtered wood ash, soap solution, ammonia solution, sodium hydroxide, hydrochloric acid, distilled water, sulphuric(VI)acid, sour milk, sodium chloride, toothpaste and calcium hydroxide into separate test tubes.

(c)Put about three drops of the extract in (a)to each test tube in (b). Record the observations made in each case.

Sample observations

 

The plant extract is able to differentiate between solutions by their nature. It is changing to a similar colour for similar solutions.

(i)Since lemon juice is a known acid, then sulphuric(VI)and hydrochloric acids are similar in nature with lemon juice because the indicator show similar colours. They are acidic in nature.

(ii)Since sodium hydroxide is a known base/alkali, then the green colour of indicator shows an alkaline/basic solution.

(iii) Since pure water is neutral,then the orange colour of indicator shows neutral solutions.

7. In a school laboratory, commercial indicators are used. A commercial indicator is cheap, readily available and easy to store. Common indicators include: Litmus, phenolphthalein, methyl orange, screened methyl orange, bromothymol blue.

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