INTRODUCTION TO ACIDS, BASES AND INDICATORS

. In a school laboratory:

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

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

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

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

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

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

Bases which dissolve in water are called alkalis.

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. Anacid-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.

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

Filter the extract into a clean 100cm₃beaker.

(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.

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.

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.

Experiment:

Using commercial indicators to determine acidic, basic/alkaline and neutral solutions

Procedure

Place 5cm₃ of the solutions in the table below. Add three drops of litmus solution to each solution.

Repeat with phenolphthalein indicator, methyl orange, screened methyl orange and bromothymol blue.

The universal indicator

The universal indicator is a mixture of other indicator dyes. The indicator uses the pH scale.The pH scale shows the strength of bases and acids. The pH scale ranges from 1-14.These numbers are called pH values:

(i) pH values 1,2,3 shows a substance is strongly acid

(ii) pH values 4,5,6 shows a substance is a weakly acid

(iii) pH value 7 shows a substance is a neutral

(iv) pH values 8,9,10,11 shows a substance is a weak base/alkali.

(v) pH values 12,13,14 shows a substance is a strong base/alkali

The pH values are determined from a pH chart.

The pH chart is a multicolored paper with each colour corresponding to a pH value. i.e.

(i) Red correspond to pH 1, 2, 3 showing strongly acidic solutions.

(ii) Orange/ yellow correspond to pH 4, 5, 6 showing weakly acidic solutions.

(iii) Green correspond to pH 7 showing neutral solutions.

(iv) Blue correspond to pH 8, 9, 10, 11 showing weakly alkaline solutions.

(v)Purple/dark blue correspond to pH 12, 13, 14 showing strong alkalis.

The universal indicator is available as:

(i) Universal indicator paper/pH paper

(ii) Universal indicator solution.

When determining the pH of an unknown solution using

(i) pH paper then the pH paper is dipped into the unknown solution.

It changes/turn to a certain colour.

The new colour is marched/compared to its corresponding one on the pH chart to get the pH value.

(ii) Universal indicator solution then about 3 drops of the universal indicator solution is added into about 5cm₃ of the unknown solution in a test tube. It changes/turn to a certain colour.

The new colour is marched/compared to its corresponding one on the pH chart to get the pH value.

Experiment: To determine the pH value of some solutions

(a)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.

(b)Put about three drops of universal indicator solution or dip a portion of a piece of pH paper into each. Record the observations made in each case.

(c)Compare the colour in each solution with the colours on the pH chart provided. Determine the pH value of each solution.

NOTE

All the mineral acids Hydrochloric, sulphuric(VI) and nitric(V)acids are strong acids
Two alkalis/soluble bases, sodium hydroxide and potassium hydroxide are strong bases/alkali. Ammonia solution is a weak base/alkali. All other bases are weakly alkaline.
Pure/deionized water is a neutral solution.
Common salt/sodium chloride is a neutral salt.
When an acid and an alkali/base are mixed, the final products have pH 7 and are neutral.