Page 15 - 8-Sci-6 ACIDS, BASES/ALKALIS AND SALTS

6. ACIDS, BASES/ALKALIS AND SALTS eLearn.Punjab

You will observe that different shades of colour appear on each strip of pH paper. By
comparing the colours with the chart provided with the pH paper you can find the pH of
different solutions. Strong acids have pH value 0 to 2. pH of weak acids is in between 3 and
6. pH of strong alkalis is 12 to 14.

Activity 6.6

Material required: Universal indicator paper, dilute NaOH, dilute NH OH,

dilute HCl, 4 dilute H SO , vinegar, distilled water

Procedure:

• Take 1cm3 of dilute HCl, dilute H2SO4, dilute CH3COOH (vinegar), dilute NaOH
distilled water in different test tubes.

• Add 1.5 cm3 of distilled water.

• Dip separate universal indicator papers in each tube and match the colour

with colour given on the strip.

• Note the observations.

Sample Colour of univeral indicator paper pH of the solution

Dilute HCl

Dilute H2SO4
Dilute CH3COOH
Dilute NaOH

Dilute NH4OH
Dilute H2O

6.4.1 pH and its Range (0 –14) in Aqueous Medium

Pure water ionizes very slightly into hydrogen (H+) and hydroxide (OH–) ions. However,
the concentrations of hydrogen ions (H+) and hydroxide ions (OH–) in pure water are
equal. Hydrogen ion concentration increases, when acids are dissolved in water. Alkalis
on dissolving in water decrease the concentration of hydrogen ions in water as compared
to hydroxide ion. The greater the concentration of hydrogen ions (H+) in a solution, the
stronger the acid it is. The lesser the concentration of hydrogen ions as compared to
hydroxide ions in a solution, the stronger the alkali it is.
Hence, the scale which is used to measure the strength of an acid or alkali in an aqueous
solution is based on the concentration of hydrogen ions (H+) which is termed as pH.

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