Page 5 - Understanding NCERT Geography 10th

5. How is the concentration of hydronium ions (H 3 O ) affected when a solution of an acid is
diluted?
Ans. On dilution, the concentration of hydronium ions decreases.
6. How is the concentration of hydroxide ions (OH ) affected when excess base is dissolved in
a solution of sodium hydroxide?
Ans. When excess base is dissolved in a solution of sodium hydroxide, the concentration of hydroxide
ions increases.
2.3 HOW STRONG ARE ACID OR BASE SOLUTIONS?

l A universal indicator is a mixture of several indicators. It shows different colours at different
concentrations of hydrogen ions in a solution. – +
l An easier way to measure the strength of an acid or base solution was worked out by the
Danish biochemist, S. Sorensen in 1909.
l A scale for measuring hydrogen ion concentration in a solution known as pH scale (In German
‘p’ stands for ‘potenz’ meaning power). pH means Potenz of Hydrogen ion concentration.
l The scale ranges from 0 to 14 and the characteristics of the scale are: . .
Features of this DIGI SMART Book.
Generally paper impregnated with the universal indicator is used for measuring pH.

 Acids have a pH value of less than 7.

 Higher the concentration of hydronium ion concentration of an acid, lower is the pH
Based on the NCERT Textbook
value.
Goyal Brothers Prakashan
 Neutral solutions (e.g., water) have a pH value of 7.
 Alkalis have a pH value of more than 7.
–
 As the pH value increases from 7 to 14, there is an increase in OH ion concentration
in the solution, i.e., increase in the strength of alkali.
Neutral
Acidic nature increasing Basic nature increasing
0 7 14
H +
OH –
+
+
Increase in H ion concentration Decrease in H ion concentration
[Courtesy: NCERT]
Study Notes Activity 2.11: To test the pH values of given solutions and identify their nature.
on each topic/ Observations:
Solution
subtopic/activity S.No. Saliva (before meal) Colour of pH paper pH value (approx.) Nature of substance
1
7
Green
Neutral
published in the 2 Saliva (after meal) Yellow 5 Acidic
NCERT textbook 3 Lemon juice Orange 2 Acidic
along with 4 Colourless aerated drink Lime Green 6 Slightly acidic
separate VIDEO 5 Carrot juice Lime Green 6 Slightly acidic
EXPLANATION 6 Coffee Yellow 5 Acidic
Beige
Acidic
Tomato juice
7
4
for each item 8 Tap water Green 7 Neutral
9 1M NaOH Purple 14 Basic
10 1M HCl Red 1 Acidic
+
–
l Strong acids and strong bases produce a greater number of H and OH ions respectively. While, weak acids and weak
bases produce a comparatively smaller number of corresponding ions.
30 Science Class X

Comprehensive
Explanation of
each and every
Intext Question and
Questions given
in the exercise in
the book published
by NCERT with
separate VIDEO
EXPLANATION for
each question