Saturday, September 18, 2021

Acids, Bases and Salts | Class 10 CBSE | Web Notes | Part 3 | How strong are Acid or Base solutions?

ACIDS, BASES AND SALTS


HOW STRONG ARE ACID OR BASE SOLUTIONS?


Amount of H+ ions present in a solution can be measured by using a universal indicator. It is a mixture of several indicators. It shows different colours at different concentrations of hydrogen ions.
A scale for measuring H+ ion concentration in a solution is called pH scale (p = ‘potenz’ in German = power).

pH is measured from 0 (very acidic) to 14 (very alkaline).

Higher the H+ ion concentration, lower is the pH value.

pH of a neutral solution is 7.

Values less than 7 represent an acidic solution.

Values more than 7 represent an alkaline solution.

pH value from 7 to 14 represents an increase in OH ion concentration, i.e., increase in the strength of alkali.

Generally, paper impregnated with the universal indicator is used for measuring pH.



pH values of various solutions:

Solution

Colour of pH paper

Approximate pH value

Nature of substance

1.    Saliva (before meal)

Green

6.8 – 7.4

Slightly acidic to basic

2.    Saliva (after meal)

Yellow green

5.8

Acidic

3.    Lemon juice

Orange

2.2

Acidic

4.    Colourless aerated drink

Yellow

4.0

Acidic

5.    Carrot juice

Yellow green

6.0

Acidic

6.    Coffee

Yellow

4.5

Acidic

7.    Tomato juice

Yellow

4.3

Acidic

8.    Tap water

Green

6 – 8.5

Varied

9.    1M NaOH

Dark blue

14

Basic

10. 1M HCl

Red

0

Acidic


pH of some common substances shown on a pH paper


The strength of acids and bases depends on the number of H+ ions and OH ions produced, respectively. E.g. 1 molar hydrochloric acid and 1 molar acetic acid (same concentration) produce different amounts of H+ ions.
Strong acids: Produce more H+ ions. E.g. HCl, H2SO4.
Weak acids: Produce less H+ ions. E.g. CH3COOH.
Strong bases: Produce more OH- ions. E.g. NaOH, KOH.
Weak bases: Produce less OH- ions. E.g. NH4OH, Ca(OH)2.


Importance of pH in Everyday Life


Plants and animals are pH sensitive. They can survive only in a narrow range of pH change.

Our body works at the pH range of 7.0 to 7.8.

When pH of rain water is less than 5.6, it is called acid rain. When it flows into rivers, it lowers the pH of river water. It adversely affects the survival of aquatic life.


What is the pH of the soil in your backyard?


To find out the pH for healthy growth of a plant, collect the soil from various places and check their pH as given below.

  • Put about 2 g soil in a test tube and add 5 mL water.
  • Shake the contents of the test tube.
  • Filter the contents and collect the filtrate in a test tube.
  • Check the pH of filtrate using universal indicator paper.

Ideal soil pH for the growth of plants = 6 to 7.


pH in our digestive system


Stomach produces hydrochloric acid.


During indigestion, the stomach produces too much acid and causes pain and irritation.


To get rid of pain, bases called antacids are used. They neutralise the excess acid. E.g. Magnesium hydroxide (Milk of magnesia), a mild base.


pH change as the cause of tooth decay

Tooth enamel is made up of calcium hydroxyapatite (a crystalline form of calcium phosphate). It is the hardest substance in the body. It does not dissolve in water.


Bacteria in the mouth produce acids by degradation of sugar and food particles. Thus enamel is corroded at the pH below 5.5. It is called tooth decay.


Using toothpaste (basic) can neutralise the excess acid and prevent tooth decay.


Self-defence by organisms through chemical warfare

  • Bee-sting leaves formic acid which causes pain and irritation. Applying a mild base like baking soda on the stung area gives relief.
  • Stinging hair of nettle leaves inject methanoic acid causing burning pain. A traditional remedy is rubbing the area with the leaf of the dock plant.

Some naturally occurring acids

Natural source

Acid

Natural source

Acid

Vinegar

Acetic acid

Sour milk (Curd)

Lactic acid

Orange

Citric acid

Lemon

Citric acid

Tamarind

Tartaric acid

Ant sting

Methanoic acid

Tomato

Oxalic acid

Nettle sting

Methanoic acid

Acids in other planets: The atmosphere of Venus is made up of thick white and yellowish clouds of sulphuric acid.


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