PH scale-strength of acids and bases

  • Measurement of strength of acid and base solution

Acidic and basic solution

  • Water on ionization gives H+ and OH- ions
  • In pure water the concentration of hydrogen ions and hydroxide ions are equal, so water is said to be a neutral or amphoteric solvent
  • When we add acid to the water, the concentration of hydrogen ion. In increases in other words acidic solutions have an excess of hydrogen ions
  • When the base is added to water, the concentration of hydroxide ion. In it increases, in other words, the basic solution has an excess of hydroxide ions
  • Both the acidic and basic solution contains hydrogen and hydroxide ions with varying concentrations respectively

Ph scale

  • In the year 1909 Sorensen, devised a scale called ph. scale on which the strength of acid solutions and basic solutions could be represented by making power of hydrogen ion concentration in them
  • The ph. of a solution is inversely proportional to the concentration of hydrogen ion
  • The strength of an acid or base is measured on a scale of numbers called the ph. scale
  • The ph. scale has values from 0-14
  • Ph is a pure number and has no units

Calculation of ph.

  • Neutral solutions have ph. of exactly 7, the ph. of water is 7. A substance having ph. of 7 will not affect litmus paper or any other common indicators such as methyl orange or phenolphthalein etc
  • Acids or acidic solutions have a ph. of less than 7, the more acidic the solution is, the lower the ph.
  • The stronger the acid is the lower the ph.
  • Bases or basic solutions have ph. of more than 7, more basic solution higher will be the ph.
  • Stronger the base higher the ph.

Ph values of some common substance

  • Concentrated hydrochloric acid- o
  • Gastric juice(stomach)- 1.4
  • Pure water-7
  • Human blood -7.4
  • Con. Sodium hydroxide- 14

Universal indicators:

  • A common method that is used to measure the ph. of a solution is the universal indicator method
  • A universal indicator is a mixture of many different indicators that give different colours at different ph. values of the entire ph. scale
  • When an acid or base solution is added to the universal indicator, the indicator produces a new colour
  • The colour produced by the universal indicator is used to fund the ph. value

The values and their colour representation

0Dark red
1Red
2Red
3Orange-red
4Orange
5Orange- yellow
6Greenish yellow
7Green
8Greenish-blue
9Blue
10Navy blue
11Purple
12Dark blue
13Violet
14Violet

Importance of ph. in our daily life

Ph in our digestive system

  • Our stomach produces hydrochloric acid, which helps in the digestion of food
  • Acidity in the stomach is caused due to overeating sometimes causes indigestion
  • Being basic in nature antacids react with excess acid in the stomach and neutralises it
  • Examples of antacids are milk of magnesium (magnesium hydroxide), baking soda (sodium hydrogen carbonate) that cure acidity

Ph and tooth decay

  • When eating food containing sugar, the bacteria present in our mouth breaks down the sugar to form acids (lactic acid)
  • Tooth decay starts when the ph. of the mouth falls below 5.5
  • To maintain the ph. of the mouth and to prevent tooth decay rinse the mouth thoroughly with water

 Ph variation in Plants and animals

  • Most of the plants grow well when the ph. of the soil is equal to 7
  • When the soil is too acidic, it is treated with materials like quick lime/calcium carbonate
  • When a honeybee stings a person, it injects an acidic liquid into the skin which irritates, rubbing a mild amount of baking soda relives it
  • An ant sting injects methanolic acid into the skin causing burning pain that can be neutralised by rubbing a mild amount of baking soda
  • Wasp sting injects an alkaline solution into a person’s skin causing immense pain
  • Nettle plant have the stinging hair which injects methanolic acid in the skin of a person causing burning pain can be neutralised by adding baking  soda

Ph change in aquatic life:

  • The high acidity of the lake or river water can kill the aquatic living beings
  • Calcium carbonate is added to neutralize the excess acidity

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