Chapter 2 Acids, Bases and Salts PAGE-2

Physical properties of bases:                    

*  Bases are bitter in taste

*  Bases turns red litmus to blue

Examples of bases are sodium hydroxide potassium hydroxide magnesium hydroxide and calcium hydroxide

Chemical properties of bases:

* Bases produce OH- ions when dissolved in water .Bases soluble in water are called alkalis.

* It conducts electricity when dissolved in water.

* It is highly exothermic hence it should be added dropwise in water with constant stirring so that the heat spreads over in water.

Decrease in OH- ions concentration per unit volume results in formation of dilute bases.

Chemical reactions of bases:

Reaction with metal (only with reactive metals like sodium and potassium).

Base + metal ➡️salt + hydrogen gas

2NaOH+Zn➡️Na2ZnO2 + H2.

Reaction with water

NaOH(s) ➡️ Na+(aq) + OH-(aq) in presence of water.

KOH(s)➡️K+(aq) + OH-(aq)

Reaction with non-metallic oxide.

Base+non- metallic oxide➡️salt + water.

Ca(OH)2 + CO2➡️CaCO3 + water.

Reaction of acids and bases with each other:

Acid + base ➡️ salt + water.

HCl + NaOH➡️ NaCl + H2O.

The reaction in which acids react with base to form salt in water is neutralisation reaction.

How strong are acid or base solutions?

To measure the concentration of hydrogen ion we require a universal indicator ,which is a mixture of several indicators. It shows different colours at different concentrations of hydrogen ion in a solution.

A scale used for measuring the hydrogen ion concentration in a solution is called a pH scale.

                                     pH                            

Here p stands for 'potenz' in German meaning 'power' and H stands for hydrogen. Hence the meaning of pH is power of hydrogen

                                                                   

FIGURE : pH colour chart showing the colour of universal indicator paper changes with solutions of different pH values.

Strong acids: Acids that give rise to more H+ ions are called strong acids.
Weak acids:  Acids that give rise to less H+ ions are called weak acids.
Strong bases: Bases that give rise to more OH-ions are called strong bases.
Weak bases: Bases that give rise to less OH-ions are called weak bases.

Importance of PH in everyday life

* Our body (human body )works within the pH range of 7.0 to 7.8

* pH of rainwater is less than 5.6  is called acid 🌧️

Acid rain flowing in river water lowers the pH of river water resulting difficulty in survival of aquatic life.

* Plants need specific range of pH of soil.

If the pH of soil is 7 then the plants grow well in it.

If the pH is less than 7 that is acidic the growth of the plant is affected.

If the pH is more than 7 that is basic the growth of the plant gets affected.

* Acids produced by stomach helps in digestion .If too much acid is formed it causes pain and irritation due to indigestion .To get rid of it people take antacid such as magnesium hydroxide (milk of magnesia )a mild base to neutralize the excess acid.

* Tooth decay starts when PH in mouth is lower than 5.5 ,it results in corrosion of tooth enamel.

Using toothpaste which are basic helps in neutralising the excess acid and prevents tooth decay.

* Bee-sting produce acids which causes pain and irritation .Use of baking soda on the stung area gives relief.

* Stinging hair of nettle leaves inject methanoic acid causing burning pain.

Test paper to check your learning skill

1. What is strong acid?

2. Define strong base ,weak acid and weak base.

3. What is neutralisation reaction ?give example.

4. Why milk and sour food substances are not stored in copper and brass containers and metal containers?

5. Give reasons:

a) acids do not show acidic behaviour in absence of water.

b) bases do not show basic behaviour in absence of water.

6. What are antacids?

7. What are olfactory indicators? Give examples.

8. What is meant by acid rain? How it is harmful to aquatic organisms?

9. Write the acids present in the following substances:

Vinegar ,lemons and oranges ,milk or curd, tamarind and unripe grapes ,ant sting and nettle sting and tomatoes.

10. Draw the diagram showing the reaction of zinc granules with dilute sulphuric acid and testing hydrogen gas by burning and label it.

11. Draw the diagram showing acid solution in water conducts electricity.

12. Name the chemical used as antioxidant or antacids.

13. Name the chemical present in the tooth enamel.

14. Which acid is produced in our stomach ?What happens if there is an excess of acid in the stomach ?How can its effect be cured?

15. What happens during a bee or wasp sting ?what is its remedy?

16. Explain the pH change as the cause of tooth decay .How can tooth decay caused by pH change be prevented?

17. What is our tooth enamel made up of? Sweet tooth may lead to tooth decay. Explain why. What is the role of toothpaste in preventing cavities?

18. What is the reason for the acidity of soil and how can we correct it?

19. List the pH values of:

Gastric juice, lemon juice ,pure water ,blood ,milk of magnesia ,sodium hydroxide solution and human body

20. List the properties of acids and bases.