KEY NOTES -pH Scale-Class 10 – Science -Chemistry -chapter 2- Acids, Bases and Salt

pH Scale:

The pH scale measures the acidity or basicity of a solution. It ranges from 0 to 14.

A pH less than 7 indicates acidity, a pH of 7 is neutral, and a pH greater than 7 indicates alkalinity (basicity).

pH Sensitivity of Plants & Animals:

Human body works in a narrow range of pH 7 to 7.8. Acidity can be lethal for plants and animals.

pH of Digestive System: Stomach secretes HCl to kill bacteria in the food. The inner lining of stomach protects vital cells from this acidic pH.

pH and tooth decay: Lower pH because of sour food and sweet food can cause tooth decay. The pH of mouth should always be more than 5.5.

pH as self defence mechanism in plants & animals: Certain animals like bee and plants like nettle secrete highly acidic substance for self defence.

pH of Acid Rain : When pH of rain water is less than 5.6 it is called acid rain. Flow of acidic rain in water bodies makes them acidic causing a threat to the survival of aquatic life.It also results in damage of structures made with marble like Taj Mahal.

pH of Soil : Plants require a specific range of pH for their healthy growth. If pH of soil of any particular place is less or more than normal then the farmers add suitable chemicals to it. The addition of these chemicals of presences of excessive damage the nutrients of the soil and decrease its natural fertility.

Time's up

Salts:

Properties of Salts:

Often form crystalline solids.

High melting and boiling points.

Conduct electricity in molten form or aqueous solution.

Examples of Salts:

Sodium chloride (NaCl), potassium nitrate (KNO₃), and calcium sulphate (CaSO₄).

Common Salt

(NaCl)Sodium chloride is known as common salt. Its main source is sea water. It is also existing in the form of rocks and is called rock salt. Common salt is an important component of our food. It is also used for preparing sodium hydroxide, baking soda, washing soda etc.

Sodium hydroxide

 (NaOH) Prepared by Chlor Alkali process :Electricity is passed through an aqueous solution of Sodium chloride (called brine). Sodium chloride decomposes to form sodium hydroxide. Chlorine gas is formed at the anode, and hydrogen gas at the cathode. Sodium hydroxide solution is formed near the  cathode. 2NaCl(aq) + 2 H2O (l) → 2NaOH(aq) + Cl2(g) + H2(g)

Bleaching Powder (Calcium Oxychloride, CaOCl2)

Preparation: Bleaching powder is produced by the action of chlorine on dry slaked lime [Ca(OH)2].

Ca(OH)₂ + Cl₂→ CaOCl₂ + H₂O

Uses:

Bleaching powder is used –

(i) for bleaching cotton and linen in the textile industry, for bleaching wood pulp in paper factories and for bleaching washed clothes in laundry;

(ii) as an oxidising agent in many chemical industries; and

(iii) to make drinking water free from germs

Baking Soda (Sodium Hydrogen Carbonate, NaHCO3)

Preparation:

The chemical name of the compound is sodium hydrogen carbonate (NaHCO3). It is produced by the reaction of brine with carbon dioxide and ammonia. This is known as Solvay process.

NaCl  +  H₂O  +  CO₂   +   NH₃   →     NH₄Cl        +    NaHCO₃

Plaster of Paris:

Calcium sulphate hemihydrate CaSO4. ½ H2O Prepared by heating Gypsum at 373K. CaSO4. 2H2O(Heat at 373K) → CaSO4. ½ H2O + 1½ H2O

pH of Salts

A salt of a strong acid and a strong base will be neutral in nature. pH = 7 (approx.).

A salt of a weak acid and a strong base will be basic in nature. pH > 7.

A salt of a strong acid and a weak base will be acidic in nature. pH < 7.

The pH of a salt of a weak acid and a weak base is determined by conducting a pH test.

Washing Soda

Preparation: Recrystallisation of sodium carbonate.

It is a basic salt used in

manufacture of Borax.

glass, soap and paper industries

cleansing agent for domestic purposes.

removing permanent hardness of water.

Water of Crystallization: fixed number of water molecules present in on formula unit of a salt.

Importance in Daily Life:

Acids, bases, and salts have crucial roles in daily life, from household cleaning products to food preservation and industrial processes.