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Notes of Ch 2 Acids, Bases and Salts: Class 10th Science

 

Notes of Ch 2 Acids, Bases and Salts: Class 10th Science


Introduction

  • The sour and bitter tastes of food are due to acids and bases are present in them.
  • Acids are sour in taste and change the colour of blue litmus to red.
  • Litmus solution is a purple dye, which is extracted from lichen. When the litmus solution is neither acidic nor basic, its colour is purple.
  • Other natural materials like red cabbage leaves, turmeric, coloured petals of some flowers such as Hydrangea, Petunia and Geranium, which indicate the presence of acid or base in a solution.

Properties of Acids
  •  They release H+ ions in aqueous solution.
  •  Acids have sour taste.
  •  They turn blue litmus solution red.
  •  They give H+ ions in aqueous solution.

Strong Acids: HCl, H2 SO4 , HNO3
Weak Acids: CH3COOH, Oxalic acid, Lactic acid
Concentrated Acids: More amount of acid + Less amount of water
Dilute Acids: More amount of water + Less amount of acid

Properties of Bases
  • They have a bitter taste.
  • They are soapy to touch.
  • They turn red litmus to blue.
  • They conduct electricity in solution form.
  • They release OH– ions in aqueous solution

Strong Bases: NaOH, KOH, Ca(OH)2
Weak Bases: NH4OH
Alkalis: These are bases which are soluble in water. Examples: NaOH, KOH, Ca(OH)2.

Strong Acids
An acid which completely dissociates into its ions in aqueous solution.
For example: Hydrochloric acid (HCl), Sulphuric acid (H2SO4), Nitric acid (HNO3)

Strong Base
A base which completely dissociates into its ions in aqueous solution.
For example: Sodium hydroxide (NaOH), Potassium hydroxide (KOH)

Weak Acids
An acid which does not completely dissociate into
its ions in aqueous solutions. 
For example: Acetic acid (CH3COOH), Carbonic acid (H2CO3)

Weak Base

A base which does not completely dissociate into its ions in aqueous solution.

For example: Ammonium hydroxide (NH4OH).


Types of Indicators and its properties

Indicators: Substances which change their colour/smell in different types of substances (like acids and bases).

Types of Indicators:
(i) Natural indicators
(ii) Synthetic indicators
(iii) Olfactory indicators

(i) Natural indicators: Found in nature in plants. Examples: Litmus, red cabbage leaves extract, flowers of hydrangea plant, turmeric.

(ii) Synthetic indicators: These are chemical substances. Examples: Methyl orange, phenolphthalein.

(iii) Olfactory indicators: These substances have different odour in acid and bases.



Reaction of Acids and Bases with Metals

→ Reaction of Acids with Metals

• Acids react with metal to form metal salt and releases Hydrogen Gas.
Acid  + Metal → Salt + Hydrogen Gas

• Example: Zinc granules react with dilute Hydrochloric acid in a test tube.
2HCl + Zn → ZnCl2 + H2

→ Reaction of Bases with Metals

• Bases react with metal to evolve hydrogen Gas. Also, note that all metals do not react with bases. The metal must be more reactive than the metals present in the base for the reaction to take place.
Base + Metal → Salt + Hydrogen gas

• Example: Zinc granules react with NaOH solution to form sodium zincate and evolve hydrogen gas.
2NaOH + Zn → Na2ZnO2 + H2

• Hydrogen gas released can be tested by bringing burning candle near gas bubbles, it burst with pop sound.

Reaction of Acids with Metal Carbonates and Metal Hydrogencarbonates

• Acids reacts with Metal Carbonates and Metal Hydrogencarbonates to form Salt, Carbon dioxide and water.
Metal carbonate/Metal hydrogen carbonate + Acid → Salt + Carbon dioxide + Water

• Examples: (i) 2HCl + Na2CO3 → 2NaCl + CO2 + H2O
(ii) HCl + NaHCO3 → NaCl + CO2 + H2O
• CO2 can be tested by passing it through lime water. It turns lime water milky.
Ca(OH)2 + CO2 → CaCO3 + H2O

• When excess CO2 is passed, milkiness disappears.
CaCO3 + CO2 + H2O → Ca(HCO)3

• Bases do not react with Metal Carbonates and Metal Hydrogencarbonates.
Base + Metal Carbonate/Metal Hydrogen Carbonate → No Reaction

Reaction of Acids and Bases with each other

• Acids and Bases react to form salt and water.
Acid + Base → Salt + H2O

• Neutralisation Reaction: Reaction of acid with a base is called as neutralization reaction.
Example: HCl + NaOH → NaCl + H2O

• Strong Acid + Weak Base → Acidic salt + H2O

• Weak Acid + Strong Base → Basic salt + H2O

• Strong Acid + Strong Base → Neutral salt + H2O
• Weak Acid + Weak Base → Neutral salt + H2O

Reaction of Metallic Oxides with Acids

→ Metallic oxides are basic in nature.

Example: CaO, MgO are basic oxides.
Metallic Oxide + Acid → Salt + H2O
CaO + 2HCl → CaCl2 + H2O

Reaction of Non-metallic Oxides with Bases

→ Non-metallic oxides are acidic in nature.

→ Non-metallic Oxide + Base → Salt + H2O
CO2 + Ca(OH)2 → CaCO3 + H2O

Reaction of Acid

(i) Acid + Metal Carbonate → Salt + CO2 + Water
(ii) Acid + Metal → Salt + H2
(iii) Acid + Metal Hydrogen Carbonate → Salt + CO2 + H2O
(iv) Acid + Metallic oxide → Salt +H2O
(v) Acid + Base → Salt + H2O
Reaction Of Base

(i) Base + Metal → Salt + H2
(ii) Base + Metal Carbonate → No Reaction
(iii) Base + Metal Hydrogen Carbonate → No Reaction
(iv) Base + Acid → Salt + H2O
(v) Base + Non Metallic oxide → Salt + H2O

Similarities between all Acids and all Bases

→ All acids have H+ ions in common. All acids produce H+ ions
→ Acids produce H+ ions in solution which are responsible for their acidic properties.
→ All bases have OH- (hydroxyl ions) in common. All bases produce OH- ions

Acid or Base in Water Solution

→ Acids produce H+ ions in presence of water.

→ H+ ions cannot exist alone, they exist as H3O+ (hydronium ions).
H+ + H2O → H3O+
HCl + H2O → H3O+ + Cl-

→ Bases when dissolved in water gives OH − ions.

→ Bases soluble in water are called alkali.

→ While diluting acids, it is recommended that the acid should be added to water and not water to acid because the process of dissolving a acid or a base in water is highly exothermic.

Strength of Acids and Base solutions

→ Strength of acid or base can be estimated using universal indicator.

→ Universal indicator: It is a mixture of several indicators. It shows different colours at different concentrations of H+ ions in the solution.

→ pH Scale: A scale for measuring H+ ion concentration in a solution. p in pH stands for ‘potenz’ a German word which means power.

For water or neutral solutions : pH = 7

For acidic solutions : pH < 7

For basic solutions : pH > 7


Importance of pH in everyday life

(i) pH in our digestive system: Our stomach produces hydrochloric acid that helps in the digestion of food. During indigestion, the stomach produces too much acid and this causes pain and irritation. To get rid of this pain, antacids like magnesium hydroxide [Mg(OH)2] also known as milk of magnesia and sodium hydrogen carbonate (baking soda) are used to neutralize excess acid.

(ii) Tooth decay caused by acids: Bacteria present in the mouth produce acids by degradation of sugar and food particles remaining in the mouth after eating. When the pH of acid formed in the mouth falls below 5.5, tooth-decaying starts. The best way to prevent this is to clean the mouth after eating food. Using toothpastes, which are generally basic, for cleaning the teeth can neutralise the excess acid and prevent tooth decay.

(iii) pH of soil and plant growth: Most plants require a specific pH range (close to 7) for their healthy growth. If the soil is too acidic or basic, the plants grow badly or do not grow at all. pH of the soil can be adjusted by using certain chemicals. For example, if the soil is too acidic then it is treated with materials like quicklime or slaked lime. On the other hand, if the soil is too alkaline then alkalinity can be reduced by adding decaying organic matter.

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