Class 10 Science - Acids, bases and salts

 Acids:

• An acid is a hydrogen containing substance that is capable of donating proton (hydrogen ion) to another substance.
• Acids are sour in taste.
• Acids change the colour of blue litmus to red.
• They conduct electricity when dissolved in water.
• Examples: HCl, H2SO4 etc.

1. Some naturally occurring acids are:

Natural Source	Acids
Vinegar	Acetic acid
Orange and lemon	Citric Acid
Tamarind (इमली)	Tartaric acid
Tomato	Oxalic acid
Curd (sour milk)	Lactic acid
Ant's sting and nettle's sting	Methanoic/formic acid

Bases:

• Bases are the chemical substances that produces hydroxide ions when dissolved in water.
• They are bitter in taste and soapy to touch.
• Some bases are soluble in water and some are not.
• They turn red litmus to blue.
• They conduct electricity when dissolved in water.
• Examples: NaOH -----> Na⁺ (aq) + OH ^-
• Alkalis: Bases that are soluble in water are called alkalis. Example: Sodium Hydroxide, Potassium Hydroxide, Calcium Hydroxide. 

Salts: They are produced due to the reaction between acids and bases. 

Indicators: They tell us whether the substance is acidic or basic.

Indicators	Bases	Acids
Litmus paper (purple)	Blue	Red
Turmeric (yellow)	Red	Yellow
Phenolphthalein (colourless)	Pink	Colourless
Methyl Orange (orange)	Yellow	Red

Abbreviation to learn the above table: LaBouR TRY PoPCorn with MaYoR where all the capital letters denote the first letter of each in above table. 

• Litmus is a natural indicator extracted from plant, lichen. 
• When the litmus solution is neither acidic nor basic, its colour is purple.

Olfactory indicators: 

• Indicators whose smell (odour) changes when in acidic or basic medium. Examples: onion, clove oil, vanilla essence. 
• These indicators retains smell in acidic whereas loses smell when in basic

Chemical properties of Acids:

• Reaction with Water:

Acids produce H+ ions in water / aq solution

• HCl +H2​O ------>​H+(aq)+Cl−

• H2​SO4​ +H2​O​ ------>2H+(aq)+SO42−​

In dry state, no H+ ions are produced.

• HCl(dry)​⟶ No H+ ions

H+ of acid combines with H2​O (water) to form H3​O+ (Hydronium ion).

• H++H2​O⟶H3​O+

NOTE:

• So, we say acid gives H3​O+ (Hydronium ion) or H+ (aq)

• This H+ (aq) ion or H3​O+ (Hydronium) gives common properties to all acids.

Preparation of HCl gas:

• $2NaCl(aq)+H_{2}S{O}_4(conc.)⟶N{a}_{2}S{O}_4(aq)+2HCl(g)$

• $HCl(g)\; with\stackrel{drybluelitmus,\; there\; is}{}\text{o colour change}$

  $HCl(g)\; with\stackrel{moistbluelitmus}{}\text{ed colour}$

  $HCl(g)+H_{2}O⟶H_3{O}^{+}(aq)+C{l}^{-}$

• Sodium Chloride + Sulphuric acid ⟶ Sodium sulphate + Hydrogen chloride gas

Diluting an Acid

• Acid + Water is highly exothermic reaction.

• If we add water is added to a concentrated acid, the heat generated may cause the mixture to splash out and cause burns. The glass container can also break due to heat given out.

NOTE: 

1. Never add water to acid
2. Always add acid slowly to water with constant stirring.

Electric current through the solution is carried out by ions. 

Solution of acids like HCl, H2SO4 etc. and alkalis like NaOH, KOH etc generate ions and hence conduct electricity. 

Glucose, Alcohol do not generate ions and hence they do not conduct electricity. 

• Reaction with metals:

1. Acids like dilute HCl react with metals to form salts and evolve H2 gas.
2. Metal + Dilute Acid -------> Salt + Hydrogen gas
3. Eg: Zn(s) + H2SO4 (aq) -------> ZnSO4 (aq) + H2(g)
4. Test for hydrogen gas: Hydrogen gas burns with the pop sound
5. Less reactive metals like Ag, Au an Pt don't react with acids.
6. Exception: HNO3 + Metal ------>  Salt + H2O

• Reaction with metal carbonates and metal hydrogen carbonates (bicarbonates): 

1. Acids with metal carbonates and bicarbonates to produce metal salts, water and evolve carbon dioxide gas with brisk effervescence.
2. Metal carbonate/ metal hydrogen carbonate + Acid ------> Salt + Carbon dioxide + Water                                                                                                                        (Brisk effervescence)
3. Eg: CaCO3 (s) + 2HCl (aq) ------> CaCl2 (aq) + H2O (aq) + CO2 (g) 
4. Eg: 2NaHCO3(s) + H2SO4 (aq) ------> Na2SO4 + H2O + 2CO2
5. Test for CO2 gas: When CO2 gas is passed through lime water, it turns lime water milky due to the formation of white precipitate of CaCO3.

Ca(OH)2 (aq) + CO2 (g) ---> CaCO3 (s) + H2O (l) 

Lime water + Carbon dioxide ----> Calcium carbonate + Water

But if CO2 is passed in excess, milkiness disappears due to the formation of Ca(HCO3)2 which is soluble in water. 

CaCO3 (s) + H2O (l) + CO2 (g) -----> Ca(HCO3)2 (aq)

Calcium carbonate + water + carbon dioxide ------> calcium hydrogen carbonate/ bicarbonate

• Reaction with metallic oxides:

1. Acids react with metallic oxides to form salt and water
2. Metal oxide + Acid ------> Salt + Water
3. Eg: CuO(s) + 2HCl(aq) -----> CuCl2(aq) + H2O
4. Copper oxide (Black solid) + Hydrochloric Acid ------> Copper Chloride (Blue green)+ Water
5. Metal oxides are basic in nature because they react with acid to form salt and water, so they are also known as basic oxides.

Chemical Properties of bases:

• Reaction with metals:

1. Bases react with metals to give hydrogen gas. Thus, these bases should not be kept in metal containers.
2. Metal + Base ----> Salt + Hydrogen Gas
3. Eg: Zn(s) + 2NaOH(aq) ----> Na2ZnO2(s) + H2 (g)
4. Zinc + Sodium hydroxide ------> Sodium zincate + Hydrogen Gas

• Reaction with non- metallic oxides: 

1. Bases react with non- metallic oxides to form salt and water.
2. Non- metallic oxide + Base -----> Salt + Water
3. Eg: CO2 (g)  + Ca(OH)2 (aq) ------> CaCO3 (s) + H2O (l)
4. Carbon- dioxide + Calcium hydroxide ------> CaCO3 (s) + H2O (l)
5. Non- metallic oxides are acidic in nature because they react with acid to form salt and water, so they are also known as acidic oxides.

Reaction between acids and bases

• Acids react with bases to form salt and water. 

Importance of pH in our everyday life

Plants and Animals are pH Sensitive

Living organisms can survive only in a narrow range of pH change. Our body works normally within the pH range of 7.0 to 7.8. When pH of rain water goes below 5.6, it is called acid rain. When acid rain flows into the rivers, it lowers the pH of the river water and makes survival of aquatic life difficult.

pH of the Soil

Every type of plant requires a specific pH range for their healthy growth. Therefore, the nature of soil is known first by testing its pH and then a particular crop is grown in it. It is also suitable for selecting the fertiliser for a particular crop by knowing the pH of the soil.

• Acidic soil is treated with quick lime (CaO) or slaked lime (Ca(OH)2​).

• Basic soil is treated with organic matter or gypsum (CaSO4​) to bring pH to neutral range.

pH in Our Digestive System

Our stomach produces hydrochloric acid (HCl), which helps in the digestion of food.

During indigestion, the stomach produces too much acid, which causes pain and irritation. To correct the disturbed pH range, magnesium hydroxide or milk of magnesia (a mild base) is used as a medicine, which is also called antacid as it neutralises the effect of excess acid (or acidity).

Tooth Decay Prevention

Bacteria in mouth feed on sugar and produce lactic acid, which lowers pH and corrodes enamel. Toothpaste is basic in nature and neutralizes the acid to protect teeth.

Honey bee bite remedy

Bee stings inject acidic venom. Baking soda (a base) neutralises the acid and provides relief.

SALTS

Salts are ionic compounds composed of positively charged ions (cations) and negatively charged ions (anions). These ions are held together by ionic bonds.

• Salts are formed through the reaction between an acid and a base. This reaction is known as Neutralisation reaction.

Salts and their pH

The pH of salts depends on the acids and bases used to form them:

1. Neutral Salts: Formed from a strong acid and a strong base (e.g., NaCl). Their pH is 7.

2. Acidic Salts: Formed from a strong acid and a weak base (e.g., NH4​Cl). Their pH is less than 7.

3. Basic Salts: Formed from a strong base and a weak acid (e.g., Na2​CO3​). Their pH is more than 7.

COMMON SALT

Chemical Formula: NaCl

Chemical Name: Sodium chloride

• Common salt is made of sodium ions (Na+) and chloride ions (Cl−).

• It is obtained from sea water or salt mines.

Reaction for Formation

• It is formed by a neutralization reaction between an acid and a base:

  $HCl+NaOH\to NaCl+H_{2}O$

Uses of Common Salt:

1. Used in cooking as a seasoning.

2. Helps in food preservation, e.g., pickling.

3. Raw material for making products like sodium hydroxide, baking soda, and washing soda.

CAUSTIC SODA

Chemical formula : NaOH

Chemical name: Sodium Hydroxide

Production: Sodium hydroxide is industrially produced by passing electricity through sodium chloride (NaCl) solution called brine. This electrolysis process is called chlor- alkali process because of the products formed—chlor for chlorine and alkali for sodium hydroxide.

PRODUCTS AND USES

1. Sodium hydroxide: Used in soap, paper, and textiles, degreasing metals, and oil refining.

2. Chlorine: Used in making bleaching powder, disinfectants, for water treatment and manufacture of plastics (like PVC).

3. Hydrogen: Used in hydrogenation of oils, as a fuel, and in making ammonia for fertilizers.

BLEACHING POWDER

Chemical formula: $CaOC{l}_2$

Chemical name: Calcium oxychloride

Production: The chlorine gas reacts with dry slaked lime [Ca(OH)2​] produces bleaching powder.

$Ca(OH{)}_2+C{l}_2\to CaOC{l}_2+H_{2}O$

USES OF BLEACHING POWDER

1. Used for bleaching:

   • Cotton and linen in the textile industry

   • Washed clothes in laundries

   • Wood pulp in paper factories

2. Used as an oxidising agent in chemical industries.

3. Used to make drinking water free from germs (disinfectant).

BAKING SODA

Chemical formula: $NaHC{O}_3$

Chemical name: SODIUM HYDROGEN CARBONATE

Production: Sodium hydrogen carbonate can be prepared by the reaction between sodium chloride, water, carbon dioxide, and ammonia:

$NaCl+H_{2}O+C{O}_2+N{H}_3\to N{H}_{4}Cl+NaHC{O}_3$

$2NaHC{O}_3\stackrel{Heat}{}N{a}_{2}C{O}_3+H_{2}O+C{O}_2$

USES OF BAKING SODA

1. To make baking powder: For making baking powder, which is a mixture of baking soda and a mild edible acid such as tartaric acid. 

       2. Baking: Sodium hydrogen carbonate is widely used in baking as a leavening agent.

            2NaHCO3​ -------> (Heat​) Na2​CO3​+H2​O+CO2​

       3. Preservation of milk: Used as preservative for milk to keep the milk safe from getting sour.

       4. Antacids: Used as an ingredient of antacids.

       5. Fire - extinguisher: Used as fire- extinguisher as it releases Carbon- dioxide gas

WASHING SODA 

Chemical formula: $Na2C{O}_3$

Chemical name: SODIUM CARBONATE DECAHYDRATE

Production:

1. First, sodium carbonate is obtained by heating baking soda.
2. Further, the recrystallisation of sodium carbonate gives washing soda. It is also a basic salt.

        Reaction involved:

        Na2CO3(s) + 10 H2O(l) ------> Na2CO3.10H2O(s)

Uses of washing soda:

1. Cleans stains and grease in households.
2. Softens hard water.
3. Used in glass, soap, and paper industries.
4. Helps manufacture chemicals like borax.
5. Enhances detergents for laundry cleaning.

PLASTER OF PARIS

Chemical formula: CaSO4. 1/2 H2O

Chemical name: CALCIUM SULPHATE HEMIHYDRATE

Production:

1. It is produced by heating gypsum (CaSO4.2H2O) at 373 K. At this temperature, gypsum loses water molecules and forms calcium sulphate hemihydrate (Plater of Paris).
2. CaSO4.2H2O ------> (373 K / heat ) CaSO4. 1/2 H2O + 1 1/2 H2O. 

Uses of Plaster of Paris:

1. Construction and Building Materials: Plaster of Paris is widely used in construction for making decorative elements, molds, and casts. It is commonly used for creating intricate designs on ceilings and walls.

2. Medical Casts: Plaster casts are often used in the medical field to set broken bones. The plaster sets and hardens, providing support to the injured area.

3. Art and Craft: Plaster of Paris is a popular material in art and craft projects. It can be molded into various shapes and is easy to work with.

4. Dental Molds: In dentistry, plaster of Paris is used for making dental molds and impressions.

Dead Burnt Plaster: Formed when POP is heated above 200∘C. All water of crystallization is lost → completely anhydrous

WATER OF CRYSTALLIZATION

Some salts contain water of crystallization, which is a fixed number of water molecules in their structure. Example: Copper Sulphate (${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$)

When heated, it loses water and turns white:

$${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}\stackrel{\text{heat}}{}{\text{CuSO}}_4+5{\text{H}}_2\text{O }$$(${\text{CuSO}}_4\cdot 5{\text{H}}_2\text{O}$ is Blue; ${\text{CuSO}}_4$ is White)

Adding water restores its blue color.

Other Examples:

• Gypsum (${\text{CaSO}}_4\cdot 2{\text{H}}_2\text{O}$): Contains two water molecules.

• Plaster of Paris (CaSO4​⋅1/2 ​H2​O): Made by heating gypsum at 373 K. It hardens when mixed with water.