Acids, bases and Salts

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Chapter 2: Acids, Bases and Salts

Acids

Definition:

Acids are substances that:

• Have a sour taste.
• Turn blue litmus red.
• Release H⁺ ions in aqueous solution.

Natural Source                              Acid

Vinegar                                           – Acetic Acid

Orange and Lemon                      – Citric Acid

Tamarind                                        – Tartaric Acid

Tomato                                           – Oxalic Acid

Curd                                                – Lactic acid

Ant’s sting and Nettle sting         – Methanoic Acid

Types of Acids:

• Mineral acids (inorganic): Hydrochloric acid (HCl), Sulphuric acid (H₂SO₄), Nitric acid (HNO₃)
• Organic acids: Acetic acid (CH₃COOH), Citric acid, Lactic acid

Strength:

• Strong acids: Completely ionise in water (e.g., HCl, H₂SO₄)
• Weak acids: Partially ionise in water (e.g., CH₃COOH)

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Chemical Properties of Acids

1. Reaction with Metals:

Acids react with metals to produce salt and hydrogen gas.

General Reaction:

Acid + Metal → Salt + Hydrogen gas

Example:

Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)

Test for Hydrogen Gas:

• Bring a burning matchstick near the mouth of the test tube.
• Hydrogen burns with a ‘pop’ sound.

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2. Reaction with Metal Carbonates and Metal Hydrogen Carbonates:

(a) Reaction with Metal Carbonates:

General Reaction:

Acid + Metal Carbonate → Salt + Water + Carbon dioxide

Example:

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

(b) Reaction with Metal Hydrogen Carbonates:

General Reaction:

Acid + Metal Hydrogen Carbonate → Salt + Water + Carbon dioxide

Example:

NaHCO₃(s) + HCl(aq) → NaCl(aq) + H₂O(l) + CO₂(g)

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Test for Carbon Dioxide Gas:

• Pass the gas through lime water.
• If carbon dioxide is present, lime water turns milky due to the formation of white precipitate i.e calcium carbonate.

Reaction:

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

(lime water)   →    (white precipitate)

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What happens when excess carbon dioxide is passed through lime water?

• When more CO₂ is passed, the milky solution becomes clear again.
• This happens due to the formation of soluble calcium hydrogen carbonate[ Ca(HCO₃)₂.]

Reaction:

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

Summary Table

Reaction Type	General Equation	Example
Acid + Metal	Acid + Metal → Salt + H₂↑	Zn + HCl → ZnCl₂ + H₂
Acid + Metal Carbonate	Acid + MCO₃ → Salt + H₂O + CO₂↑	CaCO₃ + HCl → CaCl₂ + H₂O + CO₂
Acid + Metal Hydrogen Carbonate	Acid + MHCO₃ → Salt + H₂O + CO₂↑	NaHCO₃ + HCl → NaCl + H₂O + CO₂
CO₂ + Lime water	CO₂ + Ca(OH)₂ → CaCO₃ (white ppt) + H₂O	Lime water turns milky
Excess CO₂ + Lime water	CaCO₃ + H₂O + CO₂ → Ca(HCO₃)₂ (soluble)	Milky solution becomes clear

Bases

Definition:

Bases are substances that:

• Have a bitter taste and soapy touch.
• Turn red litmus blue.
• Release OH⁻ ions in aqueous solution.

Types:

• Strong bases: Sodium hydroxide (NaOH), Potassium hydroxide (KOH)
• Weak bases: Ammonium hydroxide (NH₄OH)

Alkalis: Bases that are soluble in water are called alkalis.

Examples: NaOH, KOH, NH₄OH

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Chemical Properties of Bases

1. Reaction with Metals:

Bases react with metals to produce salt and hydrogen gas.

General Reaction:

Base + Metal → Salt + Hydrogen gas

Example:

2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

Sodium reacts with water to form sodium hydroxide and hydrogen.

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2. Reaction with Non-metallic Oxides:

Non-metallic oxides are acidic in nature. They react with bases to form salt and water.

General Reaction:

Base + Non-metallic oxide → Salt + Water

Example:

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

Calcium hydroxide reacts with carbon dioxide to form calcium carbonate (a salt) and water.

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What do Acids and Bases have in Common?

• Both release ions in water:
• Acids release H⁺ ions
• Bases release OH⁻ ions
• Both are electrolytes – they conduct electricity in aqueous solutions.
• Both show chemical reactivity with metals and oxides.
• Both change the colour of indicators.

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Effect of Dilution on an Acid or Base

• Dilution means mixing the acid or base with water.
• When an acid or base is mixed with water:
• It decreases its concentration (makes it less strong).
• It is an exothermic process (releases heat).
• Always add acid to water slowly to avoid splashing and heat burns.

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Neutralisation Reaction

Definition:

A reaction in which an acid reacts with a base to form salt and water.

General Equation:

Acid + Base → Salt + Water

Examples:

HCl + NaOH → NaCl + H₂O 

H₂SO₄ + 2KOH → K₂SO₄ + 2H₂O

Applications of Neutralisation:

• In treating indigestion (antacids neutralise stomach acid)
• In soil treatment (acidity of soil is neutralised using bases like quick lime or slaked lime)
• In treating wasp or bee stings
• In wastewater treatment to neutralise acids and bases

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Indicators

Definition:

Indicators are substances that change colour in the presence of acids or bases.

Types of Indicators:

1. Natural Indicators:

• Obtained from natural sources.
• Examples: Litmus (red in acid, blue in base), Turmeric, China rose petals

2. Synthetic Indicators:

• Man-made indicators.
• Examples:
• Phenolphthalein: Colourless in acid, Pink in base
• Methyl orange: Red in acid, Yellow in base

3. Universal Indicator:

• A mixture of indicators that shows various colours at different pH values.
• Can measure the strength of an acid or base.

4. Olfactory Indicators:

• Show change in smell with acid or base.
• Examples: Onion, clove oil, vanilla extract
• In acid: Smell retained
• In base: Smell lost

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Change of Indicators with Acids and Bases

Indicator	In Acid	In Base
Litmus	Red	Blue
Phenolphthalein	Colourless	Pink
Methyl Orange	Red	Yellow
Onion (olfactory)	Smell retained	Smell lost

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Strength of Acids and Bases

• Determined by the concentration of H⁺ or OH⁻ ions.
• Measured using the pH scale.

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pH Scale

Definition:

• pH stands for ‘potential of Hydrogen’ or ‘power of Hydrogen’.
• It measures the concentration of H⁺ ions in a solution.

Range:

• Scale ranges from 0 to 14.
• pH = 7: Neutral (e.g., pure water)
• pH < 7: Acidic
• pH > 7: Basic (alkaline)

Colour Chart of Universal Indicator:

• Red → Strong acid (pH 0–3)
• Orange/yellow → Weak acid (pH 4–6)
• Green → Neutral (pH 7)
• Blue → Weak base (pH 8–10)
• Purple → Strong base (pH 11–14)

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Importance of pH in Everyday Life

1. In Plants and Animals (Cell Activities):

• Enzymes and cell functions require specific pH levels.
• Our blood has a pH of 7.36–7.42, which must be maintained.

2. In Soil:

• pH of soil affects crop yield.
• Acidic soils are treated with bases like lime (CaO).
• Basic soils are treated with organic matter or compost.

3. In Digestive System:

• Stomach produces HCl (pH ~1–2) for digestion.
• Antacids (like milk of magnesia) neutralise excess acid during indigestion.

4. In Tooth Decay:

• Tooth enamel starts to dissolve when pH < 5.5.
• Bacteria produce acids after eating sugary food → causes decay.
• Brushing helps neutralise the acids.

5. Self Defence by Animals and Plants:

• Bee/wasp stings inject methanoic acid → painful.
• Treated with a base like baking soda.
• Nettle leaves contain acids → cause irritation.
• Rubbed with base-containing leaves like dock leaves.

🧪 SALTS

What are Salts?

• Salts are compounds formed when the hydrogen ion (H⁺) of an acid is replaced by a metal ion.
• Example:
• HCl + NaOH → NaCl + H₂O (Sodium chloride is a salt)

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 COMMON SALT products AND THEIR DETAILS

1. Caustic Soda

• Common Name: Caustic Soda
• Chemical Name: Sodium Hydroxide
• Chemical Formula: NaOH

 Preparation:

When electricity is passed through an aqueous solution of sodium chloride (called brine), it decomposes to form sodium hydroxide and chlorine. This process is named as chlor –alkali process because the products formed i.e chlor for chlorine and alkali for sodium hydroxide.

 

• Reaction:

Electric current

2NaCl(aq)  +   2H₂O                                       2NaOH (aq)    +Cl₂(g)            +          H₂(g)

Uses:

The uses of different products formed in chlor-alkali process are:

 

·       H₂ gas – used in fuels, margarine, ammonia for fertilizers.

·       Cl₂ gas – used in water treatment, swimming pools, PVC (polyvinylchloride), disinfectants, CFC’s pesticides.

·       NaOH – used for de-greasing metals, soaps and detergents, paper making, artificial fibre.

·       Chlorine and hydrogen used as a raw material for preparation of HCl acid i.e used for cleaning steel, ammonium chloride, medicines, cosmetics , etc

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2. Bleaching Powder

• Common Name: Bleaching Powder
• Chemical Name: Calcium Oxychloride
• Chemical Formula: CaOCl₂

Preparation:

• Produced by reacting chlorine gas(Cl₂) with dry slaked lime (Ca(OH)₂)
• Reaction:

Ca(OH)₂          +Cl₂     →        CaOCl₂            +H₂O  

Uses:

• Used for bleaching cotton and linen in textile industry
• Used as an oxidizing agent
• Used for disinfecting drinking water
• Used in the manufacture of chloroform

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3. Baking Soda

• Common Name: Baking Soda
• Chemical Name: Sodium Hydrogen Carbonate or Sodium Bicarbonate
• Chemical Formula: NaHCO₃

Preparation:

• Prepared by Solvay Process using sodium chloride, ammonia, and carbon dioxide
• Reaction:

NaCl(aq)   +    NH₃(g)     +      CO₂(g)  +        H₂O(l) →        NaHCO₃ (s)     +            NH₄Cl (g)

Brine                           Ammonia                                Sodium hydrogen             Ammonium chloride

carbonate

Properties:

-       It is commonly used in making crispy pakoras.

-       It is a mild non corrosive basic salt.

-       It is a major constituents of baking powder.

-       Used for faster cooking

 

 

Uses:

• Used in cooking as a baking agent (it releases CO₂ to make food soft and spongy)
• Used in soda-acid fire extinguishers
• Mild antiseptic for skin
• Ingredient in antacids to relieve acidity

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4. Washing Soda

• Common Name: Washing Soda
• Chemical Name: Sodium Carbonate Decahydrate
• Chemical Formula: Na₂CO₃·10H₂O

🔬 Preparation:

• Obtained by heating baking soda followed by recrystallization
• Reactions:

2NaHCO₃        →        Na₂CO₃           +          CO₂     +          H₂O                

Na₂CO ₃                +          10 H₂O                        →        Na2CO3⋅ 10 H₂O

Properties:

-       It is white crystalline solid

-       Its solution is alkaline in nature

-       It has a property to remove dirt and grease from dirty clothes.

 Uses:

• Used for softening hard water
• Used in the manufacture of glass, soap, and paper
• Used as a cleaning agent in laundry
• Used in laboratories as a standard reagent

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5. Plaster of Paris (POP)

• Common Name: Plaster of Paris
• Chemical Name: Calcium Sulphate Hemihydrate
• Chemical Formula: CaSO₄·½H₂O

Preparation:

• Made by heating gypsum (CaSO₄·2H₂O) at 373°C
• Reaction:

CaSO₄ ⋅2H₂O              →        CaSO4⋅½H₂O             +          1 ½ H₂O

 CaSO₄·½H₂O + 1 ½ H₂O -----------   CaSO₄·2H₂O

 

Uses:

• Used for making moulds, statues, and toys
• Used in hospitals for setting fractured bones
• Used for making decorative materials (false ceilings)
• Used in dentistry for dental impressions

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WATER OF CRYSTALLISATION

 What is Water of Crystallisation?

• Water of crystallisation is the fixed number of water molecules chemically attached to each formula unit of a salt in its crystalline form.

Example:

• Copper(II) Sulphate (Blue Vitriol):
• Chemical Formula: CuSO₄·5H₂O
• It contains 5 molecules of water of crystallisation.

 Note:

• When blue vitriol is heated, it loses water and becomes white anhydrous copper sulphate (CuSO₄), which turns blue again if water is added.