Acids, Bases And Salts

Acids, bases and salts are important chemical substances used in daily life, laboratories and industries.

This chapter explains how acids and bases are identified, how they react, what pH means, how salts are classified and why some important salts like sodium hydroxide, bleaching powder, baking soda, washing soda and Plaster of Paris are useful.

Indicators

Indicators are substances that show different colours in different media.

They are used to identify whether a solution is acidic or basic by showing a colour change.

Types Of Indicators

Indicators may be natural or synthetic.

Natural Indicators

Natural indicators are obtained from natural sources.

Examples:

• Litmus
• Turmeric
• China rose
• Red cabbage

Synthetic Indicators

Synthetic indicators are prepared artificially.

Examples:

• Methyl orange
• Phenolphthalein

Olfactory Indicators

Olfactory indicators are substances whose smell changes in acidic or basic medium.

Examples:

• Onion
• Vanilla essence
• Clove oil

Common Indicator Changes

Indicator	Acidic Medium	Basic Medium
Blue litmus	Turns red	No change
Red litmus	No change	Turns blue
Methyl orange	Red	Yellow
Phenolphthalein	Colourless	Pink
Turmeric	Yellow	Reddish brown

Acids

Acids are substances that generally have a sour taste and turn blue litmus red.

Examples:

• Hydrochloric acid – HCl
• Sulphuric acid – H₂SO₄
• Nitric acid – HNO₃
• Acetic acid – CH₃COOH

Acids release hydrogen ions, H⁺, in aqueous solution.

Since free H⁺ ions do not exist alone in water, they combine with water molecules to form hydronium ions, H₃O⁺.

H⁺ + H₂O → H₃O⁺

Physical Properties Of Acids

• Acids are sour in taste.
• Acids turn blue litmus red.
• Acidic solutions conduct electricity due to the presence of ions.
• Acids are generally corrosive in nature.

Chemical Properties Of Acids

Acids React With Metals

Acids react with metals to form salt and hydrogen gas.

Acid + Metal → Salt + Hydrogen gas

Example:

Zn + H₂SO₄ → ZnSO₄ + H₂

Hydrogen gas can be tested by bringing a burning matchstick near it. It burns with a pop sound.

Acids React With Metal Carbonates

Acids react with metal carbonates to form salt, water and carbon dioxide.

Acid + Metal carbonate → Salt + Water + Carbon dioxide

Example:

Na₂CO₃ + 2HCl → 2NaCl + H₂O + CO₂

Acids React With Metal Hydrogen Carbonates

Acids react with metal hydrogen carbonates to form salt, water and carbon dioxide.

Acid + Metal hydrogen carbonate → Salt + Water + Carbon dioxide

Example:

NaHCO₃ + HCl → NaCl + H₂O + CO₂

Acids React With Bases

Acids react with bases to form salt and water.

This reaction is called neutralisation reaction.

Acid + Base → Salt + Water

Example:

HCl + NaOH → NaCl + H₂O

Acids React With Metallic Oxides

Acids react with metallic oxides to form salt and water.

Acid + Metallic oxide → Salt + Water

Example:

CuO + 2HCl → CuCl₂ + H₂O

Black copper oxide changes to bluish green copper chloride solution when treated with dilute hydrochloric acid.

Metallic oxides are basic in nature.

Uses Of Mineral Acids

Sulphuric Acid

Sulphuric acid is known as the king of chemicals.

It is used in:

• Manufacture of fertilizers
• Petroleum refining
• Paints
• Explosives
• Synthetic fibres
• Dyes
• Drugs
• Storage batteries

Hydrochloric Acid

Hydrochloric acid is used in:

• Cleaning metals
• Textile industry
• Dyeing
• Tanning
• Printing industry

Nitric Acid

Nitric acid is used in:

• Dyes
• Plastics
• Drugs
• Fertilizers such as ammonium nitrate
• Explosives such as TNT

Acetic Acid

Acetic acid is used as:

• Vinegar
• Food preservative

Bases

Bases are substances that have a bitter taste and turn red litmus blue.

Examples:

• Sodium hydroxide – NaOH
• Ammonium hydroxide – NH₄OH
• Sodium hydrogen carbonate – NaHCO₃

Bases release hydroxide ions, OH⁻, in aqueous solution.

Physical Properties Of Bases

• Bases have bitter taste.
• Bases feel soapy to touch.
• Basic solutions turn red litmus blue.
• Basic solutions turn methyl orange from orange to yellow.
• Basic solutions turn phenolphthalein from colourless to pink.
• Basic solutions conduct electricity due to the presence of free ions.

Chemical Properties Of Bases

Bases React With Metals

Bases react with some metals to form metal salt and hydrogen gas.

Base + Metal → Metal salt + Hydrogen gas

Example:

2NaOH + Zn → Na₂ZnO₂ + H₂

Bases also react with aluminium.

2Al + 2NaOH + 2H₂O → 2NaAlO₂ + 3H₂

Sodium hydroxide reacts with aluminium, so it cannot be stored in aluminium containers.

Bases React With Acids

Bases react with acids to form salt and water.

This is called neutralisation reaction.

Base + Acid → Salt + Water

Examples:

NaOH + HCl → NaCl + H₂O

Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O

Bases React With Non-Metallic Oxides

Bases react with non-metallic oxides to form salt and water.

Base + Non-metallic oxide → Salt + Water

Examples:

2NaOH + CO₂ → Na₂CO₃ + H₂O

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

Non-metallic oxides are acidic in nature.

Industrial Uses Of Bases

Sodium Hydroxide

Sodium hydroxide is used in:

• Petroleum refining
• Manufacture of soap
• Manufacture of paper
• Synthetic fibres
• Laboratory reagent

Potassium Hydroxide

Potassium hydroxide is used in:

• Alkaline batteries
• Manufacture of soft soap

Ammonium Hydroxide

Ammonium hydroxide helps in removing grease stains from clothes.

Magnesium Hydroxide

Magnesium hydroxide is also called milk of magnesia.

It is used as an antacid to neutralise excess acid in the stomach.

Calcium Hydroxide

Calcium hydroxide is used in:

• Manufacture of bleaching powder
• Softening of hard water
• Neutralising acidic soil

Sodium Carbonate

Sodium carbonate is used for:

• Softening hard water
• Cleansing agent as washing soda

Sodium Hydrogen Carbonate

Sodium hydrogen carbonate is used as:

• Antacid
• Soda-acid fire extinguisher
• Baking soda in cooking

pH Scale

The pH scale is used to determine the strength of an acid or base solution.

The strength of an acid depends on the concentration of hydrogen ions, H⁺.

In pH, p stands for Potenz, meaning power.

pH is defined as the negative logarithm of hydrogen ion concentration.

pH = -log₁₀[H⁺]

It may also be written as:

pH = -log₁₀[H₃O⁺]

pH Values

pH Value	Nature Of Solution
pH < 7	Acidic
pH = 7	Neutral
pH > 7	Basic

As pH decreases, acidity increases.

As pH increases, basicity increases.

Example:

A solution with pH 3 is more acidic than a solution with pH 5.

When pH changes from 3 to 5, hydrogen ion concentration decreases 100 times.

Importance Of pH In Daily Life

pH In Human Body

The human body works within a narrow pH range.

The stomach contains hydrochloric acid, which helps in digestion.

When excess acid is produced, antacids such as magnesium hydroxide are used.

pH Of Soil

Plants grow properly only in a specific pH range.

If soil is too acidic, it may be treated with bases such as calcium hydroxide.

Tooth Decay

Tooth decay starts when the pH in the mouth falls below about 5.5.

Bacteria in the mouth produce acids by degradation of sugar and food particles.

Toothpaste is basic and helps neutralise excess acid.

Salts

Salts are ionic compounds formed by the neutralisation reaction between acids and bases.

General reaction:

Acid + Base → Salt + Water

Example:

HCl + NaOH → NaCl + H₂O

Types Of Salts

Normal Salts

Normal salts are formed by complete neutralisation of an acid by a base.

Example:

• NaCl
• KNO₃

Acidic Salts

Acidic salts are formed by partial neutralisation of a polybasic acid.

Example:

• NaHSO₄
• NaHCO₃

Basic Salts

Basic salts are formed by partial neutralisation of a base by an acid.

Example:

Cu(OH)₂ + HCl → Cu(OH)Cl + H₂O

Other examples:

• Pb(OH)NO₃
• Pb(OH)Cl

Mixed Salts

Mixed salts contain more than one cation or more than one anion.

Examples:

• NaKCO₃
• NaNH₄HPO₄

Double Salts

Double salts are formed from equimolar proportions when slowly crystallised from a mixture of saturated salt solutions.

Examples:

• Potash alum – K₂SO₄·Al₂(SO₄)₃·24H₂O
• Ferric alum – K₂SO₄·Fe₂(SO₄)₃·24H₂O
• Mohr’s salt – FeSO₄·(NH₄)₂SO₄·6H₂O

pH Of Salts

Type Of Salt	Nature	pH
Strong acid + Strong base	Neutral	pH = 7
Strong acid + Weak base	Acidic	pH < 7
Strong base + Weak acid	Basic	pH > 7

Some Important Salts

Sodium Hydroxide

Chemical formula: NaOH

Sodium hydroxide is produced by passing electricity through brine solution.

Brine is concentrated sodium chloride solution.

The process is called the chlor-alkali process because chlorine and alkali are produced.

Reaction:

2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂

In the cell:

• Chlorine gas is released at the anode.
• Hydrogen gas is released at the cathode.
• Sodium hydroxide solution is formed near the cathode.

Sodium hydroxide is also called caustic soda.

Bleaching Powder

Chemical name: Calcium oxychloride

Chemical formula: CaOCl₂

Bleaching powder is prepared by passing chlorine gas over dry slaked lime.

Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O

Uses Of Bleaching Powder

• Used for bleaching cotton and linen in textile industry.
• Used for bleaching wood pulp in paper industry.
• Used for bleaching washed clothes in laundry.
• Used as an oxidising agent.
• Used for disinfecting drinking water.

Baking Soda

Chemical name: Sodium hydrogen carbonate or sodium bicarbonate

Chemical formula: NaHCO₃

Baking soda is prepared from sodium chloride by passing carbon dioxide gas through a cold solution saturated with ammonia.

NaCl + H₂O + CO₂ + NH₃ → NaHCO₃ + NH₄Cl

Properties Of Baking Soda

• It is a white crystalline solid.
• It is soluble in water.
• Its aqueous solution is alkaline.
• Its pH is greater than 7.
• On heating, it gives carbon dioxide.

2NaHCO₃ → Na₂CO₃ + CO₂ + H₂O

Uses Of Baking Soda

• Used in making baking powder.
• Used as an antacid.
• Used in manufacture of aerated water.
• Used in fire extinguishers.

Baking Powder

Baking powder contains:

• Sodium hydrogen carbonate
• Mild edible acid such as tartaric acid or citric acid

Reaction with acid:

NaHCO₃ + H⁺ → CO₂ + H₂O + Sodium salt

Carbon dioxide makes cakes and bread soft and spongy.

If edible acid is absent, sodium carbonate forms and gives a bitter taste.

Washing Soda

Chemical name: Sodium carbonate decahydrate

Chemical formula: Na₂CO₃·10H₂O

Anhydrous sodium carbonate is called soda ash.

Washing soda is manufactured by the Solvay process or ammonia-soda process.

Uses Of Washing Soda

• Used as a cleansing agent.
• Used for softening hard water.
• Used in glass, soap and paper industries.
• Used in the manufacture of sodium compounds.

Plaster Of Paris

Plaster of Paris is obtained by heating gypsum.

Gypsum:

CaSO₄·2H₂O

Plaster of Paris:

CaSO₄·1/2H₂O

Reaction:

CaSO₄·2H₂O → CaSO₄·1/2H₂O + 3/2H₂O

The temperature during heating should not rise above 373 K or 100°C.

If heated above this temperature, all water is lost and anhydrous calcium sulphate, CaSO₄, is formed.

This is called dead burnt plaster.

Dead burnt plaster does not show the properties of Plaster of Paris on adding water.

Properties Of Plaster Of Paris

• It is a white powder.
• It absorbs water with evolution of heat.
• When mixed with water, it forms a paste.
• The paste sets into a hard mass.

Uses Of Plaster Of Paris

• Used as fireproofing material.
• Used in making moulds for toys.
• Used in pottery.
• Used in ceramics.

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