Everyday life is full of invisible chemical reactions happening around us – from the sour taste of lemon juice to the slippery feel of soap. These reactions are often caused by two special kinds of substances called acids and bases. Let us explore the science behind the reactions, indicators, and pH value mysteries — where each solution has a story!
Acids and bases:
| Property | Acids | Bases |
| Taste | Sour | Bitter |
| Nature | Corrosive | Slippery or soapy |
| Ions released in aqueous solution | \(H^+\) ions | \(OH^-\) ions |
| Litmus test | Turn blue litmus red | Turn red litmus blue |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red | Yellow |
| Onion, vanilla essence, clove oil | Retains the odour | Odourless |
| pH range | Less than \(7\) | Greater than \(7\) |
| Electrical conductivity | Conducts electricity due to presence of ions (\(H^+\)) | Conducts electricity due to ions (\(OH^-\)) |
- Strong acids: Completely ionise in water and release more \(H^+\) ions - \(H_2SO_4\), \(HCl\), \(HNO_3\) and most of inorganic acids.
- Weak acids: Partially ionise in water and release fewer \(H^+\) ions - \(CH_3COOH\), \(H_2CO_3\), most of organic acids.
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Strong bases: Completely ionise in water and release more \(OH^-\) ions - \(NaOH\), \(KOH\).
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Weak bases: Partially ionise in water and release fewer \(OH^-\) ions - \(NH_4OH\).
Reactions of acids and bases:
| Reaction | Acids | Bases | Example | Test |
| Metals | Form salt + hydrogen gas | Form salt + hydrogen gas |
Acid: \(Zn + 2HCl → ZnCl_2 + H_2↑\)
Base: \(2NaOH + Zn → Na_2ZnO_2 + H_2↑\)
|
Pop sound |
| Metal carbonates and bicarbonates | Produce salt + water + \(CO_2\) gas | No reaction |
\(Na_2CO_3 + 2HCl → 2NaCl + H_2O + CO_2\)
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Limewater turns milky
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| Neutralisation | Reacts with base to form salt + water | Reacts with acid to form salt + water |
\(HCl + NaOH → NaCl + H_2O\)
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Exothermic
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| Metal/non - metal oxide | React with metal oxides to form salt + water | React with non-metal oxides to form salt + water |
Acid: \(MgO + 2HCl → MgCl_2 + H_2O\)
Base: \(2NaOH + CO_2 → Na_2CO_3 + H_2O\)
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Metallic oxides are basic.
Non-metallic oxides are acidic.
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Acids and bases exhibit unique chemical properties through their reactions with metals, oxides, and carbonates, helping us understand their real-world significance. In the next session, we will explore the pH scale, its importance, and the key chemicals obtained from common salt.
Acid and base in a water solution:
Acid produces hydrogen ions in water
- Dry \(HCl\) gas: Does not have separate \(H+\) ions, fails to turn blue litmus paper to red. Do not show the acidic character of an acid.
- \(HCl\) solution: Contains separate \(H^+\) ions, turns blue litmus red. Proves the acidic character of an acid.
- Acids: \(H^+\) ions
- Bases: \(OH^-\) ions
- Guard tube: Calcium cholride - Absorb moisture from the gas.
Water is mixed with an acid or a base:
- Dissolving an acid and a base in water is an exothermic process
- Always add acid to water, never water to acid
- Adding water to an acid or a base decrease the concentration of the ions
- Concentrated acid/base: High percentage of acid/base and a low percentage of water
- Diluted acid/base: High percentage of water and a low percentage of acid/ base
Neutralisation reaction:
\(Acid\ + Base\ → Salt\ + Water\)
pH scale:
pH scale
- pH - potenz (potential) of hydrogen
- \(0\) to \(7\) - Acid
- \(7\) - Neutral
- \(7\) to \(14\) - Base
Variation of pH
Importance of pH in everday life:
| Situation | Cause | Effect | Neutralisation |
| Insect/Bee sting | Formic acid | Pain & burning | Base (Baking soda/lime paste) |
| Wasp sting | Alkaline | Pain & burning | Acid (Vinegar) |
| Nettle sting | Methanoic acid | Burning pain | Base (Rub dock leaf) |
| Tooth decay | Mouth acids | Cavity formation | Basic toothpaste |
| Acidity | Excess HCl | Heartburn/ulcer | Antacids (\(Mg(OH)_2\) |
| Acidic soil | Fertilisers | Poor plant growth | Lime (\(CaO\) or \(Ca(OH)_2\)) |
| Alkaline soil | Excess base | Poor nutrient availability | Acid (Compost) |
| Industrial waste | Acidic effluents | Environmental damage | Lime or limestone |
Salts:
- Strong acid + Strong base → Neutral Salt + Water
- Weak acid + Weak base → Neutral Salt + Water
- Strong acid + Weak base → Acidic Salt + Water
- Weak acid + Strong base → Basic Salt + Water
Chemicals from common salt:
\(NaCl\) - A raw material for chemicals:
Common salt is an important raw material for many substances that we use in our daily life, including sodium hydroxide, baking soda, washing soda, bleaching powder, and many others.
Chemicals from common salt:
1. Sodium hydroxide \(NaOH\):
The Chlor-alkali process is an industrial process that produces chlorine and sodium hydroxide by electrolysis of sodium chloride (brine) solution.
The overall reaction that occurs during the Chlor-alkali process is as follows:
\(2NaCl + 2H_2O \rightarrow 2NaOH + Cl_2 + H_2\)
The three by-products of this process are all useful.
- At cathode - \(H_2\) - Used as fuels, margarine, ammonia for fertilisers
- At anode - \(Cl_2\) - Used for water treatment, swimming pools, PVC, disinfectants, CFCs and pesticides etc.
- Sodium hydroxide \(NaOH\) is collected near the cathode, and used in soaps, detergents, paper making and artificial fibres.
| Common name | Chemical name | Formula | Uses | Method of preparation |
| Common salt | Sodium chloride |
\(NaCl\)
|
Raw material for NaOH, baking soda, washing soda; used as food preservative | Obtained from sea water & mining of rock salt |
| Caustic soda | Sodium hydroxide |
\(NaOH\)
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Soaps, detergents, paper | Chlor-alkali process: Electrolysis of brine |
| Bleaching Powder | Calcium oxychloride or Calcium hypochlorite |
\(CaOCl_2\)
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Bleaching agent, disinfectant, water treatment | Passing chlorine gas over slaked lime |
| Baking Soda | Sodium hydrogen carbonate or sodium bicarbonate |
\(NaHCO_3\)
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Baking (release of \(CO_2\)), antacid, fire extinguishers, cleaning | Solvay process |
| Washing Soda | Sodium carbonate decahydrate |
\(Na_2CO_3.10H_2O\)
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Glass, soap, paper industry, water softening | Recrystallisation of sodium carbonate/Solvay process |
Water of crystallisation:
The number of water molecules found in the crystalline substance is called water of crystallisation. These salts are known as hydrated salts.
Copper sulphate pentahydrate (Blue vitriol):
Blue vitriol contains a total of five water molecules. As a result, its water of crystallisation is \(5\). When blue copper sulphate crystals are gently heated, they lose their five water molecules and transform into colourless anhydrous copper sulphate.
(a) Crystalline hydrated salt (b) Amorphous anhydrous salt
Similarly, ferrous sulphate heptahydrate \(FeSO_4.7H_2O\) from pale green on heating loses the water of crystallisation and changes to white anhydrous salt.
Plaster of Paris:
The calcium sulphate hemihydrate is commonly known as plaster of Paris. The name is derived from the place of its origin Montmartre in the Paris district. The chemical formula is .
Preparation of plaster of Paris:
When gypsum is heated in a rotating kiln at \(373 K\), it loses all its water molecule and forms calcium sulphate hemihydrate \(CaSO_4.1/2H_2O\).
\(2CaSO_4.2H_2O \xrightarrow {373K}CaSO_4.1/2H_2O + 1.5H_2O\)
Properties of plaster of Paris:
- It is a white colour powder.
- It acts as an oxidizing agent under extreme conditions.
- It produces gypsum when mixed with water.
\(CaSO_4.\frac{1}{2}H_{2}O + 1\frac{1}{2}.H_{2}O\rightarrow CaSO_4.2H_2O\)