Chemistry Basics Notes for Competitive Exams

Chemistry is the study of matter and the changes it undergoes. In general science sections of SSC, railways, banking, defence, state-level examinations and the UPSC Prelims, questions on acids, bases, salts and metals are asked every year, because these topics connect with daily life: the lemon in your kitchen, the antacid you take after a heavy meal, the iron gate that rusts and the gold ornament that never does. This note explains acids, bases, the pH scale, common salts, and the properties, reactivity, extraction and corrosion of metals, with equations and tables for revision, in simple language.

Acids

An acid is a substance that tastes sour, turns blue litmus red and gives hydrogen ions (H⁺) in water. Acids are found in nature and in the laboratory. Strong acids like hydrochloric acid (HCl), sulphuric acid (H₂SO₄) and nitric acid (HNO₃) ionise completely in water, while weak acids like acetic acid ionise only partly. Never taste laboratory acids, because many of them are corrosive. To dilute an acid, always add the acid slowly to water and not water to acid, because the process releases a lot of heat.

Sulphuric acid is called the "king of chemicals" because of its wide use in industry, and it is used in car batteries. Nitric acid is used in fertilisers and explosives. A mixture of concentrated HCl and HNO₃ in the ratio 3:1 is called aqua regia ("royal water"), and it can dissolve gold and platinum.

Bases and Alkalis

A base tastes bitter, feels soapy, turns red litmus blue and gives hydroxide ions (OH⁻) in water. Bases that dissolve in water are called alkalis; all alkalis are bases, but not all bases are alkalis. Common bases are sodium hydroxide (NaOH, caustic soda, used in soap making), potassium hydroxide (KOH), calcium hydroxide (Ca(OH)₂, slaked lime or lime water, used in whitewashing), magnesium hydroxide (Mg(OH)₂, milk of magnesia, an antacid) and ammonium hydroxide (NH₄OH, used in window cleaners). Ammonia solution is a weak base. Antacids are mild bases that neutralise the excess acid in the stomach and give relief from acidity.

Indicators and the pH Scale

An indicator shows by a change in colour whether a substance is acidic or basic. Litmus (from lichen) is red in acid and blue in base. Phenolphthalein is colourless in acid and pink in base. Methyl orange is red in acid and yellow in base. Turmeric turns red with a base, and a China rose (hibiscus) extract turns pink in acids and green in bases. Such substances with different smells in acid and base are called olfactory indicators, for example onion and vanilla.

The pH scale measures how acidic or basic a solution is, on a scale from 0 to 14. A pH of 7 is neutral (pure water), a pH below 7 is acidic, and above 7 is basic. The lower the pH, the stronger the acid; the higher the pH, the stronger the base. Each unit change of pH means a ten-fold change in hydrogen ion concentration. Examples: gastric juice has a pH of about 1.5 to 2, lemon juice about 2, vinegar about 3, tomato juice about 4, black coffee about 5, normal rain about 5.6, milk about 6.5, pure water 7, human blood about 7.4, baking soda solution about 8.3, soap solution about 10, and household bleach about 12 to 13. Acid rain has a pH below 5.6, and it damages buildings, soil and aquatic life. Tooth decay begins when the pH in the mouth falls below 5.5, because bacteria produce acids from sugar; using toothpaste, which is basic, helps to neutralise this acid.

Reactions of Acids and Bases

• Neutralisation: acid + base gives salt + water. HCl + NaOH gives NaCl + H₂O.
• Acid with metal: acid + metal gives salt + hydrogen. Zn + H₂SO₄ gives ZnSO₄ + H₂. The hydrogen burns with a pop sound.
• Acid with metal carbonate or bicarbonate: gives salt + water + carbon dioxide. CaCO₃ + 2HCl gives CaCl₂ + H₂O + CO₂. When carbon dioxide is passed through lime water, it turns milky.
• Base with metal: some metals like zinc and aluminium react with sodium hydroxide to give hydrogen.

Important Salts

The chlor-alkali process is the electrolysis of brine (sodium chloride solution), giving sodium hydroxide, chlorine and hydrogen. Plaster of Paris sets into hard gypsum when mixed with water, and must be stored in a moisture-proof container. Washing soda is a hydrated salt (with 10 molecules of water of crystallisation), and baking soda is not.

Metals and Their Properties

About three-quarters of the known elements are metals. Typical physical properties of metals are shiny lustre, malleability (can be beaten into sheets), ductility (can be drawn into wires), good conduction of heat and electricity, high melting and boiling points, hardness, high density and a ringing sound (sonorous). There are exceptions: mercury is a liquid at room temperature; sodium and potassium are soft and can be cut with a knife; lead is a poor conductor of heat; gallium and caesium melt at very low temperatures; silver is the best conductor of electricity, followed by copper; and gold is the most malleable and ductile metal. Non-metals are generally dull, brittle and poor conductors, but graphite (a form of carbon) conducts electricity, diamond is the hardest natural substance, and iodine is shiny.

Chemical Properties and the Reactivity Series

Metals form positive ions by losing electrons. They react with oxygen to form metal oxides, which are basic in nature; aluminium oxide and zinc oxide are amphoteric, which means they react with both acids and bases. Metals react with water to different degrees: sodium and potassium react violently and are stored under kerosene, calcium reacts less vigorously, and magnesium reacts with hot water or steam. Iron reacts only with steam. Gold, silver and copper do not react with water. The reactivity series arranges metals in decreasing order of reactivity: potassium, sodium, calcium, magnesium, aluminium, zinc, iron, lead, (hydrogen), copper, mercury, silver, gold. A more reactive metal displaces a less reactive metal from its salt solution: Fe + CuSO₄ gives FeSO₄ + Cu, which is why an iron nail turns reddish-brown in a blue copper sulphate solution. The thermite reaction uses aluminium to reduce iron oxide, giving molten iron used in welding railway tracks: Fe₂O₃ + 2Al gives Al₂O₃ + 2Fe.

Extraction of Metals

The natural materials containing metals are minerals, and those from which the metal can be extracted at a profit are ores. Impurities in the ore are called gangue. The method of extraction depends on the reactivity. Highly reactive metals (K, Na, Ca, Mg, Al) are extracted by electrolysis of their molten compounds. Medium reactive metals (Zn, Fe, Pb) are obtained from their oxides by reduction with carbon after roasting (heating a sulphide ore in air) or calcination (heating a carbonate ore in limited air). Low reactive metals (Hg, Cu) can be obtained by heating alone. Metals like gold and silver occur free in nature. The metal obtained is then refined; electrolytic refining is used for copper, where pure copper is deposited at the cathode.

Alloys

Corrosion and Its Prevention

Corrosion is the slow damage of metals by air, moisture or chemicals. Rusting of iron needs both oxygen and water and forms hydrated iron oxide (reddish-brown). Silver turns black because of silver sulphide, and copper gets a green coating of basic copper carbonate. Prevention methods include painting, oiling and greasing, galvanisation (coating iron with zinc), electroplating, making alloys like stainless steel, and anodising aluminium. The loss from corrosion is huge, so protection of bridges, ships and railway tracks is an important industry.

Quick Revision

• Acid + base gives salt + water; pH 7 is neutral, below 7 acidic, above 7 basic.
• Baking soda: NaHCO₃; washing soda: Na₂CO₃·10H₂O; bleaching powder: CaOCl₂; plaster of Paris: CaSO₄·½H₂O.
• Mercury is a liquid metal; silver is the best conductor; gold is the most malleable.
• Rusting needs oxygen and water; galvanisation is zinc coating.

Chemistry becomes interesting when you link it to daily life. Revise the tables, practise writing the equations, and remember a living example for every topic, such as lemon for citric acid and the iron gate for rusting.

Related Notes to Read Next

Continue your preparation with these related notes from the same subject:

• Human Body Systems Notes for Competitive Exams
• Vitamins, Nutrition and Diseases Notes
• Everyday Science Notes for Competitive Exams
• All General Science notes

Frequently Asked Questions (FAQ)

What is an acid?

An acid is a substance that tastes sour, turns blue litmus red and releases hydrogen ions in water, such as hydrochloric acid and citric acid.

What is the difference between a base and an alkali?

A base is a substance that gives hydroxide ions, while an alkali is a base that dissolves in water. All alkalis are bases but not all bases are alkalis.

What is the pH of pure water?

The pH of pure water is 7, which is neutral.

What is neutralisation?

Neutralisation is the reaction of an acid with a base to form salt and water.

Which acid is present in vinegar?

Acetic acid is present in vinegar, citric acid is in lemon, and lactic acid is in curd.

What is the chemical name of baking soda?

Baking soda is sodium hydrogencarbonate (NaHCO₃), also called sodium bicarbonate.

What is plaster of Paris?

Plaster of Paris is calcium sulphate hemihydrate (CaSO₄·½H₂O), made by heating gypsum, and it is used for plastering fractured bones.

Which metal is liquid at room temperature?

Mercury is the only metal that is liquid at room temperature.

Which metal is the best conductor of electricity?

Silver is the best conductor of electricity, followed by copper.

Why are sodium and potassium stored under kerosene?

They are highly reactive and catch fire in air or react violently with water, so they are stored in kerosene to protect them.

What is galvanisation?

Galvanisation is the coating of iron or steel with a layer of zinc to prevent rusting.

What is an alloy?

An alloy is a homogeneous mixture of a metal with other metals or non-metals, such as brass (copper and zinc) and steel (iron and carbon).