Chemistry is all around us: in the salt in our dal, the lemon in our tea, the soap we wash with and the gas cylinder in the kitchen. Lok Sewa general science papers often ask short questions on acids, bases, chemical formulas and common gases. This note collects the most useful facts in a simple form that is easy to revise before the exam.
1. Matter and its states
Matter is anything that has mass and takes up space. It is made of tiny particles. The three common states are:
| State | Shape | Volume | Particles | Example |
|---|---|---|---|---|
| Solid | Fixed | Fixed | Tightly packed | Ice, stone |
| Liquid | Takes shape of container | Fixed | Loosely packed, can flow | Water, milk |
| Gas | No fixed shape | No fixed volume | Far apart, move freely | Air, steam |
Changes of state have special names:
- Solid to liquid: melting; liquid to solid: freezing.
- Liquid to gas: evaporation or boiling; gas to liquid: condensation.
- Solid directly to gas without becoming liquid: sublimation. Camphor, naphthalene balls, iodine and dry ice (solid carbon dioxide) sublime.
2. Elements, compounds and mixtures
- An element is a pure substance made of only one kind of atom and cannot be broken into simpler substances by chemical means. Examples: hydrogen, oxygen, iron, gold.
- A compound is formed when two or more elements combine chemically in a fixed ratio. Its properties differ from those of its elements. Examples: water (H₂O), common salt (NaCl), carbon dioxide (CO₂).
- A mixture contains two or more substances mixed in any ratio without a chemical bond; each keeps its own properties. Examples: air, salt solution, soil.
A mixture can be homogeneous (same throughout, such as sugar solution or air) or heterogeneous (not the same throughout, such as sand in water).
Common separation methods: filtration (sand from water), evaporation (salt from salt water), distillation (pure water from salty water), decantation (pouring off liquid from settled solids) and a magnet (iron filings from sulphur). Hydrogen is the lightest element.
3. Atom basics
An atom is the smallest particle of an element that takes part in a chemical reaction. It has a central nucleus with electrons moving around it.
| Particle | Charge | Location | Discovered by |
|---|---|---|---|
| Proton | Positive | Nucleus | Ernest Rutherford |
| Neutron | No charge (neutral) | Nucleus | James Chadwick (1932) |
| Electron | Negative | Outside the nucleus, in shells | J. J. Thomson (1897) |
- Atomic number = number of protons (equal to the number of electrons in a neutral atom).
- Mass number = protons + neutrons.
- Example: sodium has atomic number 11 and mass number 23, so it has 11 protons, 11 electrons and 23 − 11 = 12 neutrons.
- Isotopes are atoms of the same element with the same atomic number but different mass numbers. Hydrogen has three: protium, deuterium and tritium.
4. Acids, bases, salts and pH
Acids taste sour, turn blue litmus red and give hydrogen ions in water. Bases taste bitter, feel soapy and turn red litmus blue. Bases that dissolve in water are called alkalis. When an acid and a base react, they form salt and water; this is called neutralization.
Natural acids in daily life:
| Source | Acid present |
|---|---|
| Lemon, orange | Citric acid |
| Vinegar | Acetic acid |
| Curd, sour milk | Lactic acid |
| Tamarind | Tartaric acid |
| Apple | Malic acid |
| Ant sting, nettle (sisnu) sting | Formic acid |
| Our stomach | Hydrochloric acid |
Useful neutralization examples:
- Antacids such as milk of magnesia (magnesium hydroxide) neutralize extra stomach acid.
- A bee sting is acidic, so rubbing baking soda helps.
- Farmers add lime to acidic soil.
The pH scale runs from 0 to 14. pH 7 is neutral (pure water). A pH less than 7 is acidic; a pH more than 7 is basic. The smaller the pH, the stronger the acid.
| Substance | pH (approximate) | Nature |
|---|---|---|
| Stomach juice | 1 to 2 | Strongly acidic |
| Lemon juice | about 2 | Acidic |
| Pure water | 7 | Neutral |
| Human blood | about 7.4 | Slightly basic |
| Soap solution | about 10 | Basic |
5. Indicators
An indicator changes colour to show whether a substance is an acid or a base.
| Indicator | In acid | In base |
|---|---|---|
| Litmus | Blue turns red | Red turns blue |
| Phenolphthalein | Colourless | Pink |
| Methyl orange | Red | Yellow |
| Turmeric | Yellow (no change) | Reddish-brown |
Litmus is a natural dye obtained from lichens.
6. Metals and non-metals
| Metals | Non-metals |
|---|---|
| Shiny (lustrous) | Mostly dull |
| Good conductors of heat and electricity | Poor conductors (except graphite) |
| Can be beaten into sheets (malleable) and drawn into wires (ductile) | Brittle when solid |
| Usually solid at room temperature | Can be solid, liquid or gas |
| Form basic oxides | Form acidic oxides |
Facts often asked:
- Mercury is the only metal that is liquid at room temperature; bromine is a liquid non-metal.
- Silver is the best conductor of electricity; copper and aluminium are used for wires.
- Graphite, a form of carbon, is a non-metal that conducts electricity. Diamond, another form of carbon, is the hardest natural substance.
- Tungsten has a very high melting point, so it is used for bulb filaments.
- Sodium and potassium react strongly with air and water, so they are kept in kerosene.
- Aluminium is the most abundant metal in the Earth’s crust; oxygen is the most abundant element there.
Alloys are mixtures of a metal with other elements: brass (copper and zinc), bronze (copper and tin), steel (iron and carbon) and stainless steel (iron with chromium and nickel).
7. Common chemical names and formulas
| Common name | Chemical name | Formula | Main use |
|---|---|---|---|
| Common salt | Sodium chloride | NaCl | Food, preserving |
| Baking soda | Sodium hydrogen carbonate (sodium bicarbonate) | NaHCO₃ | Baking, antacid, fire extinguisher |
| Washing soda | Sodium carbonate decahydrate | Na₂CO₃·10H₂O | Washing, removing hardness of water |
| Bleaching powder | Calcium oxychloride | CaOCl₂ | Bleaching cloth, disinfecting drinking water |
| Plaster of Paris | Calcium sulphate hemihydrate | CaSO₄·½H₂O | Plaster for broken bones, statues |
| Gypsum | Calcium sulphate dihydrate | CaSO₄·2H₂O | Making plaster of Paris and cement |
| Quicklime | Calcium oxide | CaO | Making cement and slaked lime |
| Slaked lime | Calcium hydroxide | Ca(OH)₂ | Whitewashing, treating acidic soil |
| Limestone, marble, chalk | Calcium carbonate | CaCO₃ | Building, cement |
| Caustic soda | Sodium hydroxide | NaOH | Making soap |
Plaster of Paris sets hard when mixed with water because it turns back into gypsum. When carbon dioxide is passed through lime water (a solution of calcium hydroxide), it turns milky because calcium carbonate forms.
8. Rusting and corrosion
Corrosion is the slow eating away of a metal by air, moisture or chemicals. The corrosion of iron is called rusting. Rust is a reddish-brown hydrated iron oxide (Fe₂O₃·xH₂O).
Iron needs both oxygen (air) and water to rust. Salty water makes rusting faster, so iron rusts quickly near the sea.
Ways to prevent rusting:
- Painting, oiling or greasing.
- Galvanization: coating iron with zinc, as in galvanized roofing sheets.
- Tin plating (food tins) or chrome plating.
- Making alloys such as stainless steel.
9. Important gases
| Gas | Key fact |
|---|---|
| Nitrogen | About 78 percent of air; used to make fertilizer |
| Oxygen | About 21 percent of air; needed for breathing and burning |
| Carbon dioxide | Used by plants in photosynthesis; used in fire extinguishers and soft drinks |
| Carbon monoxide | Poisonous; forms when fuel burns in too little air; blocks haemoglobin |
| Methane | Main gas in biogas (gobar gas) and natural gas; also called marsh gas |
| LPG | Mainly butane and propane; cooking gas in cylinders |
| Nitrous oxide | “Laughing gas”; used as a mild anaesthetic |
| Ammonia | Strong smell; used to make fertilizer such as urea |
| Chlorine | Used to disinfect drinking water |
Never sleep in a closed room with a burning stove or heater, because carbon monoxide can build up.
📌 Key points to remember
- Solid, liquid and gas are the common states; camphor and iodine sublime.
- Elements have one kind of atom; compounds have a fixed ratio; mixtures do not.
- Proton is positive, electron negative, neutron neutral; atomic number equals the number of protons.
- Acids turn blue litmus red; bases turn red litmus blue; acid plus base gives salt and water.
- pH 7 is neutral; below 7 is acidic and above 7 is basic; blood is about 7.4.
- Lemon has citric acid, vinegar acetic acid, curd lactic acid and ant sting formic acid.
- Phenolphthalein is colourless in acid and pink in base.
- Mercury is a liquid metal; silver is the best conductor; graphite is a conducting non-metal.
- Baking soda is NaHCO₃, washing soda Na₂CO₃·10H₂O, bleaching powder CaOCl₂, quicklime CaO.
- Plaster of Paris is CaSO₄·½H₂O and is made from gypsum.
- Rusting needs air and water; galvanization means a zinc coating.
- Biogas is mainly methane; LPG is mainly butane and propane; carbon monoxide is poisonous.
Test yourself: Chemistry in Daily Life
55 questions on Matter and its states, Indicators and Important gases. Each attempt picks 25 at random.