Sodium chloride and other salts
Every kitchen has salt, and every sea is full of it. But the salt on your plate is only one of many salts. Chemists call a compound made from an acid and a base a salt, and sea water has many of them dissolved in it.
A simple salt (साधा क्षार) is an ionic compound with no H⁺ or OH⁻ ions and only one kind of cation and anion. Sodium chloride (NaCl, मीठ) is common table salt.
In the figure : In the figure the products of the electrolysis of brine.
Rule 1: Salts and sea water — Simple salts are ionic compounds with no H⁺ or OH⁻ ions and only one kind of cation and anion, such as Na₂SO₄, K₃PO₄ and CaCl₂. About 80 million tons of salts are added to sea water every year. Sea water contains sodium chloride, magnesium chloride, magnesium sulphate, potassium chloride, calcium carbonate and magnesium bromide.
- Easy : CaCl₂ has only Ca²⁺ and Cl⁻ and no H⁺ or OH⁻, so it is a simple salt.
- Medium : Na₂SO₄ has one kind of cation (Na⁺) and one kind of anion (SO₄²⁻): a simple salt.
- Exam-type : 80 million tons per year: in 10 years, 80 × 10 = 800 million tons are added to the sea.
Rule 2: Acidic, neutral and basic salts — A salt from a strong acid and a strong base is neutral (pH 7). A salt from a strong acid and a weak base is acidic (pH less than 7). A salt from a weak acid and a strong base is basic (pH more than 7).
- Easy : NaCl: strong acid HCl + strong base NaOH, so neutral.
- Medium : Na₂CO₃: weak acid + strong base NaOH, so basic.
- Exam-type : NH₄Cl: strong acid HCl + weak base NH₄OH, so acidic.
Sodium chloride and other salts
In the app you can move this figure with your finger.
Open in the appRule 3: Sodium chloride — Sodium chloride is a colourless crystalline ionic compound with no water of crystallization; it is neutral and salty. It is formed by neutralization of sodium hydroxide and hydrochloric acid, and is used to make Na₂CO₃ and NaHCO₃. Electrolysis of brine (a 25% solution) gives H₂ at the cathode and Cl₂ at the anode, and NaOH is formed: 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂. At about 800 °C salt melts (fused state); electrolysis of fused salt gives chlorine at the anode and liquid sodium at the cathode. Rock salt (halite, Himalayan rock salt) comes from rocks.
- Easy : In brine electrolysis, hydrogen collects at the cathode and chlorine at the anode.
- Medium : The equation 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂ has the same number of Na, Cl, H and O atoms on both sides (2 Na, 2 Cl, 4 H, 2 O).
- Exam-type : From fused salt at about 800 °C: chlorine at the anode and liquid sodium at the cathode, since Cl⁻ goes to the positive electrode and Na⁺ to the negative electrode.
Sodium chloride and other salts
In the app you can move this figure with your finger.
Open in the appStrong acid + strong base = neutral. Brine electrolysis: H₂ (cathode), Cl₂ (anode), NaOH.
Mixing up the electrodes in electrolysis. Positive ions go to the cathode; negative to the anode.
Memory clue: "Cathode = Cations come; Anode = Anions go".
Spot the difference
| Salt from | Nature |
|---|---|
| Strong acid + strong base | Neutral (pH 7) |
| Strong acid + weak base | Acidic |
| Weak acid + strong base | Basic |
Practice questions
- 1What is a simple salt?
- 2Name two salts in sea water.
- 3What is the pH of a neutral salt?
- 4Which gas forms at the cathode in brine electrolysis?
- 5Which gas forms at the anode in brine electrolysis?
- 6What forms at the cathode when fused salt is electrolysed?
- 7Classify KNO₃ (HNO₃ + KOH) and CuSO₄ (H₂SO₄ + Cu(OH)₂) as acidic, neutral or basic.
- 8Why must the reaction 2NaCl + 2H₂O → 2NaOH + Cl₂ + H₂ be balanced? Count the O atoms on both sides.
Show answers (try them yourself first)
- 1An ionic compound with no H⁺ or OH⁻ and only one kind of cation and anion.
- 2Sodium chloride and magnesium sulphate.
- 37.
- 4Hydrogen.
- 5Chlorine.
- 6Liquid sodium.
- 7KNO₃: strong + strong, neutral. CuSO₄: strong acid + weak base, acidic.
- 8Atoms are not lost, so both sides must match: 2 O on the left (2H₂O) and 2 O on the right (2NaOH).
Baking soda, bleaching powder, washing soda and soap
Baking soda makes idli and dhokla soft, bleaching powder cleans drinking water in a tank, washing soda cleans clothes, and soap lathers in bath water. Four everyday things, each with its own chemistry.
Baking soda is NaHCO₃, bleaching powder is CaOCl₂, washing soda is Na₂CO₃ and soap is the sodium or potassium salt of a fatty acid.
In the figure : In the figure everyday compounds and their uses.
Rule 1: Baking soda and bleaching powder — Baking soda (sodium hydrogen carbonate or sodium bicarbonate, NaHCO₃) is a white non-crystalline powder that turns red litmus blue, so it is basic. It is used for bread, cake and dhokla, to reduce stomach acidity, to make CO₂ in fire extinguishers and to clean an oven. Bleaching powder (calcium oxychloride, CaOCl₂) is a yellowish white solid with the smell of chlorine. It is made from chlorine gas and slaked lime: Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O. It releases chlorine slowly with the CO₂ of the air and rapidly with dilute H₂SO₄ or HCl. It is used to disinfect drinking water, swimming pools, roadsides and garbage sites, and to bleach cloth.
- Easy : Baking soda in a dough releases CO₂ when heated, which makes the dhokla spongy.
- Medium : Ca(OH)₂ + Cl₂ → CaOCl₂ + H₂O has 1 Ca, 2 O, 2 H, 2 Cl on both sides, so it is balanced.
- Exam-type : Which is used to disinfect a swimming pool, baking soda or bleaching powder? Bleaching powder, because it releases chlorine.
Rule 2: Washing soda and hard water — Washing soda (sodium carbonate) is a whitish odourless powder, hygroscopic, and litmus turns blue in its solution. It is used for washing clothes, in the glass and paper industry and for refining petrol. Hardness of water is due to chlorides and sulphates of calcium and magnesium. Na₂CO₃ forms insoluble carbonates of Mg and Ca and softens the water. Crystalline salts with water of crystallization include alum, borax, Epsom salt, barium chloride and Glauber's salt. Alum coagulates impurities so they settle; blue vitriol is used in the anaemia blood test and in the Bordeaux mixture (with slaked lime).
- Easy : Alum added to muddy water makes the dirt settle at the bottom.
- Medium : Washing soda softens hard water because it removes calcium and magnesium as insoluble carbonates.
- Exam-type : Why does washing soda keep clothes clean even in hard water? It first precipitates Ca²⁺ and Mg²⁺, so soap can then lather.
Rule 3: Soap — Soap is made by boiling oil or fat with sodium or potassium hydroxide, which gives the sodium or potassium salts of fatty acids. In hard water, insoluble Ca and Mg salts form a precipitate, so no lather forms. Bathing soap uses high grade fats and oils; washing soap uses low grade ones.
- Easy : In hard water soap forms a scum instead of a lather.
- Medium : Soap + hard water gives insoluble calcium and magnesium salts, which is why more soap is wasted.
- Exam-type : Bathing soap uses high grade fat, washing soap uses low grade fat: so the washing soap is cheaper.
NaHCO₃ baking soda; CaOCl₂ bleaching powder; Na₂CO₃ washing soda. Soap = salt of fatty acid.
Using the names baking soda and washing soda as the same. Their formulas are different.
Memory clue: "Baking = Bicarbonate (NaHCO₃); Washing = Carbonate (Na₂CO₃)".
Spot the difference
| Substance | Formula | Use |
|---|---|---|
| Baking soda | NaHCO₃ | Bread, dhokla, antacid |
| Washing soda | Na₂CO₃ | Washing, softening water |
| Bleaching powder | CaOCl₂ | Disinfecting water |
Practice questions
- 1What is the chemical name and formula of baking soda?
- 2What is the formula of bleaching powder?
- 3Which gas does bleaching powder release?
- 4What is the formula of washing soda?
- 5What causes the hardness of water?
- 6Name two crystalline salts with water of crystallization.
- 7Why does soap not lather in hard water?
- 8Which of the three is used to soften water, and how?
Show answers (try them yourself first)
- 1Sodium hydrogen carbonate, NaHCO₃.
- 2CaOCl₂.
- 3Chlorine.
- 4Na₂CO₃.
- 5Chlorides and sulphates of calcium and magnesium.
- 6Alum and borax (also Epsom salt, Glauber's salt).
- 7It forms insoluble calcium and magnesium salts.
- 8Washing soda; it forms insoluble carbonates of Mg and Ca.
Radioactivity and radiation
An X-ray of the hand and a Geiger counter that clicks near a rock are linked to invisible rays. Some elements shoot out invisible rays on their own, without any heating or pushing. This is radioactivity.
Radioactivity (किरणोत्सर्ग) is the spontaneous emission of invisible, highly penetrating, high energy radiation from the unstable nuclei of heavy elements.
In the figure : In the figure the penetrating power of alpha, beta and gamma rays.
Rule 1: Radioactivity — Elements of high atomic number (uranium, thorium, radium) spontaneously emit invisible, highly penetrating, high energy radiation, which is radioactivity. Their nucleus is unstable. Natural radioactive elements have atomic numbers 82 to 92. The Joliot-Curie couple first invented induced radioactivity (Nobel prize, 1935); elements made by bombardment in the laboratory are artificial.
- Easy : Uranium, thorium and radium are natural radioactive elements.
- Medium : An element with atomic number 90 lies in the 82 to 92 range, so it can be natural and radioactive.
- Exam-type : Elements made by bombardment in a laboratory are artificial radioactive elements, discovered first by the Joliot-Curie couple (1935 Nobel prize).
Rule 2: History — Becquerel found that uranium compounds clouded photographic plates in the dark (Becquerel rays). Madame Curie found similar properties in thorium compounds. In 1889 Rutherford found alpha and beta radiation from radium and Willard found gamma radiation. The electric-plate method was introduced by Rutherford in 1902. Rutherford (New Zealand) split the nitrogen atom with alpha particles.
- Easy : Becquerel noticed that a photographic plate near uranium went dark.
- Medium : From Rutherford's 1889 discovery to the electric-plate method of 1902 is 1902 − 1889 = 13 years.
- Exam-type : Who found alpha and beta rays from radium, and who found gamma? Rutherford; Willard.
Rule 3: Alpha, beta and gamma — Alpha: He²⁺ particles, mass 4.0028 u, charge +2, attracted to the negative plate, very high ionization. Beta: electrons, mass 0.000548 u, charge −1, attracted to the positive plate, low ionization, penetrates 2 mm of aluminium. Gamma: electromagnetic radiation, no mass, neutral, not deviated, velocity of light, very low ionization, penetrates 15 cm of lead.
- Easy : Gamma rays are not deviated by electric plates because they carry no charge.
- Medium : Alpha has charge +2 and goes to the negative plate; beta has charge −1 and goes to the positive plate.
- Exam-type : Which is most penetrating? Gamma (15 cm of lead). Which ionizes the most? Alpha.
Radioactivity and radiation
In the app you can move this figure with your finger.
Open in the appAlpha (+2): high ionization, least penetrating. Gamma: neutral, most penetrating.
Thinking gamma rays are particles. Gamma rays are electromagnetic radiation.
Memory clue: "A for Alpha: heAvy and Absorbed first; G for Gamma: Goes through".
Spot the difference
| Ray | Charge | Ionization | Penetration |
|---|---|---|---|
| Alpha | +2 | Very high | Least |
| Beta | −1 | Low | 2 mm aluminium |
| Gamma | 0 | Very low | 15 cm lead |
Practice questions
- 1What is radioactivity?
- 2Name three natural radioactive elements.
- 3What is the atomic number range of natural radioactive elements?
- 4Who first invented induced radioactivity?
- 5What is the charge on alpha, beta and gamma?
- 6Which plate attracts alpha and which attracts beta?
- 7Arrange alpha, beta, gamma in increasing order of penetration.
- 8Why are gamma rays not deviated by electric plates and why do they penetrate most?
Show answers (try them yourself first)
- 1Spontaneous emission of invisible, highly penetrating, high energy radiation from unstable nuclei.
- 2Uranium, thorium and radium.
- 382 to 92.
- 4The Joliot-Curie couple.
- 5+2, −1 and 0.
- 6Alpha: negative plate; beta: positive plate.
- 7Alpha, beta, gamma.
- 8They have no charge and no mass, and very low ionization, so they lose little energy in matter.
Uses and hazards of radioactive substances
The same hospital that treats cancer with radiation also needs workers to stay behind lead walls. Radioactivity can cure and can harm. Everything depends on how it is used.
Radioactive isotopes (किरणोत्सारी समस्थानिके) are used in research, agriculture, industry and medicine, not only in atom bombs.
In the figure : In the figure radioactivity helps and harms.
Rule 1: Uses in industry and agriculture — Industry: gamma-ray radiography (cobalt-60, iridium-192) to find cracks, thickness measurement, luminous paint (radium, promethium, tritium with a phosphor), krypton-85 in HID lamps, promethium-147 in portable X-ray units, and uranium oxide earlier in ceramic paints. Agriculture: radiation modifies the genes of seeds; cobalt-60 preserves food and stops the sprouting of onions and potatoes; strontium-90 is a tracer in crop research.
- Easy : Cobalt-60 stops onions and potatoes from sprouting, so they last longer.
- Medium : Gamma-ray radiography finds a crack inside a metal pipe without cutting it.
- Exam-type : Why is radiation used on seeds? It can change the genes and give new varieties of crops.
Rule 2: Uses in medicine — Phosphorus-32 treats polycythemia. Strontium-89, strontium-90, samarium-153 and radium-223 are used for bone cancer. Iodine-123 treats hyperthyroidism. Boron-10, iodine-131 and cobalt-60 are used for brain tumour. Arsenic-74 detects small tumours.
- Easy : Iodine-123 is used for hyperthyroidism.
- Medium : Arsenic-74 helps doctors detect small tumours early.
- Exam-type : Match: cobalt-60 → brain tumour and food preservation; phosphorus-32 → polycythemia.
Rule 3: Hazards — Hazards: damage to the central nervous system, hereditary defects from DNA damage, skin cancer, leukemia, pollution through air and sea (fishes), cancer from radioactive paint on watches, and strontium-90 entering through plants and milk. Chernobyl: on 26 April 1986 the graphite reactor exploded, and genetic and thyroid disorders increased.
- Easy : Chernobyl exploded on 26 April 1986.
- Medium : Strontium-90 is dangerous because it can reach us through plants and milk.
- Exam-type : Workers handling radioactive materials use lead shields, short working times and a safe distance.
Radioisotopes help in industry, agriculture, medicine; they also harm (DNA damage, cancer).
Thinking radioactivity is only used for bombs. It has many peaceful uses.
Memory clue: "Cobalt-60 = Cracks, Crops, Cancer".
Spot the difference
| Field | Example |
|---|---|
| Industry | Cobalt-60 radiography |
| Agriculture | Food preservation |
| Medicine | Iodine-131, phosphorus-32 |
| Hazard | DNA damage, cancer |
Practice questions
- 1What are radioactive isotopes used for?
- 2Which isotope stops sprouting of onions and potatoes?
- 3Name an isotope used for hyperthyroidism.
- 4Name an isotope used for bone cancer.
- 5What happened at Chernobyl on 26 April 1986?
- 6Name two hazards of radiation.
- 7A study was done in 2016 on the Chernobyl accident of 1986. How many years after the accident was it?
- 8Why is strontium-90 dangerous even if the reactor is far away?
Show answers (try them yourself first)
- 1Research, agriculture, industry and medicine.
- 2Cobalt-60.
- 3Iodine-123.
- 4Strontium-89 (or strontium-90, samarium-153, radium-223).
- 5A graphite reactor exploded.
- 6Skin cancer and hereditary defects (also leukemia).
- 72016 − 1986 = 30 years.
- 8It can enter plants and milk and so reach the body.
Colours, deodorants, Teflon, coatings and ceramics
The red in a sweet, the non-stick pan, the deodorant you use after sport, and the cup you drink tea in: each of them uses a special material. This section shows how colours, deodorants, Teflon, coatings and ceramics work.
Teflon is a non-stick polymer, ceramics are clay products fired in a kiln, and dyes impart colour.
In the figure : In the figure making ceramic ware.
Rule 1: Food colours and dyes — Food colours (powders, gels, pastes) can be natural (seeds, beetroot, flowers, fruit concentrate) or artificial (tetrazene, sunset yellow). Colours in pickles, jam and sauce may contain small amounts of lead and mercury, and excess artificial colours can cause ADHD in children. A dye imparts colour, is soluble in water and insoluble in oil, and a mordant fixes it. Perkin invented an artificial dye in 1856. Henna leaves are safe; the carmine of lipstick causes stomach disorders; red Rang Panchami colour contains mercury.
- Easy : Beetroot juice is a natural red colour; sunset yellow is an artificial colour.
- Medium : Natural colours from beetroot or flowers are safer than artificial colours that may carry lead or mercury.
- Exam-type : Perkin's dye was invented in 1856 and Plunkett's Teflon in 1938, so they are 1938 − 1856 = 82 years apart.
Rule 2: Deodorants, Teflon and coatings — Deodorants stop odour from bacterial decomposition of sweat. They are ordinary, antiperspirant (about 15% aluminium chlorohydrate, which clogs the sweat pores) and clinical (20 to 25% aluminium, used at night). Aluminium–zirconium compounds are the most harmful. Teflon (polytetrafluoroethene, a polymer of tetrafluoroethylene; Roy J. Plunkett, 1938) has a melting point of 327 °C, is a poor conductor, is non-stick and is unaffected by water, oil and atmosphere. Powder coating is electrostatic spray deposition of a polymer powder on metal to prevent rusting. Anodizing is electrolysis with the aluminium article as the anode, forming a hydrated aluminium oxide layer.
- Easy : A Teflon coated pan does not let food stick, and it melts only at 327 °C.
- Medium : Anodizing aluminium thickens its natural oxide layer, so the metal resists corrosion.
- Exam-type : A non-stick pan will not melt in a cooking flame of about 200 °C, since Teflon melts only at 327 °C.
Rule 3: Ceramics — Ceramics are made of clay that is shaped and fired in a kiln at 1000 to 1150 °C; the glaze removes porosity. Porcelain uses kaolin (1200 to 1450 °C); bone china adds bone ash and is harder. Advanced ceramics (alumina, zirconia, silica, SiC, B₄C) are sintered at 1600 to 1800 °C without oxygen. Ceramics are brittle, water resistant, electrical insulators and withstand high temperature.
- Easy : A tea cup is a ceramic: it is fired in a kiln and glazed.
- Medium : Ordinary clay ware is fired at 1000 to 1150 °C, but kaolin porcelain needs 1200 to 1450 °C, higher.
- Exam-type : Why are ceramic insulators used on electric poles? They are electrical insulators and withstand weather and high temperature.
Teflon: non-stick, 327 °C. Ceramics: fired in a kiln, brittle, insulating. Natural colours are safer.
Believing all food colours are harmless. Some artificial colours may contain lead or mercury.
Memory clue: "Teflon = Tough, non-sTick; Ceramic = Clay in Kiln".
Spot the difference
| Material | Key fact |
|---|---|
| Teflon | Non-stick, melts at 327 °C |
| Ceramics | Clay fired at 1000 to 1150 °C |
| Anodizing | Oxide layer on aluminium |
| Dye | Water-soluble; mordant fixes |
Practice questions
- 1Name two natural food colours.
- 2What is a dye?
- 3Who invented the first artificial dye, and when?
- 4What do deodorants stop?
- 5What is Teflon? State its melting point.
- 6What is anodizing?
- 7Name any three properties of ceramics.
- 8Why is the glaze applied on ceramic ware? Compare the temperature range of ordinary ceramics with advanced ceramics.
Show answers (try them yourself first)
- 1Beetroot and flowers (also seeds, fruit concentrate).
- 2A substance that imparts colour; soluble in water and insoluble in oil.
- 3Perkin, in 1856.
- 4Odour from bacterial decomposition of sweat.
- 5Polytetrafluoroethene, a non-stick polymer; 327 °C.
- 6Electrolysis with the aluminium article as anode, forming a hydrated aluminium oxide layer.
- 7Brittle, water resistant and electrical insulating (and withstands high temperature).
- 8To remove porosity. Ordinary: 1000 to 1150 °C; advanced: 1600 to 1800 °C, so the advanced ones are fired at higher temperature.
Quick revision
- Electrolysis of brine: H₂ at the cathode, Cl₂ at the anode, NaOH formed.
- Alpha (+2), beta (−1), gamma (neutral): gamma penetrates the most, alpha ionizes the most.
- Bleaching powder CaOCl₂, baking soda NaHCO₃, washing soda Na₂CO₃.