Chemical change and equations
When you mix a pinch of baking soda in vinegar, it fizzes at once and the white powder vanishes. Something new has been made; this is not just mixing.
A chemical change (रासायनिक बदल) forms a new substance with a different composition and different properties. A chemical equation (रासायनिक समीकरण) is a short way of writing it, with reactants (अभिक्रियाकारक) on the left and products (उत्पादिते) on the right.
In the figure : In the figure a chemical change forms a new substance. Its signs here are gas bubbles, a low sound, the vanishing of the white solid and a milder sour taste.
Rule 1: Signs of a chemical change — Change of colour, change of taste, release of gas, a new sound, heat or light can show a chemical change. In the lemon juice and baking soda activity, CO2 bubbles come out, a low sound is heard and the white particles disappear.
- Easy : Milk turning sour gives a new taste and smell: chemical change.
- Medium : Iron left in rain becomes brown rust, a new substance: chemical change.
- Exam-type : Melting of ice looks like a change but no new substance forms, so it is a physical change.
In the figure : In the figure of a word equation the reactants are on the left, the products on the right, and the arrow points towards the products.
Rule 2: Writing equations — First write the word equation with names. Replace the names by formulae to get the chemical equation. Citric acid + Sodium bicarbonate → Carbon dioxide + Sodium citrate: an acid and an alkali give CO2 and a salt, which is a neutralization reaction.
- Easy : Word equation: Magnesium + Oxygen → Magnesium oxide.
- Medium : Vinegar and baking soda: Acetic acid + Sodium bicarbonate → Sodium acetate + Water + Carbon dioxide.
- Exam-type : Check the balance of 2Mg + O2 → 2MgO: Mg is 2 on each side and O is 2 on each side.
Chemical change and equations
In the app you can move this figure with your finger.
Open in the appReactants on the left, products on the right, arrow towards the products.
Calling every change a chemical change. If no new substance forms, it is physical.
Memory clue: "Reactants Run to the Right and become Products": R on the left, P on the right.
Spot the difference
| Point | Physical change | Chemical change |
|---|---|---|
| New substance | No | Yes |
| Example | Ice melting | Rusting, souring of milk |
Practice questions
- 1What is a chemical change?
- 2Which side of the equation has the reactants?
- 3Name two signs of a chemical change.
- 4Which gas comes out when baking soda is added to lemon juice?
- 5Write the word equation for magnesium burning in oxygen.
- 6Why is melting of ice not a chemical change?
- 7A reaction between an acid and an alkali gives CO2 and a salt. What is this type of reaction called?
- 8Count the atoms to show that 2Mg + O2 → 2MgO is balanced.
Show answers (try them yourself first)
- 1A change in which a new substance with different composition and properties forms.
- 2The left side.
- 3Gas bubbles and change of colour (or taste, sound).
- 4Carbon dioxide
- 5Magnesium + Oxygen → Magnesium oxide
- 6No new substance forms.
- 7Neutralization reaction
- 8Left: Mg 2, O 2. Right: Mg 2, O 2. So both sides are equal.
Natural and man-made chemical changes
Every breath, every cooking flame and every green leaf shows a chemical change. Our own body burns food slowly to give energy, much like a stove burns gas, only gently.
Chemical changes occur in nature (respiration, photosynthesis) and are also man-made (burning of fuels).
In the figure : In the figure respiration burns glucose with oxygen in the cells, and photosynthesis forms glucose from carbon dioxide and water in sunlight.
Rule 1: Respiration and photosynthesis — Respiration: Glucose + Oxygen → Carbon dioxide + Water (C6H12O6 + O2 → CO2 + H2O). Photosynthesis in green plants in sunlight: Carbon dioxide + Water → Glucose + Oxygen. The two are opposite to each other.
- Easy : We breathe in oxygen and breathe out CO2: respiration.
- Medium : Plants take in CO2 and give out oxygen in sunlight: photosynthesis.
- Exam-type : After a long run you feel warm because respiration is releasing energy from glucose.
In the figure : In the figure breath blown through a tube into fresh lime water makes it milky because calcium carbonate forms, which shows carbon dioxide.
Rule 2: Test for carbon dioxide — Blowing air from our lungs into lime water turns it milky. A white precipitate of calcium carbonate forms, which shows the gas is carbon dioxide.
- Easy : Lime water turns milky when CO2 is passed.
- Medium : The same milkiness is seen in the gas given off when baking soda is mixed in vinegar.
- Exam-type : If clear lime water stays clear, the gas passed is not carbon dioxide.
In the figure : In the figure fuels such as wood, coal, petrol and cooking gas burn as their carbon combines with oxygen: C + O2 → CO2.
Rule 3: Combustion of fuels — Wood, coal, petrol and cooking gas burn when their carbon combines with oxygen: Carbon + Oxygen → Carbon dioxide (C + O2 → CO2). It is fast and cannot be reversed.
- Easy : A burning candle makes heat and light; wax gives CO2 and water.
- Medium : Methane (gas) burns: CH4 + 2O2 → CO2 + 2H2O. Atoms: C 1, H 4, O 4 on each side.
- Exam-type : Burning coal in a closed room is dangerous because oxygen is used up and harmful gases collect; keep rooms ventilated.
Respiration and photosynthesis are opposite. Lime water turns milky with CO2. Fuel burning: C + O2 → CO2.
Thinking photosynthesis happens only at night or respiration only in animals. Plants respire too, and all living cells respire.
Memory clue: "Photo-synthesis = Photo (light) makes food; Respiration = Release energy".
Spot the difference
| Process | Reactants | Products |
|---|---|---|
| Respiration | Glucose, oxygen | CO2, water |
| Photosynthesis | CO2, water | Glucose, oxygen |
| Fuel burning | Carbon, oxygen | CO2 |
Practice questions
- 1Write the word equation of respiration.
- 2What are the products of photosynthesis?
- 3Which test shows carbon dioxide?
- 4Which white solid makes lime water milky?
- 5Write the equation for burning of carbon.
- 6Why is the burning of fuels called a man-made chemical change?
- 7Show that CH4 + 2O2 → CO2 + 2H2O is balanced.
- 8Why are respiration and photosynthesis called opposite reactions?
Show answers (try them yourself first)
- 1Glucose + Oxygen → Carbon dioxide + Water
- 2Glucose and oxygen
- 3It turns lime water milky.
- 4Calcium carbonate
- 5C + O2 → CO2
- 6People burn fuels for their use, and new substances like CO2 form.
- 7Left: C 1, H 4, O 2 × 2 = 4. Right: C 1, H 2 × 2 = 4, O 2 + 2 = 4. Equal.
- 8The products of one are the reactants of the other.
Shahabad tile and hard water
A new Shahabad tile floor often has a white film. Housewives clean it with a little dilute acid and it fizzes. And in some villages bore-well water gives no lather with soap. Both are chemistry.
Hard water (कठीण पाणी) is water that does not form lather with soap because of dissolved salts of calcium and magnesium.
In the figure : In the figure dilute hydrochloric acid cleans a Shahabad tile: calcium carbonate gives soluble calcium chloride, carbon dioxide and water.
Rule 1: Cleaning Shahabad tile — The tile is mainly calcium carbonate. Calcium carbonate + Hydrochloric acid → Calcium chloride + Carbon dioxide + Water. Calcium chloride dissolves and is washed away, and CO2 bubbles mix with air. Use gloves because the acid can harm skin.
- Easy : Marble and limestone also fizz with dilute acid because they are calcium carbonate.
- Medium : An eggshell (calcium carbonate) fizzes in vinegar and slowly dissolves.
- Exam-type : Acid must be dilute and used carefully because strong acid can harm the skin and damage the floor.
In the figure : In the figure hard water gives no lather with soap, and washing soda gives a precipitate of insoluble carbonates and so softens the water.
Rule 2: Hard water and washing soda — Hard water contains dissolved chloride and sulphate salts of calcium and magnesium. Adding washing soda (sodium carbonate) gives a precipitate of insoluble carbonates, so the water is softened: CaCl2 + Na2CO3 → CaCO3 + 2NaCl.
- Easy : Soft water gives plenty of lather with soap.
- Medium : Kettles in hard-water areas get a white crust inside from the salts.
- Exam-type : Check the balance: CaCl2 + Na2CO3 → CaCO3 + 2NaCl has Na 2, Cl 2, Ca 1, C 1, O 3 on both sides.
Tile: calcium carbonate + acid gives CO2. Hard water is softened by washing soda.
Thinking hard water is physically hard or heavy. It is called hard because lather does not form.
Memory clue: "Hard water = No lather; Washing soda = Wipes away the salts".
Spot the difference
| Point | Hard water | Soft water |
|---|---|---|
| Lather with soap | No or little | Plenty |
| Dissolved salts | Ca, Mg chlorides and sulphates | Very little |
| Softening | Washing soda | Not needed |
Practice questions
- 1What is the main substance in a Shahabad tile?
- 2Which acid is used to clean the tile?
- 3Why does hard water not form lather?
- 4What is washing soda?
- 5Write the word equation for the cleaning of the tile.
- 6Why do bubbles form on the tile?
- 7Explain how washing soda softens hard water.
- 8A student wants to clean a marble sink with strong acid. Why is that a bad idea?
Show answers (try them yourself first)
- 1Calcium carbonate
- 2Dilute hydrochloric acid
- 3It has dissolved calcium and magnesium salts.
- 4Sodium carbonate
- 5Calcium carbonate + Hydrochloric acid → Calcium chloride + Carbon dioxide + Water
- 6Carbon dioxide gas is released.
- 7It makes insoluble carbonates of calcium and magnesium, which settle as a precipitate.
- 8Marble is calcium carbonate, so strong acid will eat into it and can also harm the skin.
Chemical bond and ionic bond
Players in a team pass the ball to a friend who can use it better. Atoms do something similar: some give electrons away and some take them, so that both end up stable.
A chemical bond (रासायनिक बंध) is the force that holds atoms together. In an ionic bond (आयनिक बंध) one atom gives electrons to another and the attraction between the oppositely charged ions holds them.
In the figure : In the figure noble gases form no bonds because their duplet (helium) or octet (neon, argon) is already complete.
Rule 1: Why atoms bond — In a chemical change, bonds in the reactants break and new bonds form in the products. The number of bonds an atom forms is its valency. Noble gases form no bonds because their octet or duplet is complete; other atoms use their valence electrons to complete it.
- Easy : Helium has 2 electrons, so its duplet is complete.
- Medium : Neon is 2, 8, so its octet is complete and it forms no bonds.
- Exam-type : Argon is 2, 8, 8: also a complete octet and therefore stable.
In the figure : In the figure sodium is 2, 8, 1 and chlorine is 2, 8, 7, so sodium can give away one electron and chlorine can accept one.
Rule 2: Electron arrangements — A metal with 1 to 3 valence electrons tends to give them away. A non-metal with 5 to 7 valence electrons tends to take electrons. Sodium (2, 8, 1) gives one; chlorine (2, 8, 7) needs one.
- Easy : Potassium (2, 8, 8, 1) has one valence electron, so it gives it away easily.
- Medium : Fluorine (2, 7) needs one more electron to make an octet.
- Exam-type : Magnesium (2, 8, 2) gives away two electrons, so its ion is Mg²⁺.
In the figure : In the figure sodium gives its valence electron to chlorine, forming Na⁺ and Cl⁻ ions, and their attraction is the ionic bond.
Rule 3: Ionic bond — Sodium gives its valence electron to chlorine and becomes Na⁺. Chlorine becomes Cl⁻. The electrostatic attraction between Na⁺ and Cl⁻ is the ionic (electrovalent) bond, and NaCl is an ionic compound. MgCl2 is another one.
- Easy : Table salt NaCl is an ionic compound.
- Medium : Potassium (K) gives one electron to chlorine: K⁺ and Cl⁻ make KCl.
- Exam-type : Magnesium gives two electrons to oxygen (2, 6): Mg²⁺ and O²⁻ make MgO, with charges 2+ and 2− balancing to zero.
Chemical bond and ionic bond
In the app you can move this figure with your finger.
Open in the appIn the figure : In the figure the valency of an ion equals the size of its charge: Na⁺ and Cl⁻ have valency 1, and Mg²⁺ and O²⁻ have valency 2.
Rule 4: Valency of ions — The valency of an ion equals the size of its charge, ignoring the sign. Na⁺ and Cl⁻: 1. Mg²⁺ and O²⁻: 2. Al³⁺: 3.
- Easy : Ca²⁺ has valency 2.
- Medium : The formula of calcium chloride is CaCl2 because one Ca²⁺ needs two Cl⁻.
- Exam-type : Aluminium oxide: two Al³⁺ (+6) and three O²⁻ (−6) give Al2O3.
Noble gases: no bonds. Ionic bond: transfer of electrons. Valency = size of charge.
Writing Na⁺ valency as +1. Valency has no sign; the charge has.
Memory clue: "Metals Lose, Non-metals Gain" (LEO-like: metals Lose electrons).
Spot the difference
| Point | Metal atom | Non-metal atom |
|---|---|---|
| Electrons | Gives away | Takes |
| Ion formed | Positive (cation) | Negative (anion) |
| Example | Na⁺, Mg²⁺ | Cl⁻, O²⁻ |
Practice questions
- 1Why do noble gases not form bonds?
- 2What is the electron arrangement of sodium?
- 3What is an ionic bond?
- 4What is the valency of Cl⁻?
- 5How is KCl formed?
- 6Why does magnesium form Mg²⁺?
- 7Why is the formula of calcium chloride CaCl2 and not CaCl?
- 8Write the formula of aluminium oxide and explain.
Show answers (try them yourself first)
- 1Their duplet or octet is already complete.
- 22, 8, 1
- 3The electrostatic attraction between oppositely charged ions.
- 41
- 5Potassium gives one electron to chlorine, giving K⁺ and Cl⁻.
- 6It has 2 valence electrons and gives both away.
- 7Ca²⁺ has charge 2+, so it needs two Cl⁻ (each 1−) to balance.
- 8Al2O3: 2 × (+3) = +6 and 3 × (−2) = −6, so the total charge is zero.
Covalent bond
Two friends buy one book and share it by reading in turns; neither owns it completely. Some atoms share electrons in the same way, and the shared pair holds them together.
A covalent bond (सहसंयुज बंध) is formed by the sharing of valence electrons between two atoms, generally of similar nature.
In the figure : In the figure of covalent molecules atoms share electrons. H2 and HCl have one bond each, H2O has two bonds, and each bond is shown by a dash.
Rule 1: Covalent bond — One covalent bond means sharing of two electrons and is drawn as a dash between the symbols. In H2 two hydrogen atoms (valency 1) share a pair and complete their duplets. In H2O oxygen (six valence electrons, valency 2) forms two bonds, one with each hydrogen. HCl has one bond.
- Easy : H–H has one bond: sharing 2 electrons.
- Medium : Cl2 (Cl–Cl): each chlorine (2, 8, 7) shares one electron and both reach an octet.
- Exam-type : In methane CH4 carbon (valency 4) makes four bonds, one with each of 4 hydrogens, so there are 4 shared pairs = 8 shared electrons.
In the figure : In the figure an ionic bond forms by the transfer of electrons and a covalent bond by the sharing of electrons.
Rule 2: Ionic and covalent bonds compared — Ionic: a metal gives and a non-metal takes, forming ions. Covalent: two non-metal atoms share electrons and form molecules.
- Easy : NaCl is ionic and HCl is covalent.
- Medium : Water is covalent and common salt is ionic.
- Exam-type : Ammonia NH3: nitrogen (2, 5, valency 3) shares one pair with each of 3 hydrogens.
Ionic: transfer. Covalent: sharing. One dash = one shared pair = 2 electrons.
Counting one covalent bond as one electron. One bond is a pair of two electrons.
Memory clue: "Ionic = Isolated give-take; Covalent = Co-share".
Spot the difference
| Point | Ionic bond | Covalent bond |
|---|---|---|
| Electrons | Transferred | Shared |
| Atoms | Metal and non-metal | Non-metals |
| Example | NaCl, MgO | H2O, HCl, CH4 |
Practice questions
- 1What is a covalent bond?
- 2How many electrons are shared in one covalent bond?
- 3How is a covalent bond shown in a drawing?
- 4How many bonds does oxygen form in water?
- 5How many bonds are in HCl?
- 6Which type of bond does NaCl have and which does H2O have?
- 7Why does carbon form four bonds in CH4?
- 8In Cl2 how many electrons does each chlorine atom have in its outer shell after bonding? Explain.
Show answers (try them yourself first)
- 1A bond formed by the sharing of valence electrons.
- 2Two
- 3By a dash between the symbols.
- 4Two
- 5One
- 6NaCl is ionic; H2O is covalent.
- 7It has 4 valence electrons (2, 4), so it shares with four hydrogens to complete its octet.
- 88. Each had 7 and shares one pair, so each counts 7 + 1 = 8.
Quick revision
- Reactants are on the left of the arrow, products on the right.
- Ionic bond: transfer of electrons; covalent bond: sharing of electrons.
- Hard water is softened with washing soda (precipitate of carbonates).