Chemical reactions and writing equations
When you melt ice, you can freeze it back into ice. When milk turns into curd, you can never get milk back. The first is a physical change; the second is a chemical change, because a new substance was made.
In a chemical change the reactants (क्रियाकारके) turn into products (उत्पादिते).
Rule 1: Chemical and physical change — In a chemical change the composition changes permanently and new substances form. In a physical change only the state or shape changes. A chemical reaction is written as reactants → products.
In the figure : In the figure a chemical change makes a new substance.
- Easy : Burning a candle's wax, souring of milk and rusting of iron are chemical changes.
- Medium : Melting of wax, breaking of a glass and dissolving of sugar in water are physical changes.
- Exam-type : Iron + sulphur, when heated, form iron sulphide (Fe + S → FeS), a black solid that is not attracted by a magnet: a new substance with new properties.
Rule 2: How we write equations — State symbols: (s) solid, (l) liquid, (g) gas, (aq) in water. ↑ shows a gas given out, ↓ a precipitate formed, Δ shows heating, and a catalyst is written over the arrow.
- Easy : CaCO₃(s) → CaO(s) + CO₂(g)↑ on heating.
- Medium : Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)↑: zinc dissolves in acid and hydrogen bubbles out.
- Exam-type : BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s)↓ + 2NaCl(aq): the white precipitate shows a chemical change.
Chemical change: new substance, permanent. Symbols: (s) (l) (g) (aq), ↑ gas, ↓ precipitate, Δ heat.
Thinking dissolving is a chemical change. Sugar in water can be got back by evaporation, so it is physical.
Memory clue: "New substance, no way back = chemical".
Spot the difference
| Change | Type |
|---|---|
| Curd from milk | Chemical |
| Melting ice | Physical |
| Rusting | Chemical |
| Dissolving sugar | Physical |
Practice questions
- 1What is a chemical change?
- 2What are reactants and products?
- 3What does the symbol (aq) mean?
- 4What does ↓ show in an equation?
- 5Is burning of paper a physical or chemical change?
- 6Write the equation for iron and sulphur forming iron sulphide.
- 7Zinc is added to dilute HCl and bubbles form. Name the gas and write the equation.
- 8Why is the heating of a platinum wire in a flame a physical change but burning of magnesium a chemical change?
Show answers (try them yourself first)
- 1A change in which new substances are formed.
- 2Reactants are the starting substances; products are the new substances formed.
- 3Dissolved in water.
- 4A precipitate is formed.
- 5Chemical.
- 6Fe + S → FeS.
- 7Hydrogen; Zn + 2HCl → ZnCl₂ + H₂↑.
- 8The wire only glows and cools back unchanged; magnesium forms new magnesium oxide.
Balancing equations
A shopkeeper packs 2 pencils and 1 eraser in each kit. If he has 10 pencils, he can make only 5 kits. Counting parts on both sides, nothing gets lost or created. A chemical equation is balanced in the same way.
A balanced equation (संतुलित समीकरण) has the same number of atoms of each element on both sides.
Rule 1: Law of conservation of mass — Mass is neither created nor destroyed in a reaction, so atoms of each element must be equal on both sides. Balance by writing numbers (coefficients) in front of formulae by trial and error; never change the formulae. Example: 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O.
In the figure : In the figure counting atoms to balance an equation.
- Easy : H₂ + Cl₂ → 2HCl: left has 2 H and 2 Cl; right (2 × 1 H and 2 × 1 Cl) also has 2 H and 2 Cl.
- Medium : Balance Mg + HCl → MgCl₂ + H₂: Cl needs 2 on the left, so write 2HCl. Now H is 2 on both sides: Mg + 2HCl → MgCl₂ + H₂.
- Exam-type : Balance Al + O₂ → Al₂O₃. O is 2 on the left and 3 on the right; make both 6: 3O₂ and 2Al₂O₃. Now Al on the right is 4, so 4Al + 3O₂ → 2Al₂O₃.
Balancing equations
In the app you can move this figure with your finger.
Open in the appAtoms equal on both sides. Change only the coefficients, never the formulae.
Changing a subscript, such as writing H₂O₂ for water, to balance. That changes the substance.
Memory clue: "Count, match, coefficient, check".
Spot the difference
| Step | What to do |
|---|---|
| 1 | Write the skeleton equation |
| 2 | Count atoms on both sides |
| 3 | Put coefficients in front |
| 4 | Count again to check |
Practice questions
- 1What does the law of conservation of mass say?
- 2What is a coefficient in an equation?
- 3Balance: H₂ + O₂ → H₂O.
- 4Balance: N₂ + H₂ → NH₃.
- 5Balance: Zn + HCl → ZnCl₂ + H₂.
- 6Balance: KClO₃ → KCl + O₂.
- 7Balance: Fe + O₂ → Fe₂O₃.
- 8Why must we not change a formula (e.g. H₂O to H₂O₂) while balancing?
Show answers (try them yourself first)
- 1Mass is neither created nor destroyed in a chemical reaction.
- 2The number written before a formula to balance the atoms.
- 32H₂ + O₂ → 2H₂O.
- 4N₂ + 3H₂ → 2NH₃.
- 5Zn + 2HCl → ZnCl₂ + H₂.
- 62KClO₃ → 2KCl + 3O₂.
- 74Fe + 3O₂ → 2Fe₂O₃.
- 8A different formula is a different substance.
Types of reactions
A cooking stove gives heat when gas burns, but a cold pack for a sprained ankle takes heat from your skin. Both are chemical changes, but heat goes out in one and comes in in the other. Reactions can also be sorted by what the reactants do.
Four types: combination, decomposition, displacement and double displacement.
Rule 1: Four types of reactions — Combination (संयोग): two or more substances form one (2Mg + O₂ → 2MgO). Decomposition (अपघटन): one substance splits into simpler ones (CaCO₃ → CaO + CO₂). Displacement (विस्थापन): a more reactive element pushes out a less reactive one (CuSO₄ + Zn → ZnSO₄ + Cu). Double displacement (द्विविस्थापन): two compounds exchange parts (AgNO₃ + NaCl → AgCl↓ + NaNO₃).
In the figure : In the figure four types of chemical reactions.
- Easy : Hydrogen burns in oxygen: 2H₂ + O₂ → 2H₂O (combination).
- Medium : Electrolysis of water: 2H₂O → 2H₂ + O₂ (decomposition). Iron nail in copper sulphate: Fe + CuSO₄ → FeSO₄ + Cu (displacement).
- Exam-type : Identify BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl: two compounds swapped partners and a precipitate formed, so it is double displacement.
Rule 2: Exothermic and endothermic reactions — Exothermic (ऊष्माक्षेपी) reactions release heat; endothermic (ऊष्माशोषी) reactions absorb heat.
In the figure : In the figure heat given out or taken in.
- Easy : Burning of fuel and neutralisation are exothermic.
- Medium : Quicklime in water: CaO + H₂O → Ca(OH)₂ gives so much heat that the vessel becomes hot (exothermic).
- Exam-type : Heating limestone CaCO₃ needs constant heat to split into CaO and CO₂, so it is endothermic.
Four types: combination, decomposition, displacement, double displacement. Exothermic gives heat; endothermic takes heat.
Confusing displacement with double displacement. In displacement one element is swapped; in double displacement two compounds exchange parts.
Memory clue: "Join, split, one swaps, two swap".
Spot the difference
| Type | Pattern |
|---|---|
| Combination | A + B → AB |
| Decomposition | AB → A + B |
| Displacement | A + BC → AC + B |
| Double displacement | AB + CD → AD + CB |
Practice questions
- 1Name the four types of chemical reactions.
- 2Give an example of a decomposition reaction.
- 3What is an exothermic reaction?
- 4Classify: 2Mg + O₂ → 2MgO.
- 5Classify: Zn + CuSO₄ → ZnSO₄ + Cu.
- 6Classify: AgNO₃ + NaCl → AgCl↓ + NaNO₃.
- 7Is the dissolving of quicklime in water exothermic or endothermic? Why?
- 8Why does iron displace copper from copper sulphate solution?
Show answers (try them yourself first)
- 1Combination, decomposition, displacement, double displacement.
- 2CaCO₃ → CaO + CO₂.
- 3A reaction that releases heat.
- 4Combination.
- 5Displacement.
- 6Double displacement.
- 7Exothermic, because a lot of heat is released and the vessel becomes hot.
- 8Iron is more reactive than copper.
Rate of reaction
Whole potatoes take longer to boil than thin chips, and food goes bad faster in summer than in the fridge. Reactions can be quick or slow, and we can control the speed.
Rate of reaction (अभिक्रियेचा वेग) is how fast the reactants turn into products.
Rule 1: Factors that change the rate — Rate depends on the nature of the reactants, particle size, concentration, temperature and a catalyst (उत्प्रेरक), a substance that changes the rate and is not used up (MnO₂ for KClO₃). Refrigeration slows the decomposition of food.
In the figure : In the figure factors that change the rate of reaction.
- Easy : Powdered chalk reacts with acid faster than a lump, because more surface is exposed.
- Medium : Raising the temperature or the concentration of the acid makes the reaction faster.
- Exam-type : Hydrogen peroxide decomposes slowly: 2H₂O₂ → 2H₂O + O₂. A pinch of MnO₂ makes it bubble quickly, and the MnO₂ can be recovered unchanged.
Rate depends on nature, size, concentration, temperature, catalyst. Catalyst is not used up.
Thinking a catalyst is a reactant. It speeds up the change but remains chemically unchanged.
Memory clue: "Smaller, stronger, hotter, catalyst = faster".
Spot the difference
| Change | Effect on rate |
|---|---|
| Smaller particles | Faster |
| Higher temperature | Faster |
| Lower concentration | Slower |
| Catalyst added | Faster |
Practice questions
- 1Define rate of reaction.
- 2What is a catalyst?
- 3Name the catalyst used with KClO₃.
- 4Why is food kept in a refrigerator?
- 5Which reacts faster with acid: a marble chip or marble powder?
- 6Will a lower temperature increase or decrease the rate?
- 7A reaction takes 20 s at 50 °C. Will it take more or less time at 20 °C?
- 8Why does a powdered substance burn faster than a lump?
Show answers (try them yourself first)
- 1How fast reactants change into products.
- 2A substance that changes the rate of a reaction and is not used up.
- 3MnO₂.
- 4The low temperature slows the reactions that spoil it.
- 5Marble powder.
- 6It decreases the rate.
- 7More time, since it is slower at a lower temperature.
- 8Powder has more surface area, so more particles meet oxygen at once.
Oxidation, reduction, corrosion and rancidity
A bicycle left in the rain turns reddish-brown on its chain, and a cut apple becomes brown. Oil in a packet of old snacks smells bad. All these are air and water slowly changing materials.
Oxidation (ऑक्सिडीकरण) and reduction (क्षपण) always happen together and are called redox reactions.
Rule 1: Oxidation and reduction — Oxidation: gain of oxygen, loss of hydrogen or loss of electrons. Reduction: gain of hydrogen or loss of oxygen.
- Easy : 2Cu + O₂ → 2CuO: copper gains oxygen, so copper is oxidised.
- Medium : CuO + H₂ → Cu + H₂O: CuO loses oxygen (reduced), and H₂ gains oxygen (oxidised).
- Exam-type : In ZnO + C → Zn + CO, zinc oxide loses oxygen so it is reduced, and carbon gains oxygen so it is oxidised: a redox reaction.
In the figure : In the figure iron rusts when both air and water are present.
Rule 2: Corrosion and rancidity — Rust is Fe₂O₃·xH₂O. It forms only when both air and water are present. Rancidity is the oxidation of oils and fats by air, giving a bad smell and taste; it is prevented by antioxidants.
- Easy : Painting an iron gate keeps air and water away and slows rusting.
- Medium : Galvanising coats iron with zinc; chip packets are flushed with nitrogen so that the oil does not turn rancid.
- Exam-type : Experiment: nails in three test tubes: (1) dry with air, (2) boiled water with oil layer, (3) tap water open to air. Only the third rusts, proving both air and water are needed.
Oxidation = gain of oxygen; reduction = loss of oxygen. Rust needs air and water. Antioxidants prevent rancidity.
Thinking oxidation and reduction can happen alone. They always occur together in a redox reaction.
Memory clue: "OIL RIG: Oxidation Is Loss of electrons, Reduction Is Gain".
Spot the difference
| Term | Meaning |
|---|---|
| Oxidation | Gain of O or loss of H or e⁻ |
| Reduction | Gain of H or loss of O |
| Rusting | Needs air and water |
| Rancidity | Oil oxidised by air |
Practice questions
- 1Define oxidation.
- 2Define reduction.
- 3What is the chemical formula of rust?
- 4What is rancidity?
- 5How can rancidity be prevented?
- 6In CuO + H₂ → Cu + H₂O name the substance oxidised and the one reduced.
- 7Why do we apply paint or grease to iron tools?
- 8Why does a nail kept in boiled water covered with oil not rust?
Show answers (try them yourself first)
- 1Gain of oxygen, loss of hydrogen or loss of electrons.
- 2Gain of hydrogen or loss of oxygen.
- 3Fe₂O₃·xH₂O.
- 4Oxidation of oils and fats by air, giving bad smell and taste.
- 5By antioxidants, airtight containers or nitrogen flushing.
- 6H₂ is oxidised; CuO is reduced.
- 7It keeps air and water away and prevents rusting.
- 8Boiling removes dissolved air and the oil stops fresh air from entering, so air is missing.
Quick revision
- Balanced equation: equal atoms on both sides.
- 4 types of reactions; rate factors; redox; corrosion; rancidity.