Chemical Reactions and Equations | CBSE Class 10 Science Notes

This note covers chemical changes, word and chemical equations, balancing equations, physical states and reaction conditions, combination and decomposition, energy changes, displacement and double displacement, precipitation, oxidation and reduction, corrosion, and rancidity.

How can we recognise a chemical reaction?

A chemical reaction takes place when a chemical change occurs. The nature and identity of the initial substance change. Milk left at room temperature during summers and food being cooked provide familiar situations in which such changes occur.

An iron pan or nail left in a humid atmosphere is another example. Across these situations, the initial substances have somewhat changed in nature and identity. Looking closely at the substances before and after a change helps us recognise what has happened.

What observations provide evidence?

A change in state, a change in colour, evolution of a gas, or a change in temperature helps us determine whether a reaction has occurred. Evolution of a gas means that gas is produced during the change.

Magnesium ribbon burns in air with a dazzling white flame. It becomes a white powder, magnesium oxide, formed through the reaction of magnesium with oxygen in the air. The product differs from the original ribbon.

1. Clean a magnesium ribbon about 3 to 4 centimetres long by rubbing it with sandpaper.
2. Hold the ribbon with tongs and burn it using a spirit lamp or burner.
3. Collect the ash in a watch-glass while keeping the burning ribbon as far from the eyes as possible.
4. Observe the dazzling white flame and the white powder formed. This activity needs the teacher's assistance; suitable eyeglasses would be better.

Tongs hold magnesium ribbon above a burner. A watch-glass underneath collects magnesium oxide. The labelled parts are the tongs, ribbon, burner, watch-glass and magnesium oxide.

See Fig. 1.1 in your NCERT textbook

Zinc granules reacting with dilute sulphuric acid produce hydrogen gas. Here, dilute describes an acid solution containing a relatively small amount of acid in water. Check for a change in the temperature of the reaction vessel as another possible sign of reaction.

What do word equations and chemical equations represent?

A word equation represents a reaction using the names of substances. Reactants are the substances that undergo chemical change. Products are the new substances formed. For burning magnesium, magnesium and oxygen are the reactants, and magnesium oxide is the product.

Magnesium + Oxygen → Magnesium oxide

The plus sign, +, separates substances on the same side. The arrow, →, points from reactants towards products and shows the direction of the reaction. Reactants occupy the left-hand side, abbreviated LHS; products occupy the right-hand side, abbreviated RHS.

How do symbols shorten the description?

A chemical equation uses chemical formulae instead of substance names. A chemical formula represents a substance using element symbols and numbers. An element is a substance consisting of one kind of atom; an atom is the basic unit counted for an element in balancing.

Here, Mg denotes magnesium and O denotes oxygen. O₂ represents oxygen, while MgO represents magnesium oxide. The small lower number, called a subscript, specifies the number of atoms of that element in the formula. Thus O₂ contains two oxygen atoms.

Mg + O₂ → MgO is a skeletal equation: it has not yet been balanced. There is one magnesium atom on each side, but two oxygen atoms on the left and one on the right.

Why is balancing necessary?

Definition: The law of conservation of mass states that mass can neither be created nor destroyed in a chemical reaction. The total mass of the elements in the products equals their total mass in the reactants.

The number of atoms of each element therefore remains the same before and after a reaction. A balanced chemical equation expresses this equality. During reactions, atoms do not change into atoms of another element, disappear from the mixture, or appear from elsewhere.

Reactions involve the breaking and making of bonds, the links between atoms, to produce new substances. Balancing records the rearrangement while keeping the number of atoms of each element equal on the two sides.

How is a chemical equation balanced step by step?

A coefficient is the number placed before a chemical formula to multiply the quantity represented by the entire formula. Balancing changes coefficients, not formulae. The hit-and-trial method uses trials to obtain the smallest whole-number coefficients that balance the equation.

Worked example 1: Is the zinc equation balanced?

For zinc reacting with sulphuric acid, Zn means zinc, H means hydrogen and S means sulphur. H₂SO₄ is sulphuric acid, ZnSO₄ is zinc sulphate, and H₂ is hydrogen gas. The equation is Zn + H₂SO₄ → ZnSO₄ + H₂.

Answer: Every element has an equal atom count on both sides, so the equation is already balanced. There is no need to change its coefficients merely because it contains several substances.

Derivation: How is the iron and water equation balanced?

Use the skeletal equation Fe + H₂O → Fe₃O₄ + H₂. Fe denotes iron, H₂O is water, and Fe₃O₄ is the iron oxide shown as the product. Initially the iron counts are 1 and 3, hydrogen counts 2 and 2, and oxygen counts 1 and 4.

1. Protect the formulae. Draw a box around each formula. Nothing inside these boxes changes during balancing.
2. List the atoms. Count each element separately on the reactant and product sides before choosing coefficients.
3. Balance oxygen. It is often convenient to begin with the compound containing the maximum number of atoms, and the element with the maximum number within it. Choose Fe₃O₄ and oxygen here: Fe + 4H₂O → Fe₃O₄ + H₂.
4. Balance hydrogen. Four water molecules contain eight hydrogen atoms. Put 4 before H₂: Fe + 4H₂O → Fe₃O₄ + 4H₂. A molecule is a group of bonded atoms represented here by H₂O or H₂.
5. Balance iron. Fe₃O₄ contains three iron atoms, so put 3 before Fe: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
6. Check every element. Both sides now contain three iron atoms, eight hydrogen atoms and four oxygen atoms. The equation is balanced.
7. Add physical states. Once the atom counts agree, state symbols can make the equation more informative, as explained next.

Answer: $3\mathrm{Fe} + 4\mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{Fe}_{3}\mathrm{O}_{4} + 4\mathrm{H}_{2}$. The coefficients are 3, 4, 1 and 4, with the coefficient 1 left unwritten.

Note: Use 4H₂O to balance oxygen in this example. Replacing H₂O with H₂O₄ or (H₂O)₄ changes the written formula instead of supplying the correct coefficient. It is often convenient, rather than compulsory, to start with the largest compound.

Derivation: How is the magnesium and oxygen equation balanced?

Start with the skeletal equation $\mathrm{Mg} + \mathrm{O}_{2} \rightarrow \mathrm{MgO}$. Apply conservation of mass by making the number of atoms of each element equal on both sides. Keep the chemical formulae unchanged and adjust their coefficients.

1. Count the atoms. There is one magnesium atom on each side. Oxygen has two atoms on the left but only one on the right, so the equation is unbalanced.
2. Balance oxygen. Put 2 before magnesium oxide: $\mathrm{Mg} + \mathrm{O}_{2} \rightarrow 2\mathrm{MgO}$. There are now two oxygen atoms on each side, but the right side also contains two magnesium atoms.
3. Balance magnesium. Put 2 before magnesium: $2\mathrm{Mg} + \mathrm{O}_{2} \rightarrow 2\mathrm{MgO}$. Changing this coefficient leaves the oxygen count unchanged.
4. Check both elements. Each side contains two magnesium atoms and two oxygen atoms. The coefficients 2, 1 and 2 are the smallest whole numbers that balance the equation.

Answer: $2\mathrm{Mg} + \mathrm{O}_{2} \rightarrow 2\mathrm{MgO}$. The coefficient 1 before oxygen is left unwritten.

How do state symbols and conditions make equations more informative?

An equation can be balanced yet leave out the physical states of its substances. Physical state identifies whether the substance is solid, liquid or gas, while the aqueous notation indicates that it is present as a solution in water.

The iron reaction becomes 3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g). Water is used as steam here, so its symbol is (g). Writing (l) would describe liquid water instead of the steam involved in this reaction.

Usually physical states are not included in a chemical equation unless it is necessary to specify them. Their purpose is to provide information, alongside the identities and balanced quantities of the substances.

Where are reaction conditions written?

Sometimes conditions such as temperature, pressure or a catalyst are indicated above or below the reaction arrow. A catalyst is a substance that changes a reaction's rate without being consumed overall. Conditions describe how the reaction takes place.

For example, carbon monoxide, CO, reacts with hydrogen to form methanol, CH₃OH. C denotes carbon. The equation is CO(g) + 2H₂(g) → CH₃OH(l), with a pressure of 340 atm indicated at the arrow. Here atm means atmosphere, a unit of pressure.

Another example uses carbon dioxide, CO₂, and water to form glucose, C₆H₁₂O₆, and oxygen. Chlorophyll is the green pigment associated with this light-driven process. Sunlight and chlorophyll are the conditions shown at the arrow.

6CO₂(aq) + 12H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(aq) + 6H₂O(l)

Retain the water on both sides when reading this form of the equation. The stated conditions and physical states add information beyond the balance of carbon, hydrogen and oxygen atoms.

What happens in combination reactions and whitewashing?

Definition: A combination reaction forms a single product from two or more reactants. The substances combining may be elements or compounds, where a compound contains chemically combined elements.

Calcium oxide is also called quick lime. Its formula is CaO, where Ca denotes calcium. It reacts vigorously with water to produce calcium hydroxide, Ca(OH)₂, called slaked lime, and releases a large amount of heat.

CaO(s) + H₂O(l) → Ca(OH)₂(aq) + Heat

The bracket in Ca(OH)₂ groups O and H together; the outside subscript 2 applies to both. There is one calcium atom, two oxygen atoms and two hydrogen atoms in this formula. The single chemical product is calcium hydroxide.

A hand touches the side of a beaker. The diagram labels the beaker, water and calcium oxide, with the calcium oxide shown near the bottom of the vessel.

See Fig. 1.3 in your NCERT textbook

Why does a whitewashed wall acquire a shiny finish?

A solution of slaked lime is used for whitewashing, coating walls with the lime preparation. Calcium hydroxide reacts slowly with carbon dioxide in air. This forms a thin layer of calcium carbonate, CaCO₃, on the wall.

Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

Calcium carbonate forms after two to three days of whitewashing and gives a shiny finish. Marble also has the chemical formula CaCO₃. Keep the preparation of slaked lime and its later reaction with carbon dioxide as two separate reactions.

Which other examples show combination?

Burning coal is represented by C(s) + O₂(g) → CO₂(g). Carbon and oxygen form the single product carbon dioxide. Formation of water is represented by 2H₂(g) + O₂(g) → 2H₂O(l), again with a single product.

Burning magnesium also forms a single product. Its balanced equation is 2Mg + O₂ → 2MgO. The coefficient 2 before magnesium oxide records its quantity; it does not mean that two different products are formed.

How do exothermic and endothermic reactions differ?

An exothermic reaction releases heat along with the formation of products. An endothermic reaction absorbs energy. These terms describe energy changes, while combination and decomposition describe the pattern of change between reactants and products.

The reaction of quick lime with water is exothermic. The large amount of heat released makes the reaction mixture warm. It is also a combination reaction because calcium oxide and water form a single product, calcium hydroxide.

Why is respiration exothermic?

Respiration, in this example, is the reaction in which glucose combines with oxygen in body cells and provides energy. Food supplies the material needed for this process. During digestion, the process of breaking food down into simpler substances, these simpler substances become available.

Rice, potatoes and bread contain carbohydrates, which are broken down during digestion to form glucose. Glucose then combines with oxygen, producing carbon dioxide, water and energy.

C₆H₁₂O₆(aq) + 6O₂(aq) → 6CO₂(aq) + 6H₂O(l) + energy

The release of energy explains why respiration is considered exothermic. The decomposition of vegetable matter into compost, the material formed as that matter breaks down, is another example of an exothermic reaction.

What do fuel burning and decomposition show?

Burning natural gas is also exothermic. Its equation uses methane, CH₄: CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g). Here the products include both carbon dioxide and water.

Decomposition reactions considered here require energy as heat, light or electricity to break down reactants. Do not replace this with the claim that every process described as decomposition absorbs heat: vegetable matter forming compost provides an exothermic example.

How does heat cause decomposition reactions?

In a decomposition reaction, a single reactant breaks down into simpler products. It is the opposite pattern to combination, where reactants join to form one product. When decomposition is carried out by heating, it is called thermal decomposition.

What happens to ferrous sulphate crystals?

Ferrous sulphate crystals are green and have the formula FeSO₄·7H₂O. Here crystals are solids with an ordered arrangement of particles, and the dot indicates water associated with the crystalline salt. The 7H₂O records seven water molecules with each FeSO₄ unit. On heating, the crystals lose water and their colour changes. Ferrous sulphate then decomposes further.

2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g), on heating.

FeSO₄ is ferrous sulphate, Fe₂O₃ is ferric oxide, SO₂ is sulphur dioxide and SO₃ is sulphur trioxide. Ferric oxide is a solid, while the two sulphur compounds are gases. An odour characteristic of burning sulphur is produced.

A student holds a sloping boiling tube over a burner and gently wafts gas towards the nose. Labels identify the tube, ferrous sulphate crystals and burner. The mouth of the tube is directed away from people.

See Fig. 1.4 in your NCERT textbook

What happens to limestone and lead nitrate?

Calcium carbonate, shown as limestone, decomposes on heating to calcium oxide and carbon dioxide: CaCO₃(s) → CaO(s) + CO₂(g). Quick lime has several uses, including the manufacture of cement. This gives the reaction industrial importance.

Heating lead nitrate produces brown fumes of nitrogen dioxide. Pb denotes lead and N denotes nitrogen. Lead nitrate is Pb(NO₃)₂, lead oxide is PbO, and nitrogen dioxide is NO₂.

2Pb(NO₃)₂(s) → 2PbO(s) + 4NO₂(g) + O₂(g), on heating.

The lead nitrate activity uses about 2 grams of powder in a boiling tube held with tongs. Its gas products are nitrogen dioxide and oxygen. Identify the brown fumes specifically as nitrogen dioxide, rather than describing every gas produced as brown.

All three equations show one reactant giving more than one product. The supplied heat is a condition for decomposition; it does not replace any of the chemical substances in the balanced atom count.

How do electricity and light cause decomposition?

Decomposition can use electricity or light as the energy supply. These examples extend the idea beyond heating: the reactant breaks down, but the way energy reaches it differs. Water and silver chloride illustrate these two routes.

How is water decomposed using electricity?

Electrolysis means chemical decomposition using electric current. In the water activity, carbon electrodes connect the liquid to a battery. An electrode is a conducting part through which current enters or leaves the liquid.

1. Fit rubber stoppers into two holes in the base of a plastic mug, and insert carbon electrodes through them.
2. Connect the electrodes to a 6 volt battery, where volt is the unit of electric potential difference supplied by the battery.
3. Fill the mug until the electrodes are immersed in water, and add a few drops of dilute sulphuric acid.
4. Invert two water-filled test tubes over the electrodes, switch on the current and leave the apparatus undisturbed.
5. Observe bubbles at both electrodes. The gases displace water from the inverted test tubes as they collect.

The gases are hydrogen and oxygen. The balanced decomposition equation is 2H₂O(l) → 2H₂(g) + O₂(g), with electricity supplied. The hydrogen collected has twice the volume of the oxygen. Any test with a burning candle must be performed carefully by the teacher.

Two inverted test tubes stand over graphite rods inside a plastic mug of water. Rubber stoppers support the rods. Wires, a switch and a 6 V battery complete the arrangement; V abbreviates volt.

See Fig. 1.6 in your NCERT textbook

The figure labels oxygen at the anode, the positive electrode in this apparatus, and hydrogen at the cathode, its negative electrode. Graphite is the form of carbon used for the rods.

What happens to silver salts in sunlight?

White silver chloride turns grey in sunlight because it decomposes into silver and chlorine. Ag denotes silver and Cl denotes chlorine. The equation is 2AgCl(s) → 2Ag(s) + Cl₂(g), with sunlight at the arrow.

Silver bromide behaves in the same way. Br denotes bromine; AgBr is silver bromide and Br₂ is bromine. In sunlight, 2AgBr(s) → 2Ag(s) + Br₂(g). These reactions are used in black and white photography.

The colour change of silver chloride is evidence of the formation of a different substance. Keep the condition sunlight with the equation: it identifies the energy responsible for this decomposition.

What does a displacement reaction show?

A displacement reaction occurs when an element removes another element from its compound. Iron placed in copper sulphate solution provides a visible example: the nail becomes brownish and the blue colour of the solution fades.

Cu denotes copper. Copper sulphate is CuSO₄, while FeSO₄ is iron sulphate, also called ferrous sulphate. The equation is Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s). Iron displaces copper from copper sulphate.

How are the observations compared?

1. Clean three iron nails with sandpaper. Put about 10 millilitres of copper sulphate solution into each of two test tubes labelled A and B.
2. Tie two nails with thread and immerse them in tube B for about 20 minutes. Keep the third nail aside.
3. Take out the immersed nails after 20 minutes and compare them with the nail kept aside.
4. Compare the blue colour of the solution in tube A with the colour of the reaction mixture in tube B.

The untouched nail and solution provide comparisons for judging the changes. The deposited copper explains the brownish appearance of the immersed nails. The fading blue solution accompanies the replacement of copper sulphate by iron sulphate.

Which other elements displace copper?

Here, more reactive describes an element's greater tendency to undergo the relevant chemical change. Zinc also displaces copper: Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s). Zinc sulphate forms in solution, and copper is produced. Zinc is more reactive than copper.

Lead displaces copper from copper chloride. Copper chloride is CuCl₂, and lead chloride is PbCl₂. The reaction can be written Pb + CuCl₂ → PbCl₂ + Cu. Lead is also more reactive than copper.

In these examples, zinc and lead displace copper from its compounds. Follow which element enters the compound and which element is released when identifying displacement.

How do double displacement and precipitation differ?

In a double displacement reaction, ions are exchanged between reactants. An ion is an atom or group of atoms carrying an electric charge. The exchange produces new combinations of the ions.

A precipitate is an insoluble substance formed in a reaction. Insoluble means that it does not dissolve in the water present. A reaction producing a precipitate is called a precipitation reaction. One reaction can fit both descriptions.

What happens when sodium sulphate and barium chloride solutions mix?

Na denotes sodium and Ba denotes barium. Sodium sulphate is Na₂SO₄, barium chloride is BaCl₂, barium sulphate is BaSO₄, and sodium chloride is NaCl. Mixing the first two solutions produces a white insoluble substance.

Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)

The white precipitate is barium sulphate. Sodium chloride remains in solution. The solid notation beside barium sulphate and the aqueous notation beside sodium chloride make this difference explicit in the equation.

The sulphate ion, SO₄²⁻, combines with the barium ion, Ba²⁺. The superscripts 2− and 2+ represent two negative and two positive charges respectively. Their combination forms the insoluble barium sulphate.

One hand pours sodium sulphate solution from a test tube into another test tube containing barium chloride solution. A white deposit is drawn at the bottom of the receiving tube.

See Fig. 1.9 in your NCERT textbook

How does this differ from displacement?

Double displacement describes the exchange, while precipitation describes the insoluble product. The barium sulphate reaction demonstrates both. Identify the actual product that leaves the solution rather than treating every product as a precipitate.

How are oxidation and reduction identified?

Oxidation is gain of oxygen or loss of hydrogen. Reduction is loss of oxygen or gain of hydrogen. In an oxidation-reduction reaction, also called a redox reaction, one reactant is oxidised while another is reduced.

What happens when copper is heated?

Heating copper powder in air coats its surface with black copper(II) oxide, CuO. The Roman numeral II identifies copper's oxidation state as +2 in this compound. An oxidation state is the charge assigned to an atom for chemical accounting.

2Cu + O₂ → 2CuO, on heating. Copper gains oxygen, so copper is oxidised. The black coating records the formation of copper oxide on the original copper surface.

If hydrogen passes over the heated copper oxide, the black coating turns brown and copper is obtained. The equation is CuO + H₂ → Cu + H₂O, with heat supplied.

These are two descriptions of the same reaction. Do not stop after finding the substance that gains oxygen: also identify the substance that loses it. In this example, name copper oxide as the substance reduced, rather than copper, which is the product.

What do other redox examples show?

Zinc oxide, ZnO, reacts with carbon according to ZnO + C → Zn + CO. Carbon gains oxygen and is oxidised to carbon monoxide. Zinc oxide loses oxygen and is reduced to zinc.

Manganese dioxide, MnO₂, reacts with hydrochloric acid, HCl, to form manganese chloride, MnCl₂, water and chlorine. Mn denotes manganese. The balanced equation is MnO₂ + 4HCl → MnCl₂ + 2H₂O + Cl₂.

In this reaction, hydrochloric acid is oxidised to chlorine, while manganese dioxide is reduced to manganese chloride. Hydrochloric acid loses hydrogen in the change to chlorine. This example shows why oxidation must not be restricted to oxygen gain alone.

Burning magnesium gives another oxygen-gain example: magnesium becomes magnesium oxide and is oxidised. Follow the change in a named substance from reactant to product to identify oxidation or reduction reliably.

How do corrosion and rancidity affect everyday life?

Corrosion occurs when a metal is attacked by surrounding substances such as moisture or acids. It changes the surface of the metal and can damage objects. Familiar coatings on iron, silver and copper show different examples.

What changes are seen on metals?

New iron articles are shiny, but when left for some time they acquire a reddish brown coating. This process is commonly called rusting. A black coating on silver and a green coating on copper are other examples of corrosion.

Corrosion damages car bodies, bridges, iron railings, ships and other metal objects, especially those made of iron. Replacing damaged iron involves an enormous amount of expenditure every year. Its importance therefore extends beyond a change in appearance.

Rusting is a specific example of corrosion involving iron. Use corrosion as the broader term when discussing attack on different metals. The colours of their coatings help distinguish the examples without treating every coating as iron rust.

What happens when fats and oils become rancid?

Rancidity occurs when fats and oils are oxidised, changing their smell and taste. Food containing fat or oil and left for a long time provides a familiar setting for noticing these changes.

Antioxidants are substances that prevent oxidation. Usually they are added to foods containing fats and oils. Keeping food in airtight containers helps to slow down oxidation by reducing its contact with air.

Chips manufacturers usually flush bags of chips with a gas such as nitrogen to prevent the chips from being oxidised. Nitrogen here is the gas used in the packaging example, rather than a product of the food's oxidation.

Preserve the distinction between preventing oxidation and slowing it down. An airtight container helps to slow oxidation; this is not a claim that oxidation becomes impossible. Rancidity concerns the smell and taste of oxidised fats and oils, while corrosion concerns attack on metals.

Glossary

• Chemical reaction — A change in which initial substances undergo chemical change and new substances form.
• Reactants — Substances that undergo chemical change, written on the left side of a chemical equation.
• Products — New substances formed during a reaction, written on the right side of the arrow.
• Balanced chemical equation — An equation with equal numbers of atoms of each element on its two sides.
• Coefficient — A number before a formula that multiplies the quantity represented by the whole formula.
• Aqueous — Describes a substance present as a solution in water, represented by the notation (aq).
• Combination reaction — A reaction in which two or more reactants combine to form a single product.
• Decomposition reaction — A reaction in which one reactant breaks down to form two or more simpler products.
• Exothermic reaction — A chemical reaction that releases heat along with the formation of its products.
• Endothermic reaction — A reaction that absorbs energy, supplied in examples as heat, light or electricity.
• Displacement reaction — A reaction in which an element removes another element from its compound.
• Double displacement — A reaction involving an exchange of ions between the reacting substances.
• Precipitate — An insoluble substance formed during a reaction, such as white barium sulphate.
• Oxidation — Gain of oxygen or loss of hydrogen by a substance during a reaction.
• Reduction — Loss of oxygen or gain of hydrogen by a substance during a reaction.

Common errors and misconceptions

• Misconception: Change the subscripts to balance an equation. Correct: Change coefficients before formulae, leaving the formula of each substance unchanged.
• Misconception: An equation is balanced when the total number of atoms matches. Correct: The number of atoms of each separate element must match on both sides.
• Misconception: H₂O must have the state symbol (l). Correct: Water is written H₂O(g) in the iron and steam reaction because it is present as a gas.
• Misconception: Combination and exothermic describe competing categories. Correct: Quick lime reacting with water is both combination and exothermic: one describes products, the other heat release.
• Misconception: Every decomposition process absorbs heat. Correct: Decomposition can use heat, light or electricity, and the decomposition of vegetable matter into compost is exothermic.
• Misconception: Sodium chloride is the white precipitate when sodium sulphate and barium chloride solutions mix. Correct: Barium sulphate precipitates; sodium chloride remains dissolved.
• Misconception: Copper is reduced in CuO + H₂ → Cu + H₂O. Correct: Copper oxide is reduced by losing oxygen; hydrogen is oxidised by gaining oxygen.
• Misconception: Airtight storage makes food oxidation impossible. Correct: Airtight containers help to slow down oxidation of the fats and oils present.

Exam-style questions with model answers

Q1. Define a balanced chemical equation and explain why chemical equations need balancing. [2 marks]

1. A balanced chemical equation has equal numbers of atoms of each element on the reactant and product sides.
2. Balancing follows the law of conservation of mass: mass is neither created nor destroyed during a chemical reaction.

Q2. Balance Fe + H₂O → Fe₃O₄ + H₂, where Fe is iron, H₂O is water, Fe₃O₄ is iron oxide and H₂ is hydrogen. Show oxygen, hydrogen and iron balancing in that order, then verify all atom counts. [4 marks]

1. Balance oxygen by putting 4 before water: Fe + 4H₂O → Fe₃O₄ + H₂. This gives four oxygen atoms on each side.
2. There are now eight hydrogen atoms on the left. Put 4 before hydrogen: Fe + 4H₂O → Fe₃O₄ + 4H₂.
3. Balance iron by putting 3 before Fe. The completed equation is 3Fe + 4H₂O → Fe₃O₄ + 4H₂.
4. Check the final counts: each side has three iron, eight hydrogen and four oxygen atoms. The formulae remain unchanged.

Q3. Calcium oxide, CaO, reacts with water, H₂O, to form calcium hydroxide, Ca(OH)₂, releasing much heat. The calcium hydroxide later reacts with carbon dioxide, CO₂, to form calcium carbonate, CaCO₃, and water. Write both balanced equations, classify the first reaction in two ways, and explain the shiny finish produced on whitewashed walls after two to three days. [5 marks]

1. The first balanced equation is CaO + H₂O → Ca(OH)₂ + Heat. Calcium oxide is quick lime, and the calcium hydroxide produced is slaked lime.
2. It is a combination reaction because two reactants, calcium oxide and water, combine to form the single product calcium hydroxide.
3. It is exothermic because a large amount of heat is released with product formation. This makes the reaction mixture warm.
4. The later balanced equation is Ca(OH)₂ + CO₂ → CaCO₃ + H₂O. Calcium hydroxide reacts slowly with carbon dioxide in air.
5. A thin calcium carbonate layer forms after two to three days of whitewashing. This layer gives the wall its shiny finish.

Q4. Heating lead nitrate, Pb(NO₃)₂, produces lead oxide, PbO, brown nitrogen dioxide gas, NO₂, and oxygen, O₂. Write the balanced equation, identify the brown fumes, and classify the reaction with a reason. [3 marks]

1. The balanced equation is 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂, with heat supplied. Lead oxide, nitrogen dioxide and oxygen are the products.
2. The brown fumes are nitrogen dioxide. Oxygen is another gas produced, so the observation of brown fumes identifies a particular product.
3. This is thermal decomposition: heating causes a single reactant, lead nitrate, to break down into more than one simpler product.

Q5. Mixing aqueous sodium sulphate, Na₂SO₄, and barium chloride, BaCl₂, forms white insoluble barium sulphate, BaSO₄, and dissolved sodium chloride, NaCl. Write the balanced equation with state symbols, identify the precipitate, and explain why the reaction is double displacement. [3 marks]

1. The equation is Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq). Here (aq) means dissolved in water and (s) means solid.
2. Barium sulphate is the white precipitate because it is insoluble in water. Sodium chloride remains in solution instead of precipitating.
3. It is double displacement because ions are exchanged between the reactants. Barium ions combine with sulphate ions to form barium sulphate.

Q6. Heated copper oxide reacts with hydrogen: CuO + H₂ → Cu + H₂O, where CuO is black copper oxide, H₂ hydrogen, Cu brown copper and H₂O water. Identify the substance oxidised and the substance reduced, explain each oxygen change, and describe the colour change from copper oxide to copper. [5 marks]

1. Hydrogen is the substance oxidised in this reaction. It begins as hydrogen gas and becomes part of the water produced when it reacts with heated copper oxide.
2. Hydrogen gains oxygen in forming water. Gain of oxygen is oxidation, so this change supplies the reason for identifying hydrogen as oxidised.
3. Copper oxide is the substance reduced. Name the reactant copper oxide, because copper is the substance obtained after the reduction has occurred.
4. Copper oxide loses oxygen and forms copper. Loss of oxygen is reduction, occurring alongside the oxidation of hydrogen in the same redox reaction.
5. The black copper oxide coating turns brown as copper is obtained. This visible change accompanies the chemical conversion represented by the equation.

Q7. Define rancidity. Explain how airtight containers and nitrogen flushing help protect foods containing fats and oils. [3 marks]

1. Rancidity occurs when fats and oils are oxidised, causing changes in their smell and taste. It concerns chemical changes in these food constituents.
2. Airtight containers limit the entry of fresh air, reducing the food's contact with oxygen and helping to slow oxidation of its fats and oils.
3. Flushing packets with nitrogen displaces oxygen-containing air. Nitrogen does not readily react with the fats and oils under storage conditions, helping to prevent their oxidation and rancidity.

Q8. Iron displaces copper from copper sulphate solution: Fe + CuSO₄ → FeSO₄ + Cu, where Fe is iron, CuSO₄ copper sulphate, FeSO₄ iron sulphate and Cu copper. State the two visible changes seen in the nail and the originally blue solution. [2 marks]

1. The iron nail becomes brownish as copper is deposited on its surface during the displacement reaction.
2. The blue colour of the copper sulphate solution fades as iron sulphate forms in the solution.

Key takeaways

• Chemical equations represent reactants becoming products; balancing keeps the number of atoms of every element equal on both sides.
• Change coefficients during balancing, keep formulae unchanged, and check every element after the final coefficient is chosen.
• State symbols distinguish solids, liquids, gases and aqueous solutions; reaction conditions provide additional information beside the arrow.
• Combination forms one product from multiple reactants, while decomposition breaks one reactant into two or more simpler products.
• Exothermic reactions release heat with products; endothermic reactions absorb energy, including energy supplied as heat, light or electricity.
• Displacement replaces an element in a compound; double displacement exchanges ions, and precipitation forms an insoluble substance.
• Oxidation involves gaining oxygen or losing hydrogen; reduction involves losing oxygen or gaining hydrogen during a reaction.
• Corrosion damages metals, while rancidity changes the smell and taste of oxidised fats and oils in food.

Test yourself

What observations help identify a chemical reaction?

A change in state, colour or temperature, or the evolution of a gas, helps identify a chemical reaction.

Why does 4H₂O differ from H₂O₄?

4H₂O uses a coefficient before the unchanged water formula. H₂O₄ changes the formula itself and cannot be used to balance water in the equation.

Why is water written H₂O(g) in the iron and steam reaction?

The symbol (g) shows that water is present as steam, its gaseous form, in this reaction.

Why can quick lime reacting with water have two classifications?

It is combination because one product forms from two reactants, and exothermic because a large amount of heat is released.

What causes white silver chloride to turn grey in sunlight?

Light decomposes silver chloride into silver and chlorine; the formation of silver accompanies the grey appearance.

What makes barium sulphate a precipitate in the sodium sulphate reaction?

It forms as a white substance that is insoluble in water, while the sodium chloride product remains dissolved.

In ZnO + C → Zn + CO, which substance gains oxygen?

Carbon gains oxygen to form carbon monoxide and is oxidised. Zinc oxide loses oxygen and is reduced.

How do the corrosion coatings on iron, silver and copper differ?

Iron develops a reddish brown coating, silver a black coating, and copper a green coating.