CBSE Class 10 Science Chemical reactions and equations Notes

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Chapter 1 Chemical Reactions and Equations is a key CBSE Class 10 Science chapter. These exam notes cover chemical reactions, reactants and products, characteristics of reactions, writing and balancing chemical equations with state symbols, the types of chemical reactions (combination, decomposition, displacement, double displacement, redox, exothermic and endothermic), corrosion, rancidity, and solved intext and exercise questions.

CHEMICAL REACTIONS

Any change can be classified as a physical change or a chemical change. Physical changes can be easily reversed, but it is not easy to reverse a chemical change. In chemical changes, new substances are formed and it is difficult to regenerate the original substances. Chemical changes are more permanent than physical changes.

Chemical reactions are the processes in which new substances with new properties are formed. During a chemical reaction, atoms of one element do not change into those of another element. Only a rearrangement of atoms takes place.

Magnesium ribbon burns with a dazzling white flame and changes into a white powder. This powder is magnesium oxide. It is formed due to the reaction between magnesium and oxygen present in the air.

Magnesium (as ribbon) + Oxygen (from air) → Magnesium oxide (white powder), on heating

The burning of magnesium in air to form magnesium oxide is an example of a chemical reaction.

Reactants and Products

The substances which take part in a chemical reaction are called reactants. The new substances produced as a result of a chemical reaction are called products.

In the above reaction, there are two reactants, magnesium and oxygen, but only one product, magnesium oxide.

Characteristics of Chemical Reactions

In a chemical reaction, reactants are transformed into products. The important characteristics of a chemical reaction are:
• Evolution of a gas
• Formation of a precipitate
• Change in colour
• Change in temperature
• Change in state

Any one of these characteristics can tell us whether a chemical reaction has taken place or not.

CHEMICAL EQUATIONS

The method of representing a chemical reaction with the help of symbols and formulas of the substances involved is known as a chemical equation.

A word equation shows the change of reactants to products through an arrow placed between them. The reactants are written on the left-hand side (LHS) with a plus sign (+) between them. The products are written on the right-hand side (RHS) with a plus sign (+) between them. The arrowhead points towards the products and shows the direction of the reaction.

Example: A + B → C + D. Here A and B are reactants, and C and D are products. A condition, if any, is written above the arrow.

When hydrogen reacts with oxygen, it gives water:
Hydrogen + Oxygen → Water (word equation)
H₂ + O₂ → H₂O (chemical equation)

A chemical equation is a concise and informative way to represent a chemical reaction.

Balanced and Unbalanced Chemical Equations

A balanced chemical equation has the number of atoms of each element equal on both sides. Example: Zn + H₂SO₄ → ZnSO₄ + H₂. Here the numbers of zinc, hydrogen and sulphate are equal on both sides, so it is balanced.

An unbalanced chemical equation has a different number of atoms of one or more elements on the two sides. Example: Fe + H₂O → Fe₃O₄ + H₂. The left side has one iron atom, while the right side has three, so it is unbalanced.

Steps for Balancing a Chemical Equation

Take the unbalanced equation Fe + H₂O → Fe₃O₄ + H₂.

1. Write the number of atoms of each element in the reactants and products in a table.

2. Balance the atom which is present in the maximum number. Here oxygen is maximum on the RHS, so multiply H₂O on the LHS by 4: Fe + 4H₂O → Fe₃O₄ + H₂.
3. Now hydrogen becomes 8 on the LHS. Multiply H₂ on the RHS by 4: Fe + 4H₂O → Fe₃O₄ + 4H₂.
4. Iron is 1 on the LHS and 3 on the RHS. Multiply Fe on the LHS by 3: 3Fe + 4H₂O → Fe₃O₄ + 4H₂.

The balanced equation is 3Fe + 4H₂O → Fe₃O₄ + 4H₂.

Physical States in Chemical Equations

Writing the physical states of substances makes a chemical equation more informative:
• Gaseous state is written as (g)
• Liquid state is written as (l)
• Solid state is written as (s)
• Aqueous solution is written as (aq)

The condition under which the reaction takes place is written above and/or below the arrow.

INTEXT QUESTIONS PAGE NO. 6

Q1: Why should a magnesium ribbon be cleaned before it is burnt in air?

Answer:
Magnesium is an extremely reactive metal. When stored, it reacts with oxygen to form a layer of magnesium oxide on its surface. This layer is quite stable and prevents further reaction of magnesium with oxygen. The magnesium ribbon is cleaned with sand paper to remove this layer, so that the underlying metal is exposed to air.

Teacher's Note:
a) The oxide layer acts as a shield and stops the metal from burning properly.
b) Rub the ribbon with sand paper until the surface looks shiny.

Question 2: Write the balanced equation for the following chemical reactions.
(i) Hydrogen + Chlorine → Hydrogen chloride
(ii) Barium chloride + Aluminium sulphate → Barium sulphate + Aluminium chloride
(iii) Sodium + Water → Sodium hydroxide + Hydrogen

Answer:
(i) H₂ + Cl₂ → 2HCl
(ii) 3BaCl₂ + Al₂(SO₄)₃ → 2AlCl₃ + 3BaSO₄
(iii) 2Na + 2H₂O → 2NaOH + H₂

Teacher's Note:
a) Count the sulphate group as a unit in part (ii); three SO₄ on the left need three BaSO₄.
b) Part (iii) has two hydrogen atoms in water, so start with 2 before the water.

With state symbols:
(i) H₂(g) + Cl₂(g) → 2HCl(g)
(ii) 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 3BaSO₄(s) + 2AlCl₃(aq)
(iii) 2Na(s) + 2H₂O(l) → 2NaOH(aq) + H₂(g)

Question 3: Write a balanced chemical equation with state symbols for the following reactions.
(i) Solutions of barium chloride and sodium sulphate in water react to give insoluble barium sulphate and the solution of sodium chloride.
(ii) Sodium hydroxide solution (in water) reacts with hydrochloric acid solution (in water) to produce sodium chloride solution and water.

Answer:
(i) BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
(ii) NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O(l)

Teacher's Note:
a) Barium sulphate is insoluble, so it is written as a solid (s).
b) Water is a liquid, so it is written as (l) in equation (ii).

TYPES OF CHEMICAL REACTION

Chemical reactions can be classified into the following types:
• Combination reaction
• Decomposition reaction
• Displacement reaction
• Double displacement reaction
• Oxidation and reduction (redox) reaction

Combination Reaction

Reactions in which two or more reactants combine to form one product are called combination reactions.
• Magnesium burns in air: Mg + O₂ → 2MgO (magnesium + oxygen → magnesium oxide)
• Carbon burns in oxygen: C + O₂ → CO₂ (carbon + oxygen → carbon dioxide)
• Hydrogen reacts with chlorine: H₂ + Cl₂ → 2HCl (hydrogen + chlorine → hydrogen chloride)
• Calcium oxide reacts with water: CaO + H₂O → Ca(OH)₂ (calcium oxide + water → calcium hydroxide)
• Carbon monoxide reacts with oxygen: 2CO + O₂ → 2CO₂ (carbon monoxide + oxygen → carbon dioxide)

Decomposition Reaction

Reactions in which one compound breaks down into two or more compounds or elements are called decomposition reactions. Decomposition is the opposite of combination.
• Thermal decomposition: calcium carbonate on heating gives calcium oxide and carbon dioxide: CaCO₃ → CaO + CO₂
• Ferric hydroxide on heating: 2Fe(OH)₃ → Fe₂O₃ + 3H₂O (ferric hydroxide → ferric oxide + water)
• Lead nitrate on heating: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ (lead nitrate → lead oxide + nitrogen dioxide + oxygen)

In the examples above, the compound decomposes because of heating, so these are called thermal decomposition reactions.

Electrolytic Decomposition (Electrolysis)

Reactions in which compounds decompose into simpler compounds because electricity is passed through them are called electrolytic decomposition, or electrolysis.
Example: 2H₂O → 2H₂ + O₂ (when electricity is passed through water, it decomposes into hydrogen and oxygen)

Photolysis (Photo Decomposition)

Reactions in which a compound decomposes because of sunlight are called photolysis or photo decomposition reactions.
• 2AgCl → 2Ag + Cl₂ (silver chloride in sunlight gives silver and chlorine)
• 2AgBr → 2Ag + Br₂ (silver bromide in sunlight gives silver and bromine)

Photographic paper has a coat of silver chloride, which turns grey when exposed to sunlight. This happens because silver chloride is colourless, while silver is a grey metal.

INTEXT QUESTIONS PAGE NO. 10

Question 1: A solution of a substance 'X' is used for white washing.
(i) Name the substance 'X' and write its formula.
(ii) Write the reaction of the substance 'X' named in (i) above with water.

Answer:
(i) The substance 'X' is calcium oxide (quicklime). Its chemical formula is CaO.
(ii) CaO + H₂O → Ca(OH)₂ (calcium oxide reacts vigorously with water to form calcium hydroxide, also called slaked lime)

With state symbols: CaO(s) + H₂O(l) → Ca(OH)₂(aq)
Calcium oxide (quick lime) + Water → Calcium hydroxide (slaked lime)

Teacher's Note:
a) The solution of slaked lime, Ca(OH)₂, is used for white washing walls.
b) The reaction with water releases a large amount of heat.

Question 2: Why is the amount of gas collected in one of the test tubes in Activity 1.7 double of the amount collected in the other? Name this gas.

Answer:
Water (H₂O) contains two parts hydrogen and one part oxygen. So, the amounts of hydrogen and oxygen produced during electrolysis of water are in the ratio 2:1. Hydrogen collects in one test tube and oxygen in the other, so the gas in one tube is double the amount in the other. The gas collected in double quantity is hydrogen.

Teacher's Note:
a) The ratio of hydrogen to oxygen in water is 2:1 by volume.
b) Hydrogen is the gas collected at the cathode, and oxygen at the anode.
c) The test tube with double the gas is the hydrogen tube.

DISPLACEMENT REACTION

Reactions in which atoms or ions move from one compound to another to form a new compound are called displacement reactions. Displacement reaction is also known as substitution reaction or single displacement (replacement) reaction.

A displacement reaction takes place only when A is more reactive than B in the compound BC. If B is more reactive than A, then A will not displace B from BC, and no reaction takes place.

• Zinc reacts with hydrochloric acid: Zn + 2HCl → ZnCl₂ + H₂
• Zinc reacts with copper sulphate: Zn + CuSO₄ → ZnSO₄ + Cu
• Silver with copper nitrate: Ag + Cu(NO₃)₂ → no reaction takes place, because silver is less reactive than copper.

DOUBLE DISPLACEMENT REACTION

Reactions in which ions are exchanged between two reactants to form new compounds are called double displacement reactions.
• Barium chloride with sodium sulphate: BaCl₂ + Na₂SO₄ → BaSO₄ + 2NaCl (a white precipitate of barium sulphate is formed)
• Sodium hydroxide (a base) with hydrochloric acid: NaOH + HCl → NaCl + H₂O

A double displacement reaction in which a precipitate is formed is also known as a precipitation reaction. Neutralisation reactions are also examples of double displacement reactions.

EXOTHERMIC AND ENDOTHERMIC REACTIONS

Chemical reactions that proceed with the evolution of heat energy are called exothermic reactions. Example: N₂ + 3H₂ → 2NH₃ + Heat. All combustion reactions are exothermic.

Chemical reactions that proceed with the absorption of heat energy are called endothermic reactions.

Most combination reactions are exothermic, while most decomposition reactions are endothermic. Respiration is a reaction in which energy is released. When quicklime is added to water, it releases energy. Cooking involves endothermic reactions, because cooking is possible only with heating.

OXIDATION AND REDUCTION REACTION

Oxidation: Addition of oxygen or a non-metallic element, or removal of hydrogen or a metallic element from a compound, is called oxidation. The element or compound which undergoes oxidation is said to be oxidised.
Oxidising agent: A compound which can add oxygen or a non-metallic element, or remove hydrogen or a metallic element, is called an oxidising agent.

Reduction: Addition of hydrogen or a metallic element, or removal of oxygen or a non-metallic element from a compound, is called reduction. The compound or element which undergoes reduction is said to be reduced.
Reducing agent: A compound or element which can cause reduction is called a reducing agent.

Oxidation and reduction take place simultaneously, and such reactions are called redox reactions. In the word REDOX, 'Red' stands for reduction and 'Ox' stands for oxidation.

• Iron reacts with air to form iron oxide (rust): 4Fe + 3O₂ → 2Fe₂O₃. Iron is oxidised, and oxygen is the oxidising agent.
• Cupric oxide reacts with hydrogen: CuO + H₂ → Cu + H₂O. Cupric oxide is reduced to copper, and hydrogen is oxidised to water. Cupric oxide is the oxidising agent, and hydrogen is the reducing agent.
• Sodium hydroxide reacts with hydrochloric acid: NaOH + HCl → NaCl + H₂O. Here no oxygen is added or removed, so this is a neutralisation (double displacement) reaction and not a redox reaction in the sense of oxygen transfer.

Significance of Oxidation and Reduction in Everyday Life
• Respiration is an oxidation reaction in which food is oxidised to produce energy.
• Iron gets oxidised to form rust, which leads to corrosion of iron in the long run.
• Most metals react with atmospheric oxygen, which forms a layer on the metal article. The metal gets corroded in the long run.
• Rusting of iron can be prevented by painting the iron article. It can also be prevented by applying a layer of zinc over the iron article. This process is known as galvanization.
• Fried food gets oxidised when exposed to air. This spoils the taste of the food and makes it unfit for consumption. The spoiling of fried food because of oxidation is called rancidity. Fried food is often packed in airtight packets to prevent rancidity.
• We are able to utilise various types of fuel because of oxidation. Oxidation of fuel produces energy.

CORROSION

Corrosion is the slow and steady destruction of a metal by the environment. It results in the deterioration of the metal to form metal compounds by chemical reactions with the environment.

Corrosion is a simple electrochemical reaction. When the surface of iron is in contact with moisture and other gases in the atmosphere, an electrochemical reaction occurs. In this, the impure iron surface acts as the cathode and pure iron acts as the anode. H₂CO₃, formed from moisture and CO₂ from the air, acts as the electrolyte.

The electrochemical reactions are:
Fe → Fe²⁺ + 2e^-
O₂ + 2H₂O + 4e^- → 4OH^-

The Fe²⁺ ions are oxidised to Fe³⁺ ions. The Fe³⁺ ions combine with OH^- ions to form Fe(OH)₃. This becomes rust (Fe₂O₃.xH₂O), which is hydrated ferric oxide.

Methods of Preventing Corrosion

Corrosion of metals is prevented by not allowing them to come in contact with moisture, CO₂ and O₂. This is achieved by the following methods:
• Coating with paints: Paint keeps out air and moisture from the metal surface.
• Coating with oil and grease: Oil and grease on the surface of iron tools keep out moisture and air.
• Alloying with other metals: Alloyed metals are more resistant to corrosion. Example: stainless steel.
• Galvanization: Coating zinc on iron sheets, using electric current. The zinc forms a protective layer of zinc carbonate on the surface of iron, which prevents corrosion.
• Electroplating: Coating one metal with another by passing electric current. Example: silver plating and nickel plating. It protects the metal and enhances its metallic appearance.
• Sacrificial protection: Magnesium is more reactive than iron. When it is coated on articles made of steel, it sacrifices itself to protect the steel.

RANCIDITY

When fats and oils are oxidised, they become rancid, and their smell and taste change. Rancidity is the chemical decomposition of fats, oils and other lipids.

There are three basic types of rancidity:
• Hydrolytic rancidity: occurs when water splits fatty acid chains away from the glycerol backbone in glycerides.
• Oxidative rancidity: occurs when the double bonds of an unsaturated fatty acid react chemically with oxygen.
• Microbial rancidity: occurs when microorganisms such as bacteria use their enzymes, including lipases, to break down the chemical structure of fat.

In each case, these reactions result in undesirable odours and flavours. When a fatty substance is exposed to air, its unsaturated components are converted into hydroperoxides, which break down into volatile aldehydes, esters, alcohols, ketones and hydrocarbons, some of which have disagreeable odours.

Butter becomes rancid by this process and by hydrolysis, which liberates volatile and malodorous acids, particularly butyric acid. Saturated fats such as beef tallow are resistant to oxidation and seldom become rancid at ordinary temperatures.

Antioxidants, which prevent oxidation, are usually added to foods containing fats and oils. Keeping food in airtight containers helps to slow down oxidation. Rancidity can be avoided by:
1. Storing food in airtight containers
2. Storing food in refrigerators
3. Adding antioxidants
4. Storing food in an environment of nitrogen

INTEXT QUESTIONS PAGE NO. 13

Question 1: Why does the colour of copper sulphate solution change when an iron nail is dipped in it?

Answer:
When an iron nail is placed in copper sulphate solution, iron displaces copper from the solution and forms iron sulphate, which is green in colour. Fe + CuSO₄ → FeSO₄ + Cu. The blue colour of the copper sulphate solution fades, and a green colour appears.

Teacher's Note:
a) Blue colour is due to copper ions, and it disappears as they are removed from the solution.
b) This is a displacement reaction, since iron is more reactive than copper.

Question 2: Give an example of a double displacement reaction other than the one given in Activity 1.10.

Answer:
Na₂CO₃(aq) + CaCl₂(aq) → CaCO₃(s) + 2NaCl(aq)
Sodium carbonate and calcium chloride exchange ions to form two new compounds, calcium carbonate and sodium chloride. Hence, it is a double displacement reaction.

Teacher's Note:
a) Calcium carbonate is an insoluble precipitate, so it is written as a solid (s).
b) Check that the metals swap partners: Na goes with Cl, and Ca goes with CO₃.

Question 3: Identify the substances that are oxidised and the substances that are reduced in the following reactions.
4Na(s) + O₂(g) → 2Na₂O(s)
CuO(s) + H₂(g) → Cu(s) + H₂O(l)

Answer:
(i) Sodium (Na) is oxidised, as it gains oxygen to form sodium oxide. Oxygen is reduced.
(ii) Copper oxide (CuO) is reduced to copper (Cu), while hydrogen (H₂) is oxidised to water (H₂O).

Teacher's Note:
a) Gain of oxygen is oxidation, and loss of oxygen is reduction.
b) In a redox reaction, the oxidised and reduced substances always appear together.

EXERCISE QUESTIONS PAGE NO. 14, 15 AND 16

Question 1: Which of the statements about the reaction below are incorrect?
2PbO(s) + C(s) → 2Pb(s) + CO₂(g)
(a) Lead is getting reduced.
(b) Carbon dioxide is getting oxidised.
(c) Carbon is getting oxidised.
(d) Lead oxide is getting reduced.
(i) (a) and (b)
(ii) (a) and (c)
(iii) (a), (b) and (c)
(iv) all

Answer: (i) (a) and (b)

Teacher's Note:
a) Statement (a) is incorrect because the element lead is produced, not reduced; it is the lead oxide that is reduced.
b) Statement (b) is incorrect because CO₂ is formed by oxidation of carbon, and it is not oxidised further.
c) Statements (c) and (d) are correct, because carbon gains oxygen and lead oxide loses oxygen.

Question 2: The above reaction is an example of a
(a) combination reaction.
(b) double displacement reaction.
(c) decomposition reaction.
(d) displacement reaction.

Answer: (d) displacement reaction.

Teacher's Note:
a) Here, the reaction of Fe₂O₃ with Al is a displacement reaction in which aluminium displaces iron.
b) A combination reaction has only one product, so option (a) is wrong.

Question 3: What happens when dilute hydrochloric acid is added to iron filings? Tick the correct answer.
(a) Hydrogen gas and iron chloride are produced.
(b) Chlorine gas and iron hydroxide are produced.
(c) No reaction takes place.
(d) Iron salt and water are produced.

Answer: (a) Hydrogen gas and iron chloride are produced.
Fe(s) + 2HCl(aq) → FeCl₂(aq) + H₂(g)↑

Teacher's Note:
a) Iron is more reactive than hydrogen, so it displaces hydrogen from the acid.
b) The bubbles seen during the reaction are hydrogen gas.

Question 4: What is a balanced chemical equation? Why should chemical equations be balanced?

Answer:
A balanced chemical equation has an equal number of atoms of all the elements on both sides of the equation.
The law of conservation of mass states that mass can neither be created nor destroyed. So, in a chemical reaction, the total mass of the reactants is equal to the total mass of the products. This means the total number of atoms of each element must be equal on both sides of a chemical equation. That is why chemical equations should be balanced.

Teacher's Note:
a) The law of conservation of mass is the reason for balancing.
b) Balancing changes only the coefficients, never the formulas of the substances.

Question 5: Translate the following statements into chemical equations and then balance them.
(a) Hydrogen gas combines with nitrogen to form ammonia.
(b) Hydrogen sulphide gas burns in air to give water and sulphur dioxide.
(c) Barium chloride reacts with aluminium sulphate to give aluminium chloride and a precipitate of barium sulphate.
(d) Potassium metal reacts with water to give potassium hydroxide and hydrogen gas.

Answer:
(a) 3H₂(g) + N₂(g) → 2NH₃(g)
(b) 2H₂S(g) + 3O₂(g) → 2H₂O(l) + 2SO₂(g)
(c) 3BaCl₂(aq) + Al₂(SO₄)₃(aq) → 2AlCl₃(aq) + 3BaSO₄(s)
(d) 2K(s) + 2H₂O(l) → 2KOH(aq) + H₂(g)

Teacher's Note:
a) Each equation is first written with the correct formulas, then balanced.
b) In part (c), count the three sulphate groups carefully.
c) Water formed by burning hydrogen sulphide is a liquid, so it is written as (l).

Question 6: Balance the following chemical equations.
(a) HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + H₂O
(b) NaOH + H₂SO₄ → Na₂SO₄ + H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃
(d) BaCl₂ + H₂SO₄ → BaSO₄ + HCl

Answer:
(a) 2HNO₃ + Ca(OH)₂ → Ca(NO₃)₂ + 2H₂O
(b) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
(c) NaCl + AgNO₃ → AgCl + NaNO₃
(d) BaCl₂ + H₂SO₄ → BaSO₄ + 2HCl

Teacher's Note:
a) Part (c) is already balanced, with one atom of each element on both sides.
b) Balance the metal and the acid radical first, then hydrogen and oxygen last.

Question 7: Write the balanced chemical equations for the following reactions.
(a) Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water
(b) Zinc + Silver nitrate → Zinc nitrate + Silver
(c) Aluminium + Copper chloride → Aluminium chloride + Copper
(d) Barium chloride + Potassium sulphate → Barium sulphate + Potassium chloride

Answer:
(a) Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
(b) Zn + 2AgNO₃ → Zn(NO₃)₂ + 2Ag
(c) 2Al + 3CuCl₂ → 2AlCl₃ + 3Cu
(d) BaCl₂ + K₂SO₄ → BaSO₄ + 2KCl

Teacher's Note:
a) In part (b), silver nitrate has one nitrate group, so two AgNO₃ are needed to give Zn(NO₃)₂.
b) Aluminium and copper chloride: three copper atoms and two aluminium atoms balance the equation.
c) Remember that equal numbers of each atom must appear on both sides.

Question 8: Write the balanced chemical equation for the following and identify the type of reaction in each case.
(a) Potassium bromide (aq) + Barium iodide (aq) → Potassium iodide (aq) + Barium bromide(s)
(b) Zinc carbonate (s) → Zinc oxide (s) + Carbon dioxide (g)
(c) Hydrogen (g) + Chlorine (g) → Hydrogen chloride (g)
(d) Magnesium (s) + Hydrochloric acid (aq) → Magnesium chloride (aq) + Hydrogen (g)

Answer:
(a) 2KBr(aq) + BaI₂(aq) → 2KI(aq) + BaBr₂(aq); double displacement reaction
(b) ZnCO₃(s) → ZnO(s) + CO₂(g); decomposition reaction
(c) H₂(g) + Cl₂(g) → 2HCl(g); combination reaction
(d) Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g); displacement reaction

Teacher's Note:
a) Barium bromide is soluble in water, so it is written as (aq), not (s), in part (a).
b) Part (c) has one product from two reactants, which marks a combination reaction.
c) Part (d) is a displacement reaction because magnesium displaces hydrogen from the acid.

Question 9: What does one mean by exothermic and endothermic reactions? Give examples.

Answer:
Chemical reactions that release energy in the form of heat, light or sound are called exothermic reactions. Example: Na(s) + ½Cl₂(g) → NaCl(s) + 411 kJ of energy. Combination reactions are generally exothermic.
Reactions that absorb energy, or need energy to proceed, are called endothermic reactions. Example: in photosynthesis, plants use energy from the sun to convert carbon dioxide and water into glucose and oxygen: 6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(g) (in sunlight).

Teacher's Note:
a) Exothermic reactions give out energy, so the surroundings become warmer.
b) Endothermic reactions take in energy, so the surroundings become cooler.
c) Photosynthesis is a common example of an endothermic reaction.

Question 10: Why is respiration considered an exothermic reaction? Explain.

Answer:
Energy is required to support life, and the energy in our body comes from the food we eat. During digestion, large molecules of food are broken down into simpler substances such as glucose. Glucose combines with oxygen in the cells and provides energy: C₆H₁₂O₆(aq) + 6O₂(g) → 6CO₂(g) + 6H₂O(l) + Energy. Since energy is released in the whole process, respiration is an exothermic process.

Teacher's Note:
a) Respiration is a slow combustion of glucose inside the cells.
b) The energy released is used for growth, movement and other body activities.

Question 11: Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

Answer:
Decomposition reactions are those in which a compound breaks down to form two or more substances. These reactions require a source of energy to proceed. Combination reactions are the exact opposite, in which two or more substances combine to give a new substance with the release of energy.
Decomposition: AB + Energy → A + B; for example, 2H₂O(l) → 2H₂(g) + O₂(g) (electrolysis)
Combination: A + B → AB + Energy; for example, 2H₂(g) + O₂(g) → 2H₂O(l) + Energy

Teacher's Note:
a) A decomposition reaction needs energy input, while a combination reaction gives energy out.
b) Writing the two equations side by side makes the reverse relationship clear.
c) Electrolysis of water is a decomposition reaction; burning hydrogen is the reverse combination.

Question 12: Write one equation each for decomposition reactions where energy is supplied in the form of heat, light or electricity.

Answer:
(a) Thermal decomposition: 2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g) (ferrous sulphate on heating gives ferric oxide, sulphur dioxide and sulphur trioxide)
(b) Decomposition by light: 2AgCl(s) → 2Ag(s) + Cl₂(g) (silver chloride in light gives silver and chlorine)
(c) Decomposition by electricity: 2Al₂O₃(l) → 4Al(l) + 3O₂(g) (aluminium oxide on electrolysis gives aluminium and oxygen)

Teacher's Note:
a) Check that both sides have 2 Fe, 2 S and 8 O in part (a).
b) Aluminium oxide is electrolysed in molten form, so it is written as (l) in part (c).

Question 13: What is the difference between displacement and double displacement reactions? Write equations for these reactions.

Answer:
In a displacement reaction, a more reactive element replaces a less reactive element from a compound: A + BX → AX + B, where A is more reactive than B.
In a double displacement reaction, two atoms or groups of atoms switch places to form new compounds: AB + CD → AD + CB.
Displacement reaction: CuSO₄(aq) + Zn(s) → ZnSO₄(aq) + Cu(s)
Double displacement reaction: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq)

Teacher's Note:
a) In displacement, one element is replaced; in double displacement, two compounds swap their partners.
b) Double displacement often gives a precipitate, which is a useful sign that the reaction has taken place.

Question 14: In the refining of silver, the recovery of silver from silver nitrate solution involved displacement by copper metal. Write down the reaction involved.

Answer:
2AgNO₃(aq) + Cu(s) → Cu(NO₃)₂(aq) + 2Ag(s)

Teacher's Note:
a) Copper is more reactive than silver, so it displaces silver from its nitrate.
b) Two silver nitrate molecules are needed to balance the single copper atom.

Question 15: What do you mean by a precipitation reaction? Explain by giving examples.

Answer:
A reaction in which an insoluble solid, called a precipitate, is formed is called a precipitation reaction.
Example 1: Na₂CO₃(aq) + CaCl₂(aq) → CaCO₃(s) + 2NaCl(aq). Calcium carbonate is obtained as a precipitate.
Example 2: Na₂SO₄(aq) + BaCl₂(aq) → BaSO₄(s) + 2NaCl(aq). Barium sulphate is obtained as a precipitate.

Teacher's Note:
a) The precipitate is the insoluble solid that settles at the bottom of the container.
b) Every precipitation reaction is also a double displacement reaction.
c) Barium sulphate is a white precipitate that does not dissolve in acid.

Question 16: Explain the following in terms of gain or loss of oxygen with two examples each. (a) Oxidation (b) Reduction

Answer:
(a) Oxidation is the gain of oxygen.
(i) CO₂ + H₂ → CO + H₂O: hydrogen gains oxygen to form water, so H₂ is oxidised.
(ii) 2Cu + O₂ → 2CuO: copper gains oxygen to form copper oxide, so copper is oxidised.
(b) Reduction is the loss of oxygen.
(i) CO₂ + H₂ → CO + H₂O: carbon dioxide loses oxygen to form carbon monoxide, so CO₂ is reduced.
(ii) CuO + H₂ → Cu + H₂O: copper oxide loses oxygen to form copper, so CuO is reduced.

Teacher's Note:
a) The same equation can show oxidation of one substance and reduction of another.
b) Look for the substance that gains oxygen (oxidised) and the one that loses it (reduced).
c) In CO₂ + H₂ → CO + H₂O, hydrogen is oxidised while carbon dioxide is reduced.

Question 17: A shiny brown-coloured element 'X' on heating in air becomes black in colour. Name the element 'X' and the black coloured compound formed.

Answer:
'X' is copper (Cu), and the black-coloured compound formed is copper oxide (CuO).
2Cu(s) + O₂(g) → 2CuO(s) (on heating)

Teacher's Note:
a) Copper is shiny brown, and its oxide is black.
b) Heating supplies the energy needed for copper to combine with oxygen.

Question 18: Why do we apply paint on iron articles?

Answer:
Iron articles are painted to prevent rusting. The paint cuts off the contact of iron with moisture and air. Air and moisture are both essential for rusting to take place, so rusting is prevented.

Teacher's Note:
a) Rusting needs both air and water.
b) Paint is a barrier, not a chemical treatment.

Question 19: Oil and fat containing food items are flushed with nitrogen. Why?

Answer:
Nitrogen is an inert gas and does not easily react with food substances. Oxygen, on the other hand, reacts with oil and fat and makes them rancid. So, packets of food items are flushed with nitrogen gas to remove oxygen from inside the pack. When oxygen is not present inside the pack, rancidity of oil and fat containing food items is avoided.

Teacher's Note:
a) Nitrogen acts as a protective blanket inside the packet.
b) Rancidity is an oxidation process, so removing oxygen stops it.

Question 20: Explain the following terms with one example each.
(a) Corrosion (b) Rancidity

Answer:
(a) Corrosion: Corrosion is a process in which materials, usually metals, deteriorate as a result of a chemical reaction with air, moisture, chemicals, etc. Example: iron, in the presence of moisture, reacts with oxygen to form hydrated iron oxide, which is rust.
4Fe + 3O₂ + nH₂O → 2Fe₂O₃.nH₂O (hydrated iron oxide)
(b) Rancidity: The oxidation of fats and oils, which can be noticed by a change in taste and smell, is called rancidity. Example: the taste and smell of butter change when it is kept for a long time.

Teacher's Note:
a) Rust is hydrated iron oxide, so it contains water as well as iron and oxygen.
b) Rancidity can be avoided by storing food in airtight containers, in refrigerators, with antioxidants, or in nitrogen.

When hydrogen reacts with oxygen, it gives water. This reaction can be represented by following chemical equation:
Hydrogen + Oxygen ⇨ Water
H₂ + O₂ → H₂O
In first equation words are used and in second symbols of substances are used to write the chemical equation. For convenience, symbol of substance is used to represent chemical equations. Chemical Equation is a way to represent the chemical reaction in concise and informative way.
Chemical equation can be divided into two types – Balanced Chemical Equation and Unbalanced Chemical Equation.

Unbalanced Chemical Equation: If the number of atoms of each element in reactants is not equal to the number of atoms of each element present in product, then the chemical equation is called unbalanced chemical equation.
Example: Fe + H₂O → Fe3O₄ + H₂
In this example number atoms of elements are not equal on two sides of the reaction. For example, on the left hand side only one iron atom is present, while three iron atoms are present on the right hand side. Therefore, it is an unbalanced chemical equation.

To balance the given or any chemical equation, follow these steps:
Fe + H₂O → Fe₃O4 + H₂
Write the number of atoms of elements present in reactants and in products in a table; as shown Balance the atom which is the maximum in number; on either side of chemical equation.

In this equation, the number of oxygen atom is the maximum on the RHS.
To balance the oxygen one needs to multiply the oxygen on the LHS by 4; so that the number of oxygen atoms becomes equal on both sides.
Fe + 4 x H₂O → Fe3O₄ + H₂
Now, the number of hydrogen atoms becomes 8 on the LHS; which is more than that on the RHS. To balance it, one needs to multiply the hydrogen on the RHS by 4.

Fe + 4 x H₂O → Fe3O₄ + 4 x H₂
After that number of oxygen and hydrogen atoms becomes equal on both sides. The number of iron is one on the LHS, while it is three on the RHS. To balance it, multiply the iron on the LHS by 3.

Writing the symbols of Physical States of substances in Chemical equation:
By writing the physical states of substances a chemical equation becomes more informative.
• Gaseous state is represented by symbol ‘g’
• Liquid state is represented by symbol ‘l’
• Solid state is written by symbol ‘s’
• Aqueous solution is written by symbol ‘aq’
Writing the condition in which reaction takes place: The condition is generally written above and/or below the arrow of a chemical equation.
Thus, by writing the symbols of physical state of substances and condition under which reaction takes place, a chemical equation can be made more informative.

Reactions in which one compound decomposes in two or more compounds or element are known as DECOMPOSITION REACTION. Decomposition reaction is just opposite of combination reaction.
A general decomposition reaction can be represented as follows: