Frank Brothers Solutions for ICSE Class 9 Chemistry Chapter 5 Physical And Chemical Changes

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Question 1:
Answer: A physical change is a temporary change in which no new substance is formed and the composition or identity of the substance is not altered although certain specific physical properties may be changed.
Physical changes usually involve a change in state, like ice melting into liquid water. Even though the appearance of the substance changes, the molecules inside remain exactly the same.
Teacher's Tip: If you can get the original substance back by reversing the process, it is almost always a physical change.
Exam Tip: Always emphasize that "no new substance is formed" to secure full marks for this definition.

Question 2:
Answer: A chemical change is a permanent change in which the original substance gives rise to one or more substances with different properties.
In this type of change, the chemical bonds between atoms are broken or formed to create something entirely new. For example, when paper burns, it turns into ash and smoke, which cannot be turned back into paper.
Teacher's Tip: Think of chemical changes as "one-way streets" because they are very difficult or impossible to undo.
Exam Tip: Use the word "permanent" to describe the nature of the change in your answer.

Question 3:
Answer: The reactions in which heat is evolved are called exothermic reactions while the reactions in which heat is absorbed are called endothermic reactions.
Exothermic reactions release energy into the surroundings, often making them feel hot, like a burning match. Endothermic reactions take energy in, often making the surroundings feel cold, like an ice pack activating.
Teacher's Tip: Remember: 'Exo' means Exit (heat leaves) and 'Endo' means Enter (heat enters).
Exam Tip: Mention both types of reactions when asked to distinguish between heat-related chemical processes.

Question 4:
Answer:
1. False
2. False
3. False
4. False
5. True
These True or False questions check your fundamental understanding of the properties of matter. Carefully reading each statement helps clarify common misconceptions about physical and chemical changes.
Teacher's Tip: Go back and rewrite the false statements as true ones to test your deep understanding.
Exam Tip: Read every word of the statement carefully, as small words like "only" or "never" can change the whole meaning.

Question 5:
Answer: When wood or paper is burnt in air, it gives carbon dioxide and water leaving behind a little ash.
[C₆(H₂O)₅]n + 6nO₂ → 6n CO₂ + 5n H₂O + heat
Wood Oxygen Carbon dioxide Water
We call it as chemical change because-
(i) In above reaction new substances are formed. Impurities which are non-volatile, remain behind.
(ii) The change is permanent.
(iii) Heat and light energies are given out.
(iii) Mass of wood get changed but the total mass remains the same.
Burning wood is a complex reaction where solid matter turns into invisible gases and a bit of solid residue. This transformation completely changes the chemical identity of the starting materials.
Teacher's Tip: Even though the wood "disappears," the mass is actually still there in the form of gas and ash!
Exam Tip: When explaining why a change is chemical, always list more than one reason, such as the formation of new substances and energy release.

Question 6:
Answer: Possible conditions for a chemical change are -
1. One or more new substance is formed during reaction.
2. The change occurring during the reaction is permanent.
3. The mass of the substance undergoing a chemical change is generally altered.
4. Chemical change involves making and breaking of bond.
These conditions act as signs that a substance has undergone a transformation at the atomic level. If you see bubbles, a color change, or feel heat, a chemical reaction is likely happening.
Teacher's Tip: Think of chemical bonds like Lego bricks being pulled apart and put together in a brand-new shape.
Exam Tip: Be sure to include the "breaking and making of bonds" to show you understand the microscopic level of chemistry.

Question 7:
Answer: Reactions in which both the reactants exchange their radicals to give new compounds are called double displacement or double decomposition reaction.
The general reaction can be written as:-
A⁺B^- + C⁺D^- → w A⁺D^- + C⁺B^-
The two examples of double decomposition reaction are:-
(a) Precipitation reactions-Reactions involving the formation of a precipitate. Sodium chloride and silver nitrate react to form an insoluble white precipitate of silver chloride. For example:-
NaCl + AgNO₃ → w AgCl ↓ + NaNO₃
                                    White ppt
(b) Neutralisation reaction-Reactions in which acids and bases mix together, forming compounds i.e. salt and water.
For example:-
NaOH + HCl →w NaCl + H₂O
Base     Acid         Salt       Water
Double displacement is like two pairs of dance partners switching friends to form new pairs. This type of reaction is very common when mixing solutions in a chemistry lab.
Teacher's Tip: A "precipitate" is just a solid that falls out of a liquid solution like snow.
Exam Tip: Use the downward arrow symbol (↓) to indicate the formation of a precipitate in your chemical equations.

Question 8:
Answer: (a) Redox reaction - The reaction in which both oxidation and reduction takes place simultaneously is known as redox reaction. Oxidation is a reaction that involves the addition of oxygen or the removal of the hydrogen. Reduction is a reaction that involves the addition of hydrogen or the removal of oxygen.
2H₂ + O₂ →2H₂O
(b) Oxidation - Oxidation is a reaction that involves the addition of oxygen or the removal of the hydrogen. In electronic concept, it is defined as the process in which an atom, molecule or ion loses one or more electrons. This results in increase in the positive charge or decrease in negative charge on the resulting species.
Cu →Cu²⁺ + 2e^-
(c) Reduction - Reduction is a reaction that involves the addition of hydrogen or the removal of oxygen. In the electronic concept, it is defined as the process in which an atom, molecule or ion gains one or more electrons. This results in increase in the negative charge or decrease in positive charge on the resulting species.
H⁺ + e^- →H
Redox is a combination of the words "reduction" and "oxidation" because they always happen at the same time. You can think of it as a game of catch where one atom throws an electron (oxidation) and another catches it (reduction).
Teacher's Tip: Use the phrase "OIL RIG" to remember: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons).
Exam Tip: When defining redox, mention that both processes happen "simultaneously" for a more accurate answer.

Question 9:
Answer:
1. Exothermic reaction
2. Endothermic reaction
These two categories describe the energy flow during any chemical process. They tell us whether energy is being released to the world or taken from it.
Teacher's Tip: Exothermic reactions feel hot to the touch because heat is moving from the reaction to your hand.
Exam Tip: Be ready to provide one everyday example for each, like burning wood (exothermic) or photosynthesis (endothermic).

Question 10:
Answer:
Characteristics: Nature of change
Physical Change: Temporary
Chemical Change: Permanent

Characteristics: Reversibility
Physical Change: Reversible. It can be reversed by reversing the conditions.
Chemical Change: Not easily reversible. It cannot be reversed by reversing the conditions.

Characteristics: Chemical composition of the substance
Physical Change: The chemical composition of the substance remains unchanged.
Chemical Change: Chemical composition of the substance is changed.

Characteristics: Product of the change
Physical Change: No chemically new substance is formed. The substance obtained after the change has the same chemical compositions as that of the original substance.
Chemical Change: At least one or more new substances with different chemical compositions are formed.

Characteristics: Change in properties
Physical Change: Only physical properties of the substance change. The chemical properties remain the same.
Chemical Change: The new substances formed have physical and chemical properties different from the original substance.
This comparison table summarizes the fundamental differences between how matter changes. By looking at these five signs, you can classify any change you observe in nature.
Teacher's Tip: Focus on "Chemical composition" as the most important difference between these two types of change.
Exam Tip: Presenting differences in a table format with clear headings makes it easier for examiners to award full marks.

Question 11:
Answer: The chemical reactions which occur with the absorption of light energy are called photochemical reactions.
Examples-
Decomposition of silver nitrate takes place in the presence of light.
2AgNO₃ →{light} 2Ag + 2NO₂ + O₂
Photochemical reactions are unique because they use the energy of photons from light to break apart molecules. This is why some chemicals are stored in dark bottles to prevent them from reacting with sunlight.
Teacher's Tip: Think of "Photo" meaning light, just like in a photograph or photosynthesis.
Exam Tip: Always write the word "light" over the reaction arrow to show it is a necessary condition.

Question 12:
Answer: As the burning substance combines with oxygen, the total mass of the products should be greater than that of the burning substance. For example, when magnesium is burnt, a new substance magnesium oxide is formed, whose weight is greater than that of the original magnesium.
Experiment - A crucible is weighed containing about 0.5{ gm} of magnesium. Now crucible is heated. When magnesium begins to burn, the lid is put back on the crucible and the lid is occasionally raised to allow air to enter and burn the magnesium such that no product is lost. When, all the magnesium has been burnt up, the crucible is allowed to cool and then on weighing it we observe that there is gain in weight.
This gain in weight happens because the magnesium atoms have physically bonded with oxygen atoms from the air. This experiment is a brilliant proof that oxygen is a real material that adds weight to the final product.
Teacher's Tip: The "extra" weight you see on the scale is actually the weight of the air that got trapped in the metal.
Exam Tip: Mention that the lid is raised "occasionally" to explain how oxygen enters without letting smoke escape.

Question 13:
Answer:
Characteristic: Process nature
Respiration: It involves a series of biochemical reactions which take place in presence of enzymes, it is a slow process.
Burning: It is a single step chemical reaction which is a rapid process.

Characteristic: Temperature
Respiration: It takes place at body temperature (37°C).
Burning: This process takes place at high temperature.

Characteristic: Energy liberation
Respiration: Less energy in the form of heat and chemical energy is liberated.
Burning: More energy in the form of heat and light is liberated.
Respiration and burning both use oxygen to release energy, but they happen at very different speeds. Respiration is a controlled "slow-burn" inside our bodies, while burning is an uncontrolled "fast-release" of energy.
Teacher's Tip: Think of respiration as a slow-burning candle inside your cells and burning as a giant bonfire.
Exam Tip: Use the term "biochemical" for respiration to distinguish it from simple chemical burning.

Question 14:
Answer: Three conditions necessary for burning are -
1. The substance to be burnt must be combustible.
2. A supporter of combustion such as air or oxygen must be present.
3. A combustible substance must be heated to its ignition temperature.
To start a fire, you need fuel, air, and a spark. If you remove even one of these three things, the fire will immediately go out.
Teacher's Tip: These three conditions are often called the "Fire Triangle" - remove one side and the fire collapses!
Exam Tip: Be specific by saying "ignition temperature" instead of just "heat."

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Question 15:
Answer: (a) A reaction which gives out heat is called an exothermic reaction. For example:-
Burning of coal in air is an example of exothermic reaction.
C + O₂ →CO₂ + {heat}
(b) A reaction which proceeds in both forward as well as backward directions. They are indicated by the sign '\rightleftharpoons'.
3Fe + 4H₂O \rightleftharpoons Fe₃O₄ + 4H₂
(c) 2Mg + O₂ →2MgO + light + {heat}
(d) A reaction which takes place with the help of sunlight.
6CO₂ + 12H₂O →[{Sunlight}]{Chlorophyll} C₆H₁₂O₆ + 6H₂O + 6O₂
(e) 2H₂O →{Electricity} 2H₂ + O₂
These examples show how energy in the form of heat, light, or electricity can drive different chemical processes. Each reaction follows a specific path depending on the type of energy applied to the reactants.
Teacher's Tip: The double arrow (\rightleftharpoons) symbol is like a two-way street for chemical reactions.
Exam Tip: Learn the photosynthesis equation (d) perfectly, as it is a very common question in science exams.

Question 16:
Answer:
1. 2AgNO₃ →{Heat} 2Ag + O₂ + 2NO₂
2. 2Cu(NO₃)₂ →{Heat} 2CuO + 4NO₂ + O₂
3. 2Al(OH)₃ →{Heat} Al₂O₃ + 3H₂O
4. 2AgCO₃} →{Heat} Ag₂O + 2CO₂
5. 2KNO₃ →{Heat} 2KNO₂ + O₂
Thermal decomposition is the process where a single substance breaks down into multiple simpler ones when heated. This technique is often used in laboratories to produce specific gases like oxygen or carbon dioxide.
Teacher's Tip: "Decomposition" basically means "falling apart."
Exam Tip: Ensure your chemical equations are balanced with the correct coefficients in front of each molecule.

Question 17:
Answer: When oxidation occurs there is a loss of electrons but simultaneously there is a gain of electrons by other species which is called reduction. These both process occur simultaneously so we can say that both oxidation and reduction go hand in hand and such reactions are known as redox reaction.
You can't have oxidation without reduction because the electrons that are lost have to go somewhere! This simultaneous exchange is the foundation of many important reactions, including the ones that power batteries.
Teacher's Tip: Imagine Redox as a trade: one atom gives away a "coin" (electron) and the other atom receives it.
Exam Tip: Use the word "simultaneously" to get full marks when explaining why oxidation and reduction are linked.

Question 18:
Answer:
1. Copper is oxidized to copper sulphate while sulphur in sulphuric acid is reduced to sulphur dioxide.
2. Silver in silver oxide is reduced to silver while oxygen in hydrogen peroxide is oxidised to molecular oxygen.
These examples illustrate how oxygen can be transferred from one substance to another during a reaction. Identifying which part of the reaction is oxidation and which is reduction helps us track the flow of energy and matter.
Teacher's Tip: Look at where the oxygen atoms end up to quickly identify what was oxidized.
Exam Tip: State clearly which substance is oxidized and which is reduced to avoid confusion.

Question 19:
Answer: Four types of chemical reactions are -
(i) Direct combination or Synthesis reactions
(ii) Decomposition reactions
(iii) Displacement reactions
(iv) Double decomposition reactions

(i) Direct combination or Synthesis reactions-When two or more elements combine to form a compound, the process is called synthesis. It may be brought about by the action of heat, electricity or pressure.
For Example:-
a. Where two elements combine to form a new compound.
H₂ + Cl₂ →2HCl
(Hydrogen) (Chlorine) (Hydrogen chloride)
b. Where an element and a compound combine to form a new compound.
CO + 2H₂ →{430}°{C, 200 atm, ZnO + Cr₂O₃} CH₃OH
(Carbon monoxide) (Hydrogen) (Methyl alcohol)

(ii) Decomposition reactions-When a single compound is broken down in a chemical reaction into two or more simpler substances (elements or compounds) the reaction is called a decomposition reaction. It can be brought about by the heat, light or electricity. They are reverse of combination reaction. For example:-
a. Decomposition by heat:
MgCO₃ →{Heat} MgO + CO₂
(Magnesium Carbonate) (Magnesium oxide) (carbon dioxide)
b. Decomposition by electricity:
2H₂O →{electric current} 2H₂ + O₂
(Acidified Water) (Hydrogen) (Oxygen)

(iii) Displacement reactions - A reaction in which one element displaces another element from its compound to form a new compound is called a displacement reaction. For example:-
Zn + H₂SO₄ →ZnSO₄ + H₂
(Zinc) (sulphuric Acid) (Zinc sulphate) (hydrogen)

(iv) Double decomposition reactions-Reactions in which both the reactants exchange their radicals to give new compounds is called a double displacement reaction.
A general reaction:-
A⁺B^- + C⁺D^- →A⁺D^- + C⁺B^-
For example:-
a. Precipitation reaction
NaCl + AgNO₃ →AgCl ↓ + NaNO₃
White ppt
b. Neutralisation reaction
NaOH + HCl →NaCl + H₂O
Classifying reactions helps chemists predict what will happen when different chemicals are mixed. Whether atoms are joining, splitting, or swapping places, they always follow one of these four basic patterns.
Teacher's Tip: Synthesis is like joining two Legos together, while decomposition is like pulling them apart!
Exam Tip: Be ready to provide at least one chemical equation as an example for each reaction type.

Question 20:
Answer: (a) In activity series, Gold is placed almost at the end of the series since it is least reactive. So, it does not tend to react with other elements easily and thus found free in nature.
(b) For most of the ionic compounds the lattice enthalpy is very high therefore, they do not easily decompose on heating.
(c) On prolonged exposure to air silver react with oxygen present in air to form silver oxide which is black in colour.
Ag + O₂ →Ag₂O
Silver(grey) Silver oxide(black)
(d) The reaction between ammonium nitrate and water is endothermic. It takes away heat from the beaker. So, beaker becomes cold.
NH₄NO₃+ H₂O →{heat} NH₄NO₃ (aq)
(e) The reaction between magnesium and hydrochloric acid is an exothermic reaction which gives heat to the beaker and thus beaker becomes hot.
Mg + 2HCl →MgCl₂ + H₂ + {heat}
This section explains why some metals stay shiny while others tarnish and how some mixtures change temperature on their own. These behaviors are determined by the internal energy and reactivity of the specific elements involved.
Teacher's Tip: Gold is "lazy"—it doesn't like to work or bond with other elements, which is why it stays beautiful for thousands of years.
Exam Tip: Use the term "endothermic" to explain why a beaker becomes cold during a reaction.

Question 21:
Answer: A candle is a stick of paraffin wax with cotton wick. As a candle burns wax melts and trickles down. It gets solidified shortly. This is physical change. Paraffin wax is a mixture of hydrocarbons. When wick catches fire, paraffin wax melts, evaporates and burns in air like any hydrocarbon to give carbon dioxide and water. This is a chemical change.
Burning a candle is a special event because it involves both physical melting and chemical burning at the same time. While the wax changes shape, it also reacts with oxygen to turn into invisible gases.
Teacher's Tip: The physical change happens to the solid wax, and the chemical change happens to the wax vapor.
Exam Tip: If asked, clearly state that melting is physical and burning is chemical to show the difference.

Question 22:
Answer:
1. physical
2. chemical
3. chemical
4. physical
Correctly identifying these common changes is the first step in understanding the world like a scientist. You just have to ask yourself: "Is there something new here, or is it just the same stuff in a new shape?"
Teacher's Tip: If it's a change of state (solid/liquid/gas), it is almost always physical.
Exam Tip: Rusting and burning are the most common examples of chemical changes used in tests.

Question 23:
Answer: Two examples are:-
1. Burning of wood-carbon get oxidized and oxygen gets reduced.
2. Rusting-In it iron is oxidized.
Oxidation is what happens when elements like carbon and iron combine with oxygen from the air. In both burning and rusting, oxygen is the "attacker" that changes the original metal or fuel into an oxide.
Teacher's Tip: Rusting is just a very, very slow version of burning!
Exam Tip: Always mention that oxygen is being added when you are describing oxidation examples.

Question 24:
Answer:
1. Chromium(VI) .
2. Hydrogen peroxide
3. Barium carbonate.
4. Silver nitrate.
These are the formal chemical names for various compounds used in science and industry. The Roman numeral in brackets, like in Chromium(VI), tells us the specific valency state of the metal.
Teacher's Tip: Learning these names is like learning the vocabulary of the "Language of Chemistry."
Exam Tip: Be careful with suffixes like "-ate" and "-ide" as they represent different chemical formulas.

Question 25:
Answer: 5. Manganese dioxide.
Ignition temperature - Ignition temperature is the lowest temperature up to which temperature of a substance must be raised so that it catches fire.
A combustible substance must be heated to its ignition temperature for burning.
Every material has a specific "tipping point" where it gets hot enough to start reacting with oxygen. This is why a piece of paper lights faster than a thick log of wood—it reaches its ignition temperature sooner.
Teacher's Tip: Think of ignition temperature as the "starting line" for a fire.
Exam Tip: Use the words "lowest temperature" to make your definition scientifically accurate.

Question 26:
Answer:
Characteristic: Nature
Respiration: It involves a series of biochemical reactions which take place in presence of enzymes, it is a slow process.
Burning: It is a single step chemical reaction which is a rapid process.

Characteristic: Temperature
Respiration: It takes place at body temperature (37°C).
Burning: This process takes place at high temperature.

Characteristic: Energy release
Respiration: Less energy in the form of heat and chemical energy is liberated.
Burning: More energy in the form of heat and light is liberated.
Respiration is essentially a biological version of burning that happens inside your cells without high flames. Because it is controlled by enzymes, it releases energy slowly enough for your body to use it for life.
Teacher's Tip: Respiration is "burning food" so your body can stay warm and move!
Exam Tip: Remember that respiration is a "slow process" while burning is "rapid."

Question 27:
Answer: On heating few crystals of iodine in a test tube, the grey crystals sublimes and dense violet fumes are seen. On cooling, the vapours again form the crystals. So, a physical change can be reversed.
Iodine is one of the rare substances that skip the liquid stage entirely, moving directly from solid to gas. Because it returns to its original solid form upon cooling, it remains a pure example of a physical change.
Teacher's Tip: This "skipping" of the liquid stage is called 'sublimation'.
Exam Tip: Use the word "sublimes" to describe what happens to the iodine crystals upon heating.

Question 28:
Answer: When hydrogen burns in air, formation of water occurs.
2H₂ + O₂ →2H₂O
Two reasons are-
(i)Water is formed as a product which is different from hydrogen and oxygen. Mass of water is different from either hydrogen or oxygen but the total mass of the substance involved in the chemical reaction remains same.
(ii)The change is permanent and cannot be reversed by reversing the conditions that initially caused the change to occur.
Even though you start with two flammable gases, you end up with liquid water, which actually puts out fires! This total change in properties is a hallmark sign of a chemical transformation.
Teacher's Tip: Hydrogen is "loud" (it pops when it burns) but it makes "quiet" water!
Exam Tip: Mention the "permanent" nature of the change as one of your two required reasons.

Question 29:
Answer: When water is freezed and evaporated, these both are physical changes because -
1. The change is temporary and reversible.
2. No new substance is formed and the chemical composition of the original substance remains the same.
3. Mass of the substance remains unchanged
4. The amount of energy required to bring about a physical change is generally equal to the amount of energy required to reverse the change. Hence,there is no net energy change involved.
Freezing and boiling are just about molecules moving faster or slower; the water molecules themselves never break apart. This is why you can cycle water between ice, liquid, and steam over and over again without changing what it is.
Teacher's Tip: Think of it as water molecules just "shivering" when cold and "running around" when hot.
Exam Tip: State that the "chemical composition remains the same" to explain why state changes are physical.

Question 30:
Answer: When two or more elements combine to form a compound, the process is called synthesis. It may be brought about by the action of heat, electricity or pressure. Reactions in which direct combination of chemical substances occur are called as synthesis.
Example-
(a) When two elements combine to form a new compound.
H₂ + Cl₂ →2HCl
Hydrogen Chlorine Hydrogen chloride
(b) Where an element and a compound combine to form a new compound.
2CO + O₂ →2CO₂
Carbon monoxide Oxygen Carbon dioxide
(c) Where two compounds combine to form new compounds.
NH₃ + HCl →NH₄Cl
Synthesis is like building a tower out of separate blocks; separate pieces join to become one single structure. These reactions are fundamental in creating many of the medicines and materials we use every day.
Teacher's Tip: In a synthesis reaction, you will always find only ONE single product on the right side of the arrow.
Exam Tip: Use the term "direct combination" as a synonym for synthesis in your answers.

Question 31:
Answer: Air is necessary for burning. Incorrect amount of air in fuel combustion accounts for the largest losses in combustion system. If the fuel does not get enough air for combustion it will generate smoke and a potentially unhealthy mixture of gas products.
Without enough oxygen, fuel cannot burn completely, which creates soot and dangerous gases like carbon monoxide. This is why heaters and engines need good ventilation to work efficiently and safely.
Teacher's Tip: This is why campfires have gaps between the logs—it lets the air reach the fuel!
Exam Tip: Mention that "smoke" is a sign of incomplete combustion due to a lack of air.

Question 32:
Answer: 1. (a) Combustible substances -The substances that catch fire and burn easily. Ex- Wood, Charcoal, petrol, kerosene etc.
Non-combustible substances-Substance which cannot burn in air or oxygen are called as non- combustible substances. Ex-Nitrogen gas, carbon dioxide etc.
2. (b) Two substances other than oxygen that support combustion are 1. Hydrogen 2. Nitrogen
Knowing what burns and what doesn't is critical for fire safety and fuel engineering. Substances like nitrogen and carbon dioxide are so non-combustible that we often use them to put out fires!
Teacher's Tip: Water is the ultimate non-combustible substance—that's why it stops fire so well.
Exam Tip: Always provide at least two clear examples for both combustible and non-combustible categories.

Question 33:
Answer: 1. (a)
1. Burning of coal in air releases CO₂ in air.
2. Respiration releases carbon dioxide and water vapours.
2. (b)
1. Photosynthesis removes CO₂ from the atmosphere. Plants take carbon dioxide from the atmosphere in the presence of sunlight and use it to synthesise glucose with the liberation of oxygen.
2. Some man made chemical activities such as setting of mortar also use atmospheric carbon dioxide and helps in removing carbon dioxide.
Nature has a beautiful balance where the "waste" from animals (CO₂) is the "food" for plants. This constant giving and taking keeps the atmosphere breathable for all living things.
Teacher's Tip: Plants are like giant atmospheric vacuum cleaners that suck up CO₂!
Exam Tip: Mention photosynthesis as the primary natural way CO₂ is removed from the air.

Question 34:
Answer: Nitrogen is inert in nature and does not support combustion while oxygen supports combustion. If proportions of nitrogen and oxygen in the air were reversed then the rate of combustion of substances will increase.
Our atmosphere is mostly nitrogen, which acts as a "buffer" to keep oxygen from being too reactive. If there were more oxygen than nitrogen, even a tiny spark could cause an explosion or a fire that would be impossible to stop.
Teacher's Tip: Nitrogen is the "chill" gas that keeps the "excited" oxygen gas under control.
Exam Tip: Use the word "inert" to describe nitrogen's lack of chemical reactivity.

Question 35:
Answer: Heating of sulphur - If some powdered sulphur is heated gently in a glass test tube, it melts to a pale yellow liquid. Flame is removed to stop heating, it is quickly changed back to solid sulphur.
Since the sulfur doesn't turn into a new chemical and can be turned back into a solid just by cooling it, this is a physical change of state. The atoms are just sliding past each other as a liquid instead of being locked together.
Teacher's Tip: This is just like melting chocolate—it changes shape but it's still chocolate!
Exam Tip: Mention that the sulfur "melts" to clarify that it is a physical state change.

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Question 36:
Answer: Activity series - The arrangement of the metals in the decreasing order of their chemical reactivity is called the activity series.
In displacement reactions, a more reactive element (metal or non-metal) displaces a lesser reactive element from its compound. With the help of the activity series, it is possible to predict which metals will displace other metals from their solutions.
The activity series is like a sports leaderboard for metals, showing which ones are the strongest at winning a reaction. A metal higher up on the list will always "kick out" a metal lower on the list from a compound.
Teacher's Tip: Gold is at the bottom because it's the "lazy" metal that doesn't want to react with anyone.
Exam Tip: Explain that a "more reactive" metal displaces a "less reactive" one to get full marks on displacement questions.

Question 37:
Answer: Balance of oxygen and carbon dioxide is maintained in nature because there is a natural oxygen cycle and a natural carbon cycle operating all the time by which the desired proportions of the two gases in the air are maintained. This is also known as carbon dioxide-oxygen cycle.
This cycle is essentially a giant recycling program run by plants and animals together. By exchanging these two gases, life ensures that neither gas ever runs out completely.
Teacher's Tip: Think of it as a relay race where plants and animals keep passing the "gas baton" back and forth.
Exam Tip: Be sure to name both the "carbon cycle" and the "oxygen cycle" in your answer.

Question 38:
Answer:
(a) N₂ + O₂ →3000°C 2 NO
Combination reaction
(b) ZnCO₃ → ZnO + CO₂
Decomposition reaction
(c) Zn + FeSO₄ → ZnSO₄ + Fe
Displacement reaction
(d) Mg + 2HCl → MgCl₂ + H₂
Displacement reaction
(e) P₂O₅ + 3H₂O → 2H₃PO₄
Combination reaction
(f) 2KI + Cl₂ → 2KCl + I₂
Displacement reaction
(g) 2Cu(NO₃)₂ → 2CuO + 4NO₂ + O₂
Decomposition reaction
(h) ZnSO₄ + 2NaOH → Zn(OH)₂ + Na2SO₄
Double decomposition reaction
(i) 2Na + 2H₂O → 2NaOH + H₂
Displacement reaction
(j) 2NaOH + H₂SO₄ → Na₂SO₄ + 2H₂O
Base Acid
Double decomposition reaction
(k) Fe + S → FeS
Combination reaction
(l) Mg(HCO₃)₂ → MgCO₃ + H₂O + CO₂
Decomposition reaction
(m) CaO + H₂O → Ca(OH)₂
Combination reaction
(n) CaCl₂ + 2NH₄OH → Ca(OH)₂ + 2NH4Cl
Double decomposition reaction
(o) 3Mg + N₂ → Mg₃N₂
Combination reaction
By identifying the patterns in these equations, we can classify every chemical change into its correct family. This helps scientists understand how atoms are moving—whether they are joining, breaking apart, or switching places.
Teacher's Tip: If you start with one thing and end with two, it is Decomposition. If you start with two and end with one, it is Combination!
Exam Tip: Double-check if the atoms "swapped" twice; that is the key to identifying Double Decomposition.

Question 39:
Answer: Carbon dioxide from the atmosphere enters the plant through photosynthesis, where carbohydrates are produced. From green plants, the carbon in the form of carbohydrates, etc. enter the animal and human bodies. The atmospheric carbon dioxide gets dissolved in oceans by diffusion. Marine algae and photosynthetic bacteria obtain carbon dioxide from water.
Carbon dioxide returns to the atmosphere by respiration, combustion of fossil fuels like coal, wood, petroleum etc., weathering of rocks, volcanic eruptions etc.
Carbon is the building block of life, and it travels through air, water, and living things in a never-ending loop. This cycle ensures that carbon atoms are recycled so new life can grow.
Teacher's Tip: The carbon atom in the bread you ate today might have been in a dinosaur millions of years ago!
Exam Tip: Mention "combustion of fossil fuels" as one of the ways carbon returns to the sky.

Question 40:
Answer: The oxygen cycle is the biogeochemical cycle that describes the movement of oxygen within atmosphere, biosphere and lithosphere. The main source of atmospheric oxygen is photosynthesis, which produces sugars and oxygen from carbon dioxide and water.
6CO₂ + 6H₂O + {Energy} →Chlorophyll {Sunlight} C₆H₁₂O₆ + 6O₂
Oxygen also comes from photolysis.
Then oxygen is taken away from atmosphere by plants and animals. It is also consumed from atmosphere by chemical weathering i.e. oxidation of exposed minerals and rocks.
4FeO + O₂ →2Fe₂O₃
Oxygen is essential for life on Earth, and it moves through the atmosphere, living things, and even rocks. Without the work of green plants doing photosynthesis, the oxygen in our air would eventually run out.
Teacher's Tip: Plants are the "lungs" of our planet because they give us the oxygen we need to breathe.
Exam Tip: State clearly that photosynthesis is the "main source" of oxygen in our atmosphere.