Selina Concise Solutions for ICSE Class 8 Chemistry Chapter 6 Chemical Reactions

Chemical Reactions

Question 1:
(a) Define a chemical reaction.
(b) What happens during a chemical reaction ?
(c) What do you understand by a chemical bond ?
Answer:
(a) Any chemical change in matter which involves transformation into one or more substances with entirely different properties is called a chemical reaction.
(b) A chemical reaction involves breaking of chemical bonds between the atoms or groups of atoms of reacting substances and rearrangement of atoms making new bonds to form new substances.
(c) A chemical bond is the attractive force that holds the atoms of a molecule together, in a compound.

Chemical reactions are like recipes where you take apart ingredients and put them back together in a new way to make something different. The bonds mentioned are the invisible "glue" that keeps everything stable in a molecule.
Teacher's Tip: Think of a chemical reaction like a LEGO set: you take apart a house to build a car using the exact same bricks.
Exam Tip: Always mention that the new substances formed have "entirely different properties" to get full marks.

Question 2: Give one example each of which illustrates the following characteristics of a chemical reaction:
(a) evolution of a gas
(b) change of colour
(c) change in state
Answer:
(a) When Zinc reacts with dil. sulphuric acid. Hydrogen gas is evolved, with an effervescence
Zn + H₂SO₄ → ZnSO₄ + H₂
(zinc) (dil. sulphuric acid) (zinc sulphate) (hydrogen)
(b) When blue coloured copper sulphate reacts with hydrogen sulphide gas, a black coloured substance copper sulphide is formed.
CuSO₄ + H₂S → CuS + H₂SO₄
(copper sulphate solution, blue) (hydrogen sulphide) (copper sulphide black solid) (sulphuric acid)
(c) The reaction between hydrogen sulphide and chlorine (both gases) produces sulphur (solid) and hydrogen chloride (gas).
H₂S(g) + Cl₂(g) → 2HCl(gas) + S(s)
hydrogen sulphide chlorine hydrogen chloride sulphur yellow solid

These signs help scientists know a reaction is happening even when the atoms are too small to see. For instance, fizzing or a sudden color shift are big clues that something new has been made.
Teacher's Tip: Use the word "effervescence" instead of just "bubbles" to sound like a real chemist!
Exam Tip: When describing a color change, always write down the original color and the final color clearly.

Question 3: How do the following help in bringing about a chemical change?
(a) pressure (b) light
(c) catalyst (d) heat.
Answer:
(a) Some chemical reactions take place when reactants are subjected to high pressure. e.g: Nitrogen and hydrogen when subjected to high pressure produce ammonia gas.
N₂ + 3H₂ x→[200 atm.]450°C 2NH₃
(b) Some chemical reactions can take place in the presence of light. Ex. Photosynthesis.
e.g. 6CO₂ + 6H₂O x→[Chlorophyll]Sunlight C₆H₁₂O₆ + 6O₂ (glucose)
(c) A catalyst can either increases or decreases the rate of chemical reaction and some chemical reactions need a catalyst to change the rate of the reaction, in case it is too slow or too fast.
1. Positive catalyst: When a catalyst increase the rate of reaction finely divided iron is used as a positive catalyst in the manufacturing of ammonia from hydrogen and nitrogen.
N₂ + 3H₂ x→[450°C]Iron (Catalyst) 200 - 900 2NH₃ + Heat
2. Negative Catalyst: When a catalyst decreases the rate of reaction. Ex. Phosphoric acid act as a negative catalyst to decrease the rate of the decomposition of hydrogen peroxide.
(d) Some chemical reactions take place only in the presence of heat. e.g. When lead nitrate is heated, it breaks into lead monoxide, nitrogen dioxide and oxygen.
2Pb(NO₃)₂ x→heat 2PbO + 4NO₂ + O₂
(lead nitrate) (lead monoxide) (nitrogen dioxide)

Reactants sometimes need a "push" to start reacting, like heat from a flame or the pressure in a tank. Catalysts are special helpers that speed up the process without getting used up themselves.
Teacher's Tip: A catalyst is like a coach; it helps the team win faster but it doesn't play in the game.
Exam Tip: Be sure to distinguish between "positive" (speeds up) and "negative" (slows down) catalysts in your answers.

Question 4: (a) Define catalyst.
(b) What are (i) positive catalysts and (ii) negative catalysts? Support your answer with one example for each of them.
(c) Name three biochemical catalysts found in the human body.
Answer:
(a) Catalyst: A catalyst is a substance that either increases or decreases the rate of a chemical reaction without itself undergoing any chemical change during the reaction.
(b) (i) Positive catalyst: When a catalyst increases the rate of chemical reaction, it is called positive catalyst. e.g. when potassium chlorate heated to 700°C decomposes to evolve oxygen gas, when MnO₂ is added the decomposition takes place at 300°C.
2KClO₃ x→[300°C]MnO₂ 2KCl + 3O₂
(ii) Negative catalyst: When a catalyst decreases the rate of chemical reaction it is called negative catalyst. Example. Phosphoric acid acts as a negative catalyst to decrease the rate of the decomposition of hydrogen peroxide. Alcohol too acts as a negative catalyst in certain chemical reactions.
(c) Biochemical catalysts found in human body: 1. Pepsin 2. Tryspin 3. lipase.

Our bodies are full of natural catalysts called enzymes that help us digest food and grow. These biochemical helpers make sure life processes happen fast enough to keep us healthy.
Teacher's Tip: Remember "Pepsin" helps you "prep" your protein for digestion!
Exam Tip: When defining a catalyst, always emphasize that it remains "unchanged" at the end of the reaction.

Question 5: What do you observe when
(a) dilute sulphuric acid is added to granulated zinc?
(b) a few pieces of iron are dropped in a blue solution of copper sulphate?
(c) silver nitrate is added to a solution of sodium chloride?
(d) ferrous sulphate solution is added to an aqueous solution of sodium hydroxide.
(e) solid lead nitrate is heated?
(f) when dilute sulphuric acid is added to barium chloride solution ?
Answer:
(a) When Zinc reacts with dilute sulphuric acid, hydrogen gas is evolved with an effervescence.
Zn + dil. H₂SO₄ → ZnSO₄ + H₂.
(b) When a few pieces of iron are dropped into a blue coloured copper sulphate solution, the blue colour of the solution fades and eventually turns into green.
(c) When a solution of silver nitrate is added to a solution of sodium chloride, white insoluble ppt. of silver chloride is formed.
AgNO₃(aq) + NaCl(aq) → AgCl(ppt) + NaNO₃(aq)
(d) When ferrous sulphate solution is added to sodium hydroxide solution, a dirty green ppt. of ferrous hydroxide is formed.
FeSO₄(aq) + 2NaOH(aq) → Fe(OH)₂ ↓ + Na₂SO₄(aq)
(e) When solid lead nitrate is heated, it decomposes to produce light yellow solid lead monoxide, reddish brown nitrogen dioxide gas and colourless oxygen gas.
2Pb(NO₃)₂ x→heat 2PbO + 4NO₂ ↑ + O₂ ↑
(f) When few drops of dilute sulphuric acid is added to barium chloride solution, a white precipitate of barium sulphate is formed.
BaCl₂ + H₂SO₄ → BaSO₄ ↓ + 2HCl (Barium chloride solution) (Sulphuric acid) (Barium sulphate) (Hydrochloric acid) (White precipitate)

Observing a reaction is like being a detective looking for clues such as bubbles, color shifts, or solids appearing. These specific results help identify exactly which chemical change has occurred.
Teacher's Tip: A "precipitate" (ppt) is just a solid that doesn't dissolve and sinks to the bottom.
Exam Tip: For heating lead nitrate, mentioning the "reddish brown" color of the gas is essential for full marks.

Question 6: Complete and balance the following chemical equations:
(a) N₂ + O₂ →
(b) H₂S + Cl₂ →
(c) Na + 2H₂O →
(d) NaCl + AgNO₃ →
(e) Zn + H₂SO₄(dil) →
(f) FeSO₄(aq.) + NaOH(aq.) →
(g) Pb(NO₃)₂ x→heat
(h) BaCl₂(aq.) + H₂SO₄(aq.) →
Answer:
(a) N₂ + O₂ → 2NO(g)
(b) H₂S + Cl₂ → 2HCl + S
(c) 2Na + 2H₂O → 2NaOH + H₂ ↑
(d) NaCl + AgNO₃ → AgCl + NaNO₃
(e) Zn + H₂SO₄ → ZnSO₄ + H₂ (dil)
(f) FeSO₄(aq.) + 2NaOH(aq.) → Fe(OH)₂ + Na₂SO₄
(g) 2Pb(NO₃)₂ x→heat 2PbO + 4NO₂ + O₂ ↑
(h) BaCl₂(aq.) + H₂SO₄(aq.) → BaSO₄ ↓ + 2HCl

Balancing equations is like keeping a score sheet where the atoms on the left must equal the atoms on the right. This follows the rule that matter cannot be created or destroyed during a reaction.
Teacher's Tip: Treat the arrow like an equals sign and count each atom carefully on both sides.
Exam Tip: Use small pencil marks to count atoms during the exam so you don't miss any, then erase them later.

Exercise - II

Question 1:

1. Fill in the blanks.

(a) A reaction in which two or more substances combine to form a single substance is called a combination reaction.
(b) A catalyst is a substance which changes the rate of a chemical reaction without undergoing a chemical change.
(c) The formation of gas bubbles in a liquid during a reaction is called effervesence
(d) The reaction between an acid and a base is called neutralization reaction.
(e) Soluble bases are called alkalis.
(f) The chemical change involving iron and hydrochloric acid illustrates a displacement reaction.
(g) In the type of reaction called double decomposition reaction, ions two compounds exchange their positive and negative radicals ions respectively.
(h) A catalyst either increases or decreases the rate of a chemical change but itself remains unchanged at the end of the reaction.
(i) The chemical reaction between hydrogen and chlorine is a combinaton reaction
(j) When a piece of copper is added to silver nitrate solution, it turns blue in colour.

These terms are the vocabulary of chemistry that help us describe how matter behaves. Mastering these words is like learning the grammar of a new language.
Teacher's Tip: Remember "A-N-B" for acid-neutralization-base to link them together.
Exam Tip: Be careful with the spelling of "effervescence" and "alkalis," as they are common spelling trap words.

Question 2: Classify the following reactions as combination, decomposition, displacement, precipitation and neutralization. Also balance the equations.
(a) CaCO₃(s) x→heat CaO(s) + CO₂(g)
(b) Zn(s) + H₂SO₄ → ZnSO₄(s) + H₂(g)
(c) AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃
(d) NH₃(g) + HCl(g) → NH₄Cl(s)
(e) CuSO₄(aq) + H₂S(g) → CuS(s) + H₂SO₄(l)
(f) Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
(g) Ca(s) O₂(g) x→heat CaO(s)
(h) NaOH + HCl → NaCl + H₂O
(i) KOH + H₂SO₄ → K₂SO₄ + H₂O
Answer:
(i) Combination reactions :
(d) NH₃(g) + HCl(g) → NH₄Cl(s)
(g) 2Ca(s) O₂(g) x→heat 2CaO(s)
(ii) Decomposition reaction :
(a) CaCO₃(s) x→heat CaO(s) + CO₂(g)
(iii) Displacement reaction :
(f) Zn(s) + CuSO₄(aq) → ZnSO₄(aq) + Cu(s)
(b) Zn(s) + H₂SO₄ → ZnSO₄(s) + H₂(g)
(iv) Precipitation reaction :
(c) AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃
(e) CuSO₄(aq) + H₂S(g) → CuS(s) + H₂SO₄(l)
(v) Neutralization reaction :
(h) NaOH + HCl → NaCl + H₂O
(i) 2KOH + H₂SO₄ → K₂SO₄ + 2H₂O

Classifying reactions helps you predict what will happen when you mix two chemicals. For example, if you know it's displacement, you know one element will push out another.
Teacher's Tip: Combination reactions have two or more reactants but only ONE product.
Exam Tip: Don't forget to balance the equation *before* you decide on its type, as coefficients can give you clues.

Question 3: Define:
(a) precipitation (b) neutralization (c) catalyst
Answer:
(a) Precipitation: A chemical reaction in which two compounds in their aqueous state react to form an insoluble salt as one of the product.
Example. BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄ ↓ + 2NaCl(aq)
(Barium chloride) (Sodium sulphate) (Barium sulphate white ppt) (sodium chloride)
(b) Neutralization: A chemical reaction in which a base or an alkali reacts, with an acid to produce a salt and water only.
NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O
(sodium hydroxide) (hydro chloride acid) (sodium chloride) (water)
(c) Catalyst: A catalyst is a substance that either increases or decreases the rate of a chemical reaction without itself undergoing any chemical change.
N₂ + 3H₂ x→[450°C]Iron (Catalyst) 200 - 900 2NH₃ + Heat
here iron act as a catalyst and increases the rate of chemical reaction.

These definitions provide the rules for specific types of important reactions. Knowing them allows you to categorize almost any chemical change you see in the lab.
Teacher's Tip: Neutralization always makes two things: Salt and Water (H₂O).
Exam Tip: In the precipitation definition, the word "insoluble" is the most important word to include.

Question 4: Explain the following types of chemical reactions giving two examples for each of them.
(a) combination reaction
(b) decomposition reaction
(c) displacement reaction
(d) double decomposition reaction
Answer:
(a) Combination reaction: A reaction in which two or more substances combine to form a single substance is called combination reaction. A + B → AB. e.g (i) When iron and sulphur are heated together, they combine to form iron sulphide. Fe + S x→heat FeS. (ii) When carbon burns in oxygen to form a gaseous compound called carbon dioxide. C + O₂ x→heat CO₂ + Heat.
(b) Decomposition reaction: A reaction in which a compound breaks up due to the application of heat into two or more simple substances is called decomposition reaction. AB x→heat A + B. e.g. (i) Mercuric oxide when heated, decomposes to form two elements mercury and oxygen. 2HgO(s) x→heat 2Hg(s) + O₂(g). (ii) CaCO₃ when heated decomposes to calcium oxide and carbon dioxide. CaCO₃ x→heat CaO + CO₂.
(c) Displacement reaction: A reaction in which a more active element displaces a less active element from a compound is called displacement reaction. AB + C → CB + A. e.g. (i) Zinc, displaces copper from copper sulphate solution. Zn + CuSO₄(aq) → ZnSO₄(aq) + Cu. (ii) Iron piece when added to copper sulphate solution, copper is displaced. Fe + CuSO₄ → FeSO₄ + Cu.
(d) Double decomposition reaction: A chemical reaction in which two compounds in their aqueous state exchange their ions to form new compounds is called a double decomposition reaction. AB + CD → CB + AD. e.g. (i) AgNO₃ + HCl → AgCl + HNO₃(aq). (ii) NaOH(aq) + HCl(aq) → NaCl(aq) + H₂O.

Chemical reactions can be categorized by how the atoms move around. Some reactions build bigger molecules, while others break them down into smaller pieces.
Teacher's Tip: Think of Double Decomposition as a dance where two pairs of partners switch places.
Exam Tip: Using the letters like A + B → AB in your explanation shows you understand the general pattern of the reaction.

Question 5: Write the missing reactants and products and balance the equations.
(a) NaOH + ................ → NaCl + ......................
(b) KClO₃ x→heat ........................ + ..........................
(c) ...................... + HCl → NaCl + H₂O + ..........................
Answer:
(a) NaOH + HCl → NaCl + H₂O
(b) 2KClO₃ x→heat 2KCl + 3O₂
(c) Na₂SO₃ + 2HCl → 2NaCl + H₂O + SO₂.

Filling in missing parts is like solving a chemical puzzle by matching the elements. You check what is on one side and make sure it has a "home" on the other side.
Teacher's Tip: If you see NaCl as a product, you probably started with something containing Sodium (Na) and something containing Chlorine (Cl).
Exam Tip: After filling in the blanks, always re-count your atoms to make sure you didn't unbalance the equation.

Question 6: How will you obtain?
(a) Magnesium oxide from magnesium.
(b) Silver chloride from silver nitrate.
(c) Nitrogen dioxide from lead nitrate.
(d) Zinc chloride from zinc.
(e) Ammonia from nitrogen.
Also give balanced equations for the reactions
Answer:
(a) Magnesium when burnt in air (oxygen) Magnesium oxide is formed. 2Mg + O₂ x→heat 2MgO.
(b) When silver nitrate solution reacts with sodium chloride, silver chloride is formed. AgNO₃ + NaCl → AgCl + NaNO₃
(c) Lead nitrate when heated nitrogen oxide is obtained. 2Pb(NO₃)₂ x→heat 2PbO(s) + 4NO₂ + O₂(g).
(d) Zinc when reacts with hydrochloric acid zinc chloride and hydrogen (g) is formed. Zn + 2HCl → ZnCl₂ + H₂
(e) Nitrogen when reacts with hydrogen at 450°C and under 200 atm, ammonia is formed. N₂ + 3H₂ x→[200 atm.]450°C 2NH₃

These reactions show how we can make useful chemicals by mixing the right starting materials. Many of these require energy like heat to transform into the products we want.
Teacher's Tip: "Burnt in air" is a scientist's way of saying it reacts with Oxygen (O2).
Exam Tip: Write both the sentence explanation and the symbolic equation to get the highest score possible.

Question 7: What do you observe when
(a) Iron nail is kept in copper sulphate solution for sometime.
(b) Phenolphthalein is added to sodium hydroxide solution.
(c) Blue litmus paper is dipped in dilute hydrochloric acid.
(d) Lead nitrate is heated.
(e) Magnesium ribbon is burnt in oxygen.
(f) Ammonia is brought in contact of hydrogen chloride gas.
Answer:
(a) A brown layer of copper gets deposited on iron nail. This is due to chemical reaction. Fe(s) + CuSO₄(aq) → FeSO₄(aq) + Cu(s)
(b) Solution turns pink.
(c) Blue litmus turns red in an acid solution.
(d) The pale yellow solid is lead monoxide, the reddish brown gas is nitrogen dioxide and the colourless gas is oxygen. 2Pb(NO₃)₂(s) x→heat 2PbO(s) + 4NO₂(g) + O₂(g)
(e) Magnesium ribbon burns with a dazzling white light and produces a white powder which is magnesium oxide. 2Mg + O₂ → 2MgO (white powder)
(f) Ammonia and hydrogen chloride, both compounds, combine to form a compound, ammonium chloride. NH₃(g) + HCl(g) → NH₄Cl(s) (ammonium chloride)

Observations are the "visible proof" of chemistry happening in front of our eyes. These color changes and lights tell us that atoms are rearranging into something entirely new.
Teacher's Tip: Acid turns Blue litmus Red (A-B-R is an easy way to remember).
Exam Tip: Mentioning the "dazzling white light" for magnesium is a specific detail teachers always look for.

Question 8: Give reason:
(a) A person suffering from acidity is advised to take an antacid.
(b) Acidic soil is treated with quick lime.
(c) Wasp sting is treated with vinegar.
Answer:
(a) An antacid neutralizes stomach acidity.
(b) If the soil is acidic it can be treated with base like quick lime, to make it neutral.
(c) Wasp stings are alkaline and they can be neutralized by vinegar which is a weak acid.

Chemistry helps us solve everyday problems like stomach aches or farming issues. By using a base to cancel an acid (or vice versa), we can fix harmful imbalances.
Teacher's Tip: Wasp stings are bases, so we use vinegar (an acid). Bee stings are acidic, so we use baking soda (a base)!
Exam Tip: Use the word "neutralize" in your answers for questions about acids and bases fighting each other.

Question 9: What is meant by the metal reactivity series ? State its importance, (any two points).
Answer:
A list in which the metals are arranged in the decreasing order of their chemical reactivity is called the metal reactivity series. Special features of the activity series:
1. The ease with which a metal in solution loses electron(s) and forms a positive ion decreases down the series, i.e. from potassium to gold.
2. Hydrogen is included in the activity series because, like metals do, it too loses an electron and becomes positively charged (H^+) in most chemical reactions.
3. The series facilitates the comparative study of metals in terms of the degree of their reactivity.
4. The compounds of the metals (oxides, carbonates, nitrates and hydroxides) too can be easily compared.

The reactivity series is like a leaderboard for how active different metals are. Metals at the top are very "aggressive" and can easily push out metals that are further down the list.
Teacher's Tip: Potassium (K) is the "King" at the top, and Gold (Au) is at the very bottom.
Exam Tip: Remember that a metal can only displace another metal if it is *higher* than it on this series.

Question 10: What are oxides ? Give two examples of each of the following oxides.
(a) Basic oxide (b) Acidic oxide
(c) Amphoteric oxide (d) Neutral oxide
Answer: An oxide is a compound which essentially contains oxygen in its molecule, chemically combined with a metal or a non-metal. Following are the examples :
(a) Basic oxides - [Eg. Na₂O, CaO, MgO, etc.]
(b) Acidic oxides - [Eg. CO₂, SO₂, P₂O₅, etc.]
(c) Amphoteric - [Eg. ZnO, Al₂O₃, PbO, etc.]
(d) Neutral oxides - [Eg. H₂O, NO, CO, etc.]

Oxides are what you get when oxygen "teams up" with almost any other element. Depending on the partner, these oxides can behave like acids, bases, or stay completely neutral.
Teacher's Tip: "Amphoteric" oxides are like people who speak two languages; they can act as both acids and bases.
Exam Tip: Water (H2O) is the most important neutral oxide to remember for your tests.

Question 11: Define exothermic and endothermic reactions. Give two examples of each.
Answer:
Exothermic reactions: The chemical reaction in which heat is given out is called exothermic reactions. It causes rise in temperature. e.g. (i) When carbon bums in oxygen to form carbon dioxide, a lot of heat is produced. C + O2 → CO2 + heat. (ii) When water is added to quicklime a lot of heat is produced which boils the water. CaO + H₂O → Ca(OH)₂ + Heat.
Endothermic reaction: A chemical reaction in which heat is absorbed is called endothermic reaction. It causes fall in temperature. e.g. (i) When nitrogen and oxygen together are heated to a temperature of about 3000°C, nitric oxide gas is formed. N₂ + O₂ + heat → 2NO(g). (ii) Decomposition of calcium carbonate into carbon dioxide and calcium oxide when heated to a 1000°C. CaCO₃ + Heat → CaO(s) + CO₂(g).

Exothermic reactions release energy, like a warm bonfire, while endothermic reactions soak up energy, like an ice pack. This transfer of energy is a key part of every chemical change.
Teacher's Tip: "Ex" for "Exit" (heat goes out) and "En" for "Enter" (heat goes in).
Exam Tip: Don't forget to write "+ heat" on the correct side of the equation to show where the energy is going.

Question 12: State the effect of:
(a) an endothermic reaction
(b) an exothermic reaction on the surroundings.
Answer:
(a) Carbon dioxide present in the atmosphere is trapped by infrared radiations, gives rise to temperature which is exothermic reaction.
(b) The melting of glaciers by global warming.

Heat exchange between reactions and the environment can change our world. For instance, the way gases trap heat in our atmosphere is a massive-scale energy exchange that affects the whole planet.
Teacher's Tip: These processes are why global warming is such a serious environmental issue today.
Exam Tip: Be sure to connect "exothermic" with a *rise* in temperature and "endothermic" with a *fall* in temperature.

Question 13: What do you observe when
(a) an acid is added to a basic solution.
(b) ammonium chloride is dissolved in water.
Answer: 
(a) A chemical reaction in which a base or an alkali reacts with an acid to produce a salt and water. Acid + Base → Salt + Water
(b) Dissolution of ammonium chloride in water is an endothemic reaction in which heat energy is absorbed.

When you mix an acid and a base, they cancel each other out to make safe, neutral substances. Some processes, like dissolving certain salts, actually steal heat from the water and make it colder!
Teacher's Tip: If the test tube feels cold after you mix something in, you know it's an endothermic process.
Exam Tip: Always identify the products as "Salt and Water" for any neutralization question.

ADDITIONAL QUESTIONS

MULTIPLE CHOICE QUESTIONS
Tick the most appropriate answer.

1. Which one of the following alters the rate of the chemical reaction without itself undergoing any change?
1. temperature and Pressure
2. concentration of reactants
3. presence of catalyst
4. none of these
Answer: presence of catalyst
 

2. Heat energy is evolved in these reactions
1. Thermal decomposition reactions
2. Exothermic reactions
3. Endothermic reactions
4. None of these
Answer: Exothermic reactions
 

3. Thermal decomposition of a substance is brought about with the help of
1. reactants
2. water
3. wind
4. heat
Answer: heat
 

4. Reduction reaction is the exact opposite of
1. displacement reaction
2. decomposition reaction
3. oxidation reaction
4. none of these
Answer: oxidation reaction 
 

5. Which of the following is a reducing agent?
1. oxygen
2. chlorine
3. hydrogen
4. none of these
Answer: hydrogen
 

6. Compounds, which in their molten state allow an electric current to pass through them are called
1. electrolytes
2. non-electrolytes
3. non-conductors
4. none of these
Answer: electrolytes
 

7. Reduction is a chemical reaction in which there is
1. removal of hydrogen
2. removal of oxygen
3. addition of oxygen
4. none of these
Answer: removal of oxygen
 

8. Miscible liquids having slightly different boiling points can be separated by the
1. evaporation
2. distillation
3. fractional distillation
4. sedimentation
Answer: fractional distillation

MCQs check if you remember the quick facts that connect different parts of chemistry. Knowing these definitions helps you solve more complex problems later on.
Teacher's Tip: "Reduction" reduces the amount of oxygen in a compound.
Exam Tip: Read all options carefully; sometimes "none of these" is a real trick choice!

FILL IN THE BLANKS

1. A physical change is a temporary change.
2. In a endothermic reaction heat energy is absorbed.
3. In a double decomposition precipitation reaction both the reactants must be water soluble.
4. In a neutralization reaction, an acid reacts with a base to form salt and water as the only products.
5. Distillation is a method of obtaining pure solvant from a solution.
6. On reaction with sulphuric acid solution, barium chloride forms a white precipitate of barium sulphate.
7. Strong electrolytes dissociate completely in aqueous state and allow large amount of electricity to flow through it.
8. Negatively charged ions are called anions.
9. In a chemical change, new substances are formed.
10. The new substances formed during a chemical reaction are called the products.
11. Positively charged ions are called cations.

These blank-filling sentences summarize the most important rules of the chapter. They cover everything from the types of changes to the names of charged atoms.
Teacher's Tip: "Anion" starts with "A-N" for A Negative ion.
Exam Tip: Be careful with the spelling of technical words like "precipitate" and "electrolytes."

MATCH THE COLUMNS

1. H₂ + Cl₂ → 2HCl                                   a. displacement reaction
2. 2FeCl₃ + H₂ → 2FeCl₂ + 2HCl             b. addition of an electronegative redical
3. Fe + CuSO₄ → FeSO₄ + Cu                 c. removal of an electronegative redical
4. 2H₂O + 4NO₂ + O₂ → 4HNO₃             d. reduction by adding hydrogen
5. 3Fe + 3Cl₂ → 2FeCl₃                            e. electrolysis of water
6. FeCl₃ + 3NaOH → Fe(OH)₃ ↓ + NaCl   f. double decomposition, precipitation reaction
7. 2H₂O → 2H₂ ↑ + O₂                              g. combination or synthesis reaction
Answer: 
1. H₂ + Cl₂ → 2HCl | g. combination or synthesis reaction
2. 2FeCl₃ + H₂ → 2FeCl₂ + 2HCl | d. reduction by adding hydrogen
3. Fe + CuSO₄ → FeSO₄ + Cu | a. displacement reaction
4. 2H₂O + 4NO₂ + O₂ → 4HNO₃ | c. removal of an electronegative radical
5. 3Fe + 3Cl₂ → 2FeCl₃ | b. addition of an electronegative radical
6. FeCl₃ + 3NaOH → Fe(OH)₃ ↓ + NaCl | f. double decomposition, precipitation reaction
7. 2H₂O → 2H₂ ↑ + O₂ | e. electrolysis of water

Matching equations to their names is a great way to test if you can "read" the language of chemistry. Each equation tells a story about how atoms are moving or changing partners.
Teacher's Tip: Look for the downward arrow (↓) to quickly find precipitation reactions.
Exam Tip: Match the easiest ones (like combination or displacement) first to narrow down your choices.

TRUE / FALSE

1. A precipitation reaction takes place only in a solution. 
Answer: True.

2. Copper can displace iron from iron sulphate. 
Answer: False. Copper cannot displace iron from iron sulphate.

3. Ammonia is a good oxidizing agent. 
Answer: False. Oxygen is a good reducing agent.

4. Weak electrolytes dissociate partially in their aqueous or fused state. 
Answer: True.

5. Electrolytic refining is a method in which a thin coating of a metal is done over another metal. 
Answer: False. Electroplating is a method in which a thin coating of a metal is done over another metals are refined to purity.

6. Evaporation of water is a physical change. True or false ? 
Answer: True.

7. The evolution of a gas during a chemical reaction is shown by an arrow pointing downwards. True or false ?
Answer:  False.

8. Positively charged electrode is called the anode. True or false ? 
Answer: True.

9. Temperature at which a liquid starts boiling is known as the boiling point of that liquid. True or false ? 
Answer: True.

True and false questions challenge you to spot common mistakes or misconceptions in science. For example, knowing that gas goes *up* helps you spot that the arrow for gas evolution must point upwards.
Teacher's Tip: Remember "P-A" for Positive-Anode and "N-C" for Negative-Cathode.
Exam Tip: Always correct the "False" statements in your mind as you go; it helps you study better.

WRITE SHORT ANSWERS

Question 1: What are chemical reactions?
Answer:  Chemical reactions are defined as changes in which substances undergo an irreversible change, resulting in the formation of a new substances.
This is the basic definition of any permanent chemical change. Unlike melting ice, a chemical reaction makes something completely new that you can't easily change back.
Teacher's Tip: A chemical reaction is like baking a cake; once it's baked, you can't get the flour and eggs back.
Exam Tip: Using the word "irreversible" is key to getting full marks for this definition.

Question 2: What are biocatalysts ?
Answer:  Biocatalysts are enzymes which help our body to perform various metabolic functions, e.g. amylase, lipase etc.
These are the specialized helpers that keep living things alive and working correctly. Without them, processes like digestion would happen too slowly for us to survive.
Teacher's Tip: "Bio" means "life"—so these are catalysts for living things.
Exam Tip: Name at least one example like "lipase" to make your answer complete.

Question 3: How is precipitate formation in a chemical reaction indicated?
Answer: A precipitate is an insoluble substance formed when two substances or chemicals react in solution form. The precipitate formed is indicated by writing an arrow pointing downwards (↓) beside the precipitate. NaCl + AgNO₃ → AgCl ↓ + NaNO₃
This symbolic arrow shows that the substance is heavy and solid, so it sinks to the bottom of the test tube. It's a quick way to show the physical result of a reaction.
Teacher's Tip: Think of the arrow as "gravity" pulling the solid down.
Exam Tip: Always include the example equation with the arrow to show you know how to use the symbol.

Question 4: What are homogenous reactions ?
Answer: Homogeneous reactions are those in which the reactants and products are in the same physical state and are miscible with each other.
In these reactions, everything blends together perfectly, like mixing two different colors of juice. There are no separate layers or solid bits floating around.
Teacher's Tip: "Homo" means "same"—so everything is in the *same* state.
Exam Tip: Don't forget to mention that the substances must be "miscible" (mixable).

Question 5: Name the type of reaction which is governed by the position of a metal in the metal activity series ?
Answer: Displacement reaction.
This reaction is like a "strong" metal taking the seat of a "weaker" metal in a compound. The activity series tells us exactly who is strong enough to do the displacing.
Teacher's Tip: Think of it as a more popular kid displacing someone from a seat at lunch.
Exam Tip: Remember that displacement only happens if the free metal is *higher* on the activity series.

Question 6: Define electrolysis.
Answer: Electrolysis is the process of decomposition of an electrolyte, in its aqueous state by passing direct electric current through it.
This is a way to use electricity to "rip apart" molecules into their basic elements. We use this process to get pure metals like aluminum or gases like oxygen.
Teacher's Tip: "Lysis" means "splitting"—so electrolysis is "splitting with electricity."
Exam Tip: Be sure to specify that the electrolyte must be in an "aqueous" (liquid) state for it to work.

Question 7: Name three reducing agents ?
Answer: (i) Hydrogen (ii) Ammonia (iii) Carbon
Reducing agents are the "oxygen thieves" of chemistry—they take oxygen away from other substances. This process is used extensively in mining and industry to get pure metals.
Teacher's Tip: These three are the most common ones used in big factories.
Exam Tip: List them clearly as (i), (ii), and (iii) to make your answer look organized.

Question 8: Is kerosene an electrolyte or a non-electrolyte ?
Answer: Non-electrolyte.
This means that kerosene does not let electricity pass through it even when it is a liquid. Electrolytes need ions to carry the current, and kerosene doesn't have them.
Teacher's Tip: Most oils and fuels are non-electrolytes.
Exam Tip: If you're asked why, explain that it doesn't form ions in solution.

ANSWER IN DETAIL

Question 1: What are the characteristics of a chemical reaction ? A chemical reaction is characterised by an irreversible change in which a new substance is formed.
Answer: Characteristics of a chemical reaction:
1. Evolution of a gas: A gas may evolve during a chemical reaction. This is shown by writing an arrow pointing upwards (↑) besides the gas. Some gases have characteristic smell also. e.g. 2Na + 2HCl → 2NaCl + H₂ ↑
2. Change in colour: Change in colour takes place in some chemical reactions e.g. CuCO₃ x→Delta CuO + CO₂ (green to black).
3. Formation of precipitate: Some chemical reactions involve the formation of a precipitate which is characterised by an arrow pointing downwards (↓) beside the precipitate. e.g. NaCl + AgNO₃ → AgCl ↓ + NaNO₃ (Precipitate)
4. Change of physical state: In some chemical reactions, a change of state takes place from reactants to products, e.g. NH3 + HCl → NH4Cl (gas) (gas) (solid)
These four signs are the physical clues that tell us an invisible atomic dance is happening. By looking for these clues, we can identify what kind of reaction is taking place in our test tube.
Teacher's Tip: Color changes and state changes are the easiest signs to spot during an experiment.
Exam Tip: For each point, providing a specific example equation with symbols like ↑ or ↓ makes your answer perfect.

Question 2: Explain the type of reactions on the basis of the direction of reaction.
Answer: On the basis of whether or not a reaction goes to completion, chemical reactions are classified into two categories. These are discussed as under:
1. Irreversible reactions: A reaction in which the reactants are completely converted into products is called an irreversible reactions, e.g. C + O₂ → CO₂
2. Reversible reactions: A reaction which proceeds in both forward and reverse directions is called as a reversible reaction. e.g. N₂ + 3H₂ rightleftharpoons 2NH₃
Direction tells us if a reaction is a "one-way street" or a "two-way street." Irreversible reactions finish completely, while reversible ones can "reset" themselves back to the start.
Teacher's Tip: Think of the special double arrow (rightleftharpoons) as the "two-way street" sign.
Exam Tip: Be sure to use the correct arrow symbol for reversible reactions in your sketches.

Question 3: What are thermal decomposition reactions ? Explain with an example.
Answer: Thermal decomposition reactions are those reactions which break up or decomposes on heating to form many products. This is represented by a general formula format: A → B + C. e.g. ZnCO₃ x→heat ZnO + CO₂. Zinc carbonate on heating decomposes to form Zinc oxide and carbon dioxide.
This reaction uses heat energy like a hammer to smash a complex molecule into simpler pieces. It is a very common way to extract useful elements from minerals found in the ground.
Teacher's Tip: "Thermal" means heat, and "Decomposition" means breaking down.
Exam Tip: Draw the heat symbol (Delta) over the reaction arrow to show energy was added.

Question 4: What are chemical combination or synthesis reactions ? Chemical combination or synthesis:
Answer: In chemical combination or synthesis reactions, two or more elements or substances combine to form a single product. For example: H₂ + Cl₂ → 2HCl. When hydrogen combine with chlorine in the presence of diffused sunlight, it forms hydrogen chloride.
This is the exact opposite of decomposition—it's like two pieces of a puzzle clicking together to make one. Many important industrial chemicals are "synthesized" or built up in this way.
Teacher's Tip: If there's only one product on the right side, it's a combination reaction!
Exam Tip: Use the word "Synthesis" as it's the more professional term for combination.

Question 5: What do you mean by redox reaction ? Explain with the help of an example.
Answer: A redox reaction is defined as a reaction in which oxidation and reduction takes place simultaneously. In fact both these processes take place simultaneously. For example in the reaction of Copper oxide and Hydrogen: CuO + H₂ → Cu + H₂O. In the above process: 1. Removal of oxygen from copper oxide to form copper. 2. Addition of oxygen to hydrogen to form water.
Redox is a "team effort" reaction where oxygen moves from one atom to another. One side gets reduced (loses oxygen) while the other gets oxidized (gains oxygen) at the same time.
Teacher's Tip: REDOX is just short for Reduction and Oxidation.
Exam Tip: Draw arrows over your equation showing exactly which atom is being oxidized and which is being reduced.

Question 6: Write a short note on the information derived from a balanced chemical equation.
Answer: A chemical equation is a statement that represents a chemical change in terms of symbols and formulae. A lot of information can be obtained from balanced chemical equation.
1. It emphasizes the fact that matter can neither be destroyed nor be created.
2. Simple calculation can be done on the basis of chemical equation.
3. Problems based on percentage calculation can be solved.
A balanced equation is like a perfect map of a reaction; it tells you exactly what ingredients you need and how much of each. It's the most powerful tool a chemist has for planning experiments.
Teacher's Tip: It's just like a cooking recipe that has the exact weight of each ingredient.
Exam Tip: Mention the "Law of Conservation of Mass" as the fundamental reason why equations must be balanced.

Question 7: How is a strong electrolyte different from a weak electrolyte?
Answer: 
Strong electrolytes are compounds which dissociate completely in their aqueous or fused state. It allows electricity to flow through them. e.g. all strong acids and alkalis.
Weak electrolytes are chemical compounds which dissociate partially in their aqueous or fused state. They are not good conductors of electricity, e.g. weak acids like carbonic acid, weak alkalis like ammonium hydroxide.
Strong electrolytes are like wide superhighways for electricity, letting lots of charge through easily. Weak electrolytes are like bumpy dirt roads—some charge gets through, but it's much harder.
Teacher's Tip: "Strong" means 100% split into ions; "Weak" means only a few split.
Exam Tip: Always give an example for each, like "Sodium Hydroxide" for strong and "Vinegar" for weak.