Keywords To Remember
- Matter
Anything that has mass and occupies space. - Element
A pure substance made of only one kind of atom that cannot be broken down into simpler substances. - Compound
A substance formed when two or more elements chemically combine in a fixed ratio. - Mixture
A combination of two or more substances that are not chemically combined. - Atom
The smallest unit of an element that retains the properties of that element. - Molecule
A group of two or more atoms held together by chemical bonds. - Homogeneous Mixture
A mixture that has the same composition throughout (e.g., saltwater). - Heterogeneous Mixture
A mixture that does not have the same composition throughout (e.g., sand and iron filings). - Solution
A type of homogeneous mixture where one substance is dissolved in another. - Solute
The substance that is dissolved in a solution. - Solvent
The substance in which the solute is dissolved. - Physical Change
A change that affects the physical properties of a substance but does not change its chemical composition. - Chemical Change
A change that results in the formation of one or more new substances. - Chemical Reaction
A process in which one or more substances (reactants) are transformed into new substances (products). - Reactant
A substance that takes part in and undergoes change during a chemical reaction. - Product
A new substance formed as a result of a chemical reaction. - Exothermic Reaction
A chemical reaction that releases heat energy. - Endothermic Reaction
A chemical reaction that absorbs heat energy. - Symbol
A one- or two-letter abbreviation used to represent an element (e.g., H for hydrogen, O for oxygen). - Chemical Formula
A representation of a compound using symbols and numbers (e.g., H₂O for water). - Ion
An atom or molecule that has gained or lost one or more electrons, acquiring a charge. - Cation
A positively charged ion (formed by loss of electrons). - Anion
A negatively charged ion (formed by gain of electrons). - Decomposition Reaction
A chemical reaction in which a compound breaks down into simpler substances. - Combination Reaction
A chemical reaction in which two or more substances combine to form a new compound. - Displacement Reaction
A chemical reaction in which an element displaces another element in a compound. - Double Displacement Reaction
A reaction in which parts of two compounds switch places to form two new compounds. - Precipitate
A solid that forms and settles out of a liquid mixture during a chemical reaction. - Oxidation
A chemical reaction in which a substance gains oxygen or loses electrons. - Reduction
A chemical reaction in which a substance loses oxygen or gains electrons.
Something To Know
A. Fill in the blanks.
- Materials made up of same kind of particles are called __________ .
Answer – pure substances
- The type and number of particles of each kind present in a substance is given by its __________ .
Answer – chemical formula
- The chemical formula of water is __________ .
Answer – H₂O
- Iron gets rusted on coming in contact with __________ and__________ .
Answer – oxygen, water
- The process that leads to a chemical change is called a __________ .
Answer – chemical reaction
- In a neutralisation reaction, __________ and __________ are formed.
Answer – salt, water
B. Write True or False:
- All matter is made up of atoms.
Answer – True
- Compounds are substances consisting of two or more elements chemically combined in a fixed ratio.
Answer – True
- The symbol of element copper is Cu.
Answer – True
- Formula of sodium chloride is written as CINa.
Answer – False (Correct: NaCl)
- The chemical formula of aluminium sulphate A1,SO4 is.
Answer – False (Correct: Al₂(SO₄)₃)
- New substance formed in a chemical reaction is called product.
Answer – True
- Magnesium hydroxide is an acid.
Answer – False (It is a base)
C. Tick (✓) the correct option.
- The chemical symbol Ag represents the element—
Sodium
silver
aluminium
sulphur
Answer – silver
- One molecule of nitric acid is made up of—
two atoms of hydrogen, two atoms of nitrogen and two atoms of oxygen.
one atom of hydrogen, one atom of nitrogen and three atoms of oxygen,
one atom of hydrogen, one atom of nitrogen and two atoms of oxygen,
one atom of hydrogen, two atoms of nitrogen and three atoms of oxygen.
Answer – one atom of hydrogen, one atom of nitrogen and three atoms of oxygen
3.The chemical formula of magnesium phosphate is—
Mg(PO4)2
Mg3(PO4)2
Mg2(PO4)3
Mg(PO4)3
Answer – Mg₃(PO₄)₂
- The following reaction is an example of a—
Fe(s) + CuSO4(aq) ► FeSO4(aq) + Cu(s)
combination reaction
decomposition reaction
displacement reaction
neutralisation reaction
Answer – displacement reaction
- The chemical formula of quicklime is—
CaO
CaCO3
Ca(OH)2
CaCl2
Answer – CaO
D. Answer the following questions in brief.
- What are elements?
Answer – Elements are pure substances that consist of only one kind of atom and cannot be broken down into simpler substances.
- Give the chemical symbol of iron and chlorine.
Answer – Fe (iron), Cl (chlorine)
- Write the chemical formulae of the following compounds:
(a) Aluminium oxide
Answer – Al₂O₃
(b) Zinc acetate
Answer – Zn(CH₃COO)₂
- Balance the following equation:
Ca(OH)2 + HC1 ► CaCl2 + H2O
Answer – Balanced equation – Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O
- What does a chemical equation represent?
Answer – A chemical equation represents a chemical reaction using symbols and formulas of reactants and products.
- What are reactants and products in a chemical equation?
Answer – Reactants are substances that undergo chemical change, and products are the new substances formed.
- Give one example of a combination reaction.
Answer – Example – 2H₂ + O₂ → 2H₂O
E. Answer the following questions
- Write the steps involved in writing the chemical formula of calcium phosphate.
Answer – Step 1: Write the symbols of the ions: Ca²⁺ and PO₄³⁻
Step 2: Cross the valencies to balance charges:
Ca³(PO₄)₂ → Ca₃(PO₄)₂
- How is a chemical change different from a physical change?
Answer – A chemical change results in the formation of a new substance with different properties, whereas a physical change only affects the form or appearance without changing the substance itself.
- ‘Neutralisation reaction is a chemical change.’ Justify this statement with the help of an example.
Answer – In a neutralisation reaction, an acid reacts with a base to form salt and water, which are new substances.
Example – HCl + NaOH → NaCl + H₂O
- Define a decomposition reaction and give an example of the same.
Answer – A decomposition reaction is one where a single compound breaks down into two or more simpler substances.
Example – 2H₂O → 2H₂ + O₂
- Classify the following reactions into different types, giving reason.
(a) CaO(s) + SiO2(g) ► CaSiO3(s)
Answer – Combination reaction (two reactants form a single product)
(b) KOH(aq) + HCl(aq) ► KCl(aq) + H2O(1)
Answer – Neutralisation reaction (acid + base → salt + water)
(c) Cu(s) + 2AgNO3(aq) ► Cu(NO3)2(aq) + 2Ag(s)
Answer – Displacement reaction (copper displaces silver)
(d) BaCO3(s) BaO(s) + CO2(g)
Answer – Decomposition reaction (single compound breaks into simpler
Value Based Question
The school principal told her students that she would like them to follow a ‘special practice’ of the Japanese schools. Their schools do not keep staff members for cleaning as such. The students, themselves, work as a team and take pride in maintaining the cleanliness of their school. She went on to say that she would like them to replace their old habits by new, better habits in a way similar to a ‘displacement reaction’ in which one element replaces another in a compound.
- State two values displayed by the students of Japanese schools.
Answer – Responsibility and teamwork
- Why did the principal tell her students that the suggested ideas, in a way, similar to what happens in a displacement reaction?
Answer – Because just like one element replaces another in a compound, new habits (like cleanliness) should replace the old habits (like carelessness).
- Give one example of a displacement reaction.
Answer – Example – Zn + CuSO₄ → ZnSO₄ + Cu
Something To Do
- List any five physical and chemical changes that you see around you.
Answer – Physical Changes:
- Melting of ice
- Breaking of glass
- Dissolving sugar in water
- Boiling of water
- Cutting of paper
Chemical Changes:
- Rusting of iron
- Burning of wood
- Cooking of food
- Digestion of food
- Souring of milk
- Think of a small activity to show that rusting of iron requires both oxygen and water.
Answer –Take three test tubes with iron nails.
In the first, add only dry air (with calcium chloride).
In the second, add boiled (oxygen-free) water and oil to keep air out.
In the third, leave it open with tap water (exposed to air).
Only the third test tube will show rusting, proving both air and water are required.
Additional Questions and Answers – 01
Fill in the Blanks
- The smallest unit of a compound is called a __________.
Answer – molecule - A pure substance containing only one kind of atom is called an __________.
Answer – element - The process of breaking down a compound using electricity is called __________.
Answer – electrolysis - A __________ change is generally irreversible and involves formation of new substances.
Answer – chemical - __________ is used to represent a chemical reaction using symbols and formulae.
Answer – Chemical equation - __________ is the symbolic representation of an element.
Answer – Chemical symbol - __________ is formed when calcium reacts with water.
Answer – Calcium hydroxide - In a chemical reaction, substances that undergo the reaction are called __________.
Answer – reactants - A __________ reaction involves exchange of ions between two compounds.
Answer – double displacement - The number written at the lower right corner of a symbol in a formula is called __________.
Answer – subscript
Multiple Choice Questions (MCQs)
- Which of the following is a chemical change?
a) Melting of ice
b) Dissolving salt in water
c) Burning of paper
d) Boiling of water
Answer – c) Burning of paper - The chemical formula for ammonia is—
a) NH₄
b) NH₃
c) NO₂
d) N₂H₄
Answer – b) NH₃ - Which of these is a decomposition reaction?
a) Na + Cl₂ → NaCl
b) H₂ + Cl₂ → 2HCl
c) CaCO₃ → CaO + CO₂
d) Zn + HCl → ZnCl₂ + H₂
Answer – c) CaCO₃ → CaO + CO₂ - Which of the following is not a compound?
a) Water
b) Air
c) Carbon dioxide
d) Methane
Answer – b) Air - A reaction in which heat is released is known as—
a) Endothermic
b) Isothermal
c) Neutralisation
d) Exothermic
Answer – d) Exothermic
True/False
- All physical changes are irreversible.
Answer – False - Atoms of the same element always combine to form molecules.
Answer – True - Rusting of iron is a chemical change.
Answer – True - Elements can be broken down into simpler substances by chemical means.
Answer – False - The formula of calcium carbonate is CaCO₃.
Answer – True - Neutralisation reactions produce water and salt.
Answer – True - The formula of methane is CH₄.
Answer – True
- Short Answer Questions
- What is a molecule?
Answer – A molecule is the smallest unit of a compound that retains its chemical properties. - Define a compound.
Answer – A compound is a substance made from two or more elements that are chemically combined in a fixed ratio. - What do we mean by a balanced chemical equation?
Answer – A balanced chemical equation has an equal number of atoms for each element on both sides of the equation. - Name two types of chemical reactions.
Answer – Combination reaction and decomposition reaction. - What is a physical change?
Answer – A physical change is a change in state or appearance that does not alter the chemical composition of a substance.
Competency-Based Questions (Reasoning & Assertion)
- Assertion (A): All chemical reactions are accompanied by a change in energy.
Reason (R): Energy is either absorbed or released during a chemical reaction.
Answer – Both A and R are true and R is the correct explanation of A. - Assertion (A): Rusting can be prevented by painting iron surfaces.
Reason (R): Painting prevents the contact of iron with oxygen and moisture.
Answer – Both A and R are true and R is the correct explanation of A. - Assertion (A): Evaporation of water is a chemical change.
Reason (R): It involves a change in state but not in composition.
Answer – A is false but R is true. - Assertion (A): CO₂ is formed during respiration.
Reason (R): Respiration is a combustion reaction.
Answer – A is true but R is false. - Assertion (A): In a neutralisation reaction, the product is always acidic.
Reason (R): Acids and bases react to form neutral products.
Answer – A is false but R is true.
Higher Order Thinking Skill (HOTS) Questions
- Why does iron rust faster in coastal areas than in deserts?
Answer – Coastal areas have high humidity and moisture in the air, which increases the rate of rusting due to the presence of both water and oxygen. - If you heat a blue-coloured copper sulphate crystal and it turns white, what does this indicate?
Answer – It indicates the loss of water of crystallisation, a chemical change that alters the compound. - Why do chemical equations need to be balanced?
Answer – To follow the Law of Conservation of Mass which states that mass can neither be created nor destroyed in a chemical reaction. - If you mix lead nitrate solution with potassium iodide, a yellow precipitate is formed. What type of reaction is this?
Answer – This is a double displacement reaction. - Why do acids not show acidic properties in the absence of water?
Answer – Because acids release H⁺ ions only in aqueous solutions, which are responsible for acidic behaviour.
Long Answer Questions (50–60 words)
- Explain with an example how a chemical equation can be balanced.
Answer – A chemical equation is balanced by equalizing the number of atoms of each element on both sides. For example, in the reaction H₂ + O₂ → H₂O, we balance it as 2H₂ + O₂ → 2H₂O to ensure there are equal numbers of hydrogen and oxygen atoms on both sides. - How do elements differ from compounds?
Answer – Elements are pure substances made up of only one kind of atom, and cannot be broken down. Compounds are formed by chemical combination of two or more elements in a fixed ratio and can be decomposed into simpler substances by chemical methods. - What is a decomposition reaction? Give one example and explain.
Answer – A decomposition reaction involves breaking down of a compound into two or more simpler substances. For example, 2HgO → 2Hg + O₂, where mercury oxide decomposes into mercury and oxygen gas when heated. - Why is rusting of iron a serious problem? How can it be prevented?
Answer – Rusting weakens iron by forming flaky iron oxide, reducing its strength and durability. It can be prevented by painting, galvanizing, or applying oil to create a barrier between iron, water, and oxygen. - What are the characteristics of a chemical change?
Answer – Characteristics include change in color, temperature, evolution of gas, formation of precipitate, and formation of a new substance. For example, burning of wood releases heat and forms ash and gases, showing a chemical change.
Additional Questions and Answers – 02
Fill in the Blanks
- A pure substance made up of atoms of the same kind is called an ________.
Answer: Element
- The chemical formula of water is ________, indicating two hydrogen atoms and one oxygen atom.
Answer: H2O
- A mixture contains two or more substances that are not ________ combined.
Answer: Chemically
- The symbol for the element sodium is ________.
Answer: Na
- In a chemical equation, the substances that undergo a chemical change are called ________.
Answer: Reactants
- A chemical reaction where two or more substances combine to form a new substance is called a ________ reaction.
Answer: Combination
- The chemical formula of aluminium sulphate is ________.
Answer: Al2(SO4)3
- A reaction between an acid and a base that forms salt and water is called a ________ reaction.
Answer: Neutralisation
- In a balanced chemical equation, the number of atoms of each element is ________ on both sides.
Answer: Equal
- The process of a substance changing into another substance due to a chemical change is called a ________ reaction.
Answer: Chemical
Multiple Choice Questions (MCQs)
- Which of the following is a pure substance?
- a) Air
- b) Iron
- c) Soil
- d) Lemonade
Answer: b) Iron
- What does the chemical formula CO2 represent?
- a) One carbon atom and one oxygen atom
- b) One carbon atom and two oxygen atoms
- c) Two carbon atoms and one oxygen atom
- d) Two carbon atoms and two oxygen atoms
Answer: b) One carbon atom and two oxygen atoms
- What is the symbol for the element gold?
- a) Ag
- b) Au
- c) Cu
- d) Fe
Answer: b) Au
- In a chemical equation, where are the products written?
- a) Left-hand side
- b) Right-hand side
- c) Above the arrow
- d) Below the arrow
Answer: b) Right-hand side
- Which type of reaction occurs when limestone decomposes into quick lime and carbon dioxide?
- a) Combination
- b) Decomposition
- c) Displacement
- d) Neutralisation
Answer: b) Decomposition
- What is the chemical formula of sodium chloride?
- a) NaCl
- b) ClNa
- c) Na2Cl
- d) NaCl2
Answer: a) NaCl
- In a displacement reaction, what happens to the blue color of copper sulphate solution when an iron nail is added?
- a) It turns red
- b) It turns green
- c) It turns yellow
- d) It remains blue
Answer: b) It turns green
- What is the charge of the magnesium ion?
- a) Mg+
- b) Mg2+
- c) Mg3+
- d) Mg-
Answer: b) Mg2+
- Which of the following is an example of a combination reaction?
- a) 2H2O → 2H2 + O2
- b) Fe + CuSO4 → FeSO4 + Cu
- c) 2H2 + O2 → 2H2O
- d) NaOH + HCl → NaCl + H2O
Answer: c) 2H2 + O2 → 2H2O
- What does the notation (aq) indicate in a chemical equation?
- a) Solid state
- b) Liquid state
- c) Gaseous state
- d) Aqueous solution
Answer: d) Aqueous solution
True/False Questions
- A compound is made up of atoms of the same kind.
Answer: False (A compound is made up of molecules containing atoms of different elements)
- Mixtures are pure substances.
Answer: False (Mixtures are not pure substances)
- The chemical formula of a substance indicates the type and number of atoms present.
Answer: True
- In a chemical equation, reactants are written on the right-hand side.
Answer: False (Reactants are written on the left-hand side)
- A decomposition reaction results in the formation of a single product from multiple reactants.
Answer: False (It results in multiple products from a single reactant)
- The symbol for oxygen is O.
Answer: True
- A neutralisation reaction produces a salt and water.
Answer: True
- In a balanced chemical equation, the number of atoms of each element is equal on both sides.
Answer: True
- The chemical formula of glucose is C6H12O6.
Answer: True
- During a physical change, a substance changes into another substance.
Answer: False (A physical change does not change the substance into another)
Short Questions and Answers
- What is an element?
Answer: An element is a pure substance made up of atoms of the same kind, such as iron.
- How is a compound different from a mixture?
Answer: A compound is a pure substance with atoms of different elements in a fixed ratio, while a mixture contains substances mixed in any proportion without chemical bonding.
- What does the chemical formula HNO3 represent?
Answer: HNO3 represents nitric acid, containing one hydrogen atom, one nitrogen atom, and three oxygen atoms.
- What is a chemical reaction?
Answer: A chemical reaction is a process where a substance changes into another due to a chemical change.
- What is a combination reaction?
Answer: A combination reaction is a chemical reaction where two or more substances combine to form a new substance.
- Why must a chemical equation be balanced?
Answer: A chemical equation must be balanced to ensure the number of atoms of each element is the same on both sides, following the law of conservation of mass.
- What happens in a displacement reaction?
Answer: In a displacement reaction, one element displaces another in a compound, forming a new compound and releasing the displaced element.
- Name one example of a neutralisation reaction.
Answer: The reaction between hydrochloric acid and sodium hydroxide to form sodium chloride and water: NaOH + HCl → NaCl + H2O.
- What is the role of subscripts in a chemical formula?
Answer: Subscripts in a chemical formula indicate the number of atoms of each element in a molecule, such as the “2” in H2O showing two hydrogen atoms.
- What are ions?
Answer: Ions are positively or negatively charged particles, such as Na+ (sodium ion) or Cl- (chloride ion), present in some substances.
Competency-Based Questions (Reasoning and Assertion)
- Assertion (A): Water is a compound. Reason (R): Water is made up of hydrogen and oxygen atoms in a fixed ratio of 2:1.
- a) Both A and R are true, and R is the correct explanation of A.
- b) Both A and R are true, but R is not the correct explanation of A.
- c) A is true, but R is false.
- d) A is false, but R is true.
Answer: a) Both A and R are true, and R is the correct explanation of A.
- Assertion (A): Mixtures are pure substances. Reason (R): Mixtures contain two or more substances mixed in any proportion.
- a) Both A and R are true, and R is the correct explanation of A.
- b) Both A and R are true, but R is not the correct explanation of A.
- c) A is true, but R is false.
- d) A is false, but R is true.
Answer: d) A is false, but R is true.
- Assertion (A): A chemical equation must be balanced. Reason (R): A balanced chemical equation ensures the same number of atoms of each element on both sides.
- a) Both A and R are true, and R is the correct explanation of A.
- b) Both A and R are true, but R is not the correct explanation of A.
- c) A is true, but R is false.
- d) A is false, but R is true.
Answer: a) Both A and R are true, and R is the correct explanation of A.
- Assertion (A): The reaction between iron and copper sulphate is a decomposition reaction. Reason (R): Iron displaces copper from copper sulphate, forming iron sulphate and copper.
- a) Both A and R are true, and R is the correct explanation of A.
- b) Both A and R are true, but R is not the correct explanation of A.
- c) A is true, but R is false.
- d) A is false, but R is true.
Answer: d) A is false, but R is true. (The reaction is a displacement reaction, not decomposition.)
- Assertion (A): Sodium chloride is written as NaCl. Reason (R): In the chemical formula, the symbol of the positive ion is written first, followed by the negative ion.
- a) Both A and R are true, and R is the correct explanation of A.
- b) Both A and R are true, but R is not the correct explanation of A.
- c) A is true, but R is false.
- d) A is false, but R is true.
Answer: a) Both A and R are true, and R is the correct explanation of A.
High Order Thinking Skill (HOTS) Questions and Answers
- Why does the blue colour of copper sulphate solution change to green when an iron nail is added?
Answer: Iron displaces copper from copper sulphate, forming iron sulphate, which is green, and copper deposits as a brown coating on the nail, changing the solution’s colour.
- How does the criss-cross method help in writing the chemical formula of ionic compounds?
Answer: The criss-cross method involves exchanging the charges of positive and negative ions as subscripts, ensuring the compound is neutral, like in Na2O, where Na+ and O2- charges are swapped.
- Why is a chemical equation balanced, and what does it signify?
Answer: A chemical equation is balanced to follow the law of conservation of mass, ensuring the same number of atoms of each element on both sides, signifying that matter is neither created nor destroyed.
- How can you differentiate between a combination and a decomposition reaction?
Answer: A combination reaction involves two or more substances forming one product (e.g., 2H2 + O2 → 2H2O), while a decomposition reaction breaks one reactant into multiple products (e.g., 2H2O → 2H2 + O2).
- Why are coefficients used instead of changing subscripts when balancing a chemical equation?
Answer: Coefficients are used to balance the number of atoms without altering the chemical formula of substances, as changing subscripts would change the identity of the compound, which is incorrect.
Long Questions and Answers (50-60 words)
- Explain the difference between an element and a compound with examples.
Answer: An element is a pure substance made of atoms of the same kind, like iron (Fe). A compound is a pure substance made of molecules with atoms of different elements in a fixed ratio, like water (H2O), which contains two hydrogen atoms and one oxygen atom combined together.
- Describe the process of writing the chemical formula of sodium oxide.
Answer: To write the formula of sodium oxide, write the symbols of sodium (Na+) and oxide (O2-) with their charges. Criss-cross the charges: Na gets 2, O gets 1. Remove the common factor if any (none here). The formula becomes Na2O, where the subscript 1 for O is not written.
- What is a displacement reaction? Give an example with a chemical equation.
Answer: A displacement reaction occurs when one element displaces another in a compound. For example, when an iron nail is placed in copper sulphate solution, iron displaces copper, forming iron sulphate and copper: Fe(s) + CuSO4(aq) → FeSO4(aq) + Cu(s). The solution turns green, and copper forms a brown coating.
- Explain the significance of a balanced chemical equation with an example.
Answer: A balanced chemical equation ensures the same number of atoms of each element on both sides, following the law of conservation of mass. For example, in 2H2(g) + O2(g) → 2H2O(g), two hydrogen molecules and one oxygen molecule form two water molecules, balancing the atoms on both sides.
- What is a neutralisation reaction? Provide an example with a chemical equation.
Answer: A neutralisation reaction occurs between an acid and a base, forming a salt and water. For example, hydrochloric acid reacts with sodium hydroxide to form sodium chloride and water: NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l). This reaction is used to neutralize acidic or basic solutions.