Selina Concise Solutions for ICSE Class 6 Chemistry Chapter 4 Elements Compounds Symbols And Formulae

Elements, Compounds, Symbols And Formulae

POINTS TO REMEMBER

1. Pure substances : “A single substance of definite composition.” Pure substances are homogeneous. They are made up of only one kind of atoms and compounds or made up of only one kind of molecules.
2. Elements : An element is defined as a pure substance made up of only one kind of atoms that cannot be converted into anything simpler than itself by any physical or chemical process.
3. Metals : Most of the elements known to us are metals. Example: Sodium potassium, iron, gold, silver are elements. Metals are lustrous, hard, ductile and malleable, good conductor of electricity. Mercury (liquid) have High M.P. and High B.P. but exceptions are there.
4. Non-metals : Carbon, phosphorous, sulphur all solids, Bromine (liquid) iodine(s) and rest are gases, dull, do not shine. Cannot be drawn into wires, non malleable, have low M.P. and B.P. do not produce sound when struck, do not conduct electricity. Exceptions are there.
5. Boron, sillicon, arsenic, antimony resemble both metals and non-metals and are called metalloids.
6. Unreactive gases, also called noble gases are gases which are very unreactive i.e. He, Ne, Ar, Kr, xenon and radon.
7. Symbols are abbreviations that are used to denote a chemical element which is usually first letter of its name in English or Latin.
8. O is the symbol of element oxygen.
C is symbol of element carbon.
Cu is symbol of element copper (taken from Latin name Cuprum)
9. Compound is formed by the chemical combination of two or more elements in definite ratio (by mass).
10. Molecule is the smallest unit of a compound.
11. Elements are the basic substances from which all other substances are made.
12. Compound : Consist of two or more elements combined in definite proportion, pure and homogeneous, physical and chemical properties are entirely new and different from its consititutent elements, energy is either needed or produced when a compound is formed.
13. Atoms : Atom is the smallest unit of an element which cannot be further broken into simpler parts, may or may not have independent existence.
14. Molecule of element : When two or more atoms of the same element combine it forms a molecule of an atom. e.g. N₂, O₂
15. Molecule of compounds : When atoms of two or more element combine, they form a molecule of a compound. H₂O, HCl, CuSO₄.

EXERCISE-I

Question 1: Classify the following substances into elements and compounds.
Mercury, sulphur, sugar, water, sand, gold, coal, oxygen, alcohol.

Answer:
Element: Mercury, sulphur, gold, coal, oxygen.
Compound : Sugar, water, sand, alcohol.
Elements are pure substances consisting of only one type of atom, whereas compounds are formed when different elements bond together. Identifying these helps us understand how matter is structured at the basic level.
Teacher's Tip: Elements are usually listed by a single name or symbol, while compounds often represent things we use daily like sugar or water.
Exam Tip: Always list elements and compounds in separate columns or clearly labeled groups to ensure the examiner can read your answer easily.

Question 2: Give the symbols of: Carbon, calcium, copper, chlorine, cobalt, argon.

Answer:
Carbon is C Chlorine is Cl
Calcium is Ca Cobalt is Co
Copper is Cu Argon is Ar
Chemical symbols are shorthand notation for elements often derived from English or Latin names. These symbols make it much easier to write chemical reactions and formulas quickly.
Teacher's Tip: If a symbol has two letters, the first is always capital and the second is always lowercase.
Exam Tip: Be careful not to confuse Co (Cobalt) with CO (Carbon monoxide); the lowercase letter is vital.

Question 3: Define a pure substance. How many types of pure substances do you know ?

Answer:
Pure substances : “A substance of a definite composition which has consistent properties throughout, is called a pure substance”
Types of pure substances: Pure substances are of two types (i) Elements, (ii) Compounds.
A pure substance has a fixed chemical makeup and uniform characteristics regardless of where it is found. This category excludes mixtures where different substances are just physically blended together.
Teacher's Tip: Think of a pure substance as something that contains only one type of particle, whether it is an atom or a molecule.
Exam Tip: Always mention both elements and compounds when asked for the types of pure substances to get full marks.

Question 4: Define : (a) Elements (b) Compounds.
Name the particles from which elements and compounds are made of.

Answer:
(a) Elements : An element is defined as a pure substance made up of only one kind of atoms that cannot be converted into anything simpler than itself by any physical or chemical process.
(b) Compounds : Compounds are pure substances composed of two or more elements in definite proportion by mass and has a definite set of properties. Compound is made up of only one kind of molecules.
Elements are the fundamental materials of the universe that cannot be broken down by ordinary means. Compounds have entirely different characteristics compared to the elements that formed them.
Teacher's Tip: Remember that elements are made of atoms, while compounds are made of molecules formed from different atoms.
Exam Tip: When defining compounds, make sure to include the phrase "definite proportion by mass" as it is a key technical requirement.

Question 5: Give two examples for each of the following :
(a) Metals
(b) Non-metals
(c) Metalloids
(d) Noble gases

Answer:
(a) Metals : Iron, silver, gold.
(b) Non-metals : Carbon, sulphur, oxygen.
(c) Metalloids : Antimony, silicon, boron.
(d) Noble gases : Helium, argon, neon.
These categories help scientists organize elements based on shared physical and chemical characteristics. Metals are generally shiny and conductive, while noble gases are known for being very stable and unreactive.
Teacher's Tip: Use the Periodic Table as a map; metals are on the left, non-metals on the right, and noble gases on the far right column.
Exam Tip: Always provide three examples if the question asks for two, just in case one is incorrect.

Question 6: Name the elements which form water. How will you justify that water is a compound ?

Answer:
The elements which form water are (i) Hydrogen and oxygen.
Justification : Water has entirely different properties (i.e. is a liquid, extinguishes fire) from the elements it is made up of i.e. Hydrogen a gas catches fire oxygen a gas supporter of combustion.
1. Energy is needed to form water on combining O₂ with H₂.
2. We can not seperate the constituents of water by simple physical means.
Water is a classic example of a compound because its components lose their original identities to create a new substance. This process involves a chemical change rather than just a physical mixing.
Teacher's Tip: Remember that H₂ pops and O₂ burns, but H₂O puts out fire—that is the best way to see the change.
Exam Tip: In your justification, mention that constituents cannot be separated by physical methods like filtration.

Question 7: Give three differences between metals and non-metals.

Answer:
Metals
1. Metals are ductile i.e. can be drawn into wires.
2. Metals are malleable i.e. can be beaten to form sheets.
3. They are sonorous.
Non-metals
1. Non-metals are mostly soft solids cannot be drawn into wires.
2. They are mostly gases and are not malleable.
3. They donot produce sound when struck.
Metals possess unique physical traits like being able to be stretched into thin wires or hammered into foil. Non-metals are much more varied in their states and generally lack the strength and shine of metals.
Teacher's Tip: Think of a gold ring (malleable/lustrous) versus a piece of coal (brittle/dull) to remember the differences.
Exam Tip: Presenting differences in a T-table format is the most effective way to score full marks.

Question 8: How is sodium chloride different from its constituent elements, sodium and chlorine ?

Answer:
Sodium is a metal that is stored in kerosene oil as it reacts very fast with air and water. Chlorine is a reactive greenish yellow gas which is poisonous. When these two elements combine chemically they form common salt sodium chloride which is non poisonous colourless solid substance that we use in our food to add taste and to obtain some nutrition.
This transformation shows how dangerous elements can become safe and useful when they bond chemically. The resulting compound has a totally unique set of properties compared to its parents.
Teacher's Tip: Sodium is a soft metal you can cut with a knife, but salt is a hard crystal we eat every day.
Exam Tip: Be sure to highlight the change from "poisonous" chlorine to "safe" salt to emphasize the chemical change.

Question 9: State four important characteristics of compounds.

Answer:
1. When compound is formed energy like heat, light or electricity is either needed or produced.
2. A compound has properties entirely different from the properties of its constituents.
3. Change in weight takes place.
4. It cannot be separated into its constituents by simple physical means.
Compounds are the result of chemical reactions that bind atoms together in specific, fixed ratios. Because of this bond, you need chemical methods to pull the original elements back apart.
Teacher's Tip: Use the abbreviation "P-E-S-I" (Properties, Energy, Separation, Ingredients) to remember these four points.
Exam Tip: Mentioning that energy (like heat or light) is involved is crucial for a complete definition of a compound formation.

Question 10: Give two examples for each of the following :
(a) Non-metals which are solids
(b) Metals which are soft
(c) Non-metals which are lustrous
(d) Elements which are liquids.
(e) Inert gases
(f) Metalloids

Answer:
(a) Phosphorus, Sulphur
(b) Lead and Sodium
(c) Radium, Graphite
(d) Mercury, Bromine
(e) Helium, Neon
(f) Antimony, Arsenic
These specific examples highlight the exceptions to general rules, such as metals that are soft or non-metals that shine. Understanding these outliers is just as important as knowing the general properties of matter.
Teacher's Tip: Mercury is the only metal that is liquid at room temperature—remember it for almost every science quiz.
Exam Tip: For part (c), if you forget Radium, Iodine is another excellent example of a lustrous non-metal.

Question 11: Name the elements present
(a) Sugar
(b) Ammonia
(c) Marble
(d) Washing soda

Answer:
Compounds
(a) Sugar
(b) Ammonia
(c) Marble
(d) Washing soda
Elements present
(a) Carbon, hydrogen & oxygen
(b) Nitrogen and hydrogen
(c) Calcium, carbon & oxygen
(d) Sodium, carbon & oxygen
Every common household substance is actually a compound made of specific chemical elements. Knowing these elements helps us understand the chemical makeup of our daily environment.
Teacher's Tip: Most "white" household powders like sugar and soda contain Carbon and Oxygen as their base.

Question 12: What is the proportion of elements present in the following compounds?
(a) H₂O (b) CO₂ (c) CaO (d) NO₂

Answer:
Compounds | Elements | Proportion of elements
(a) H₂O | H:O | 1:8
(b) CO₂ | C:O | 3:8
(c) CaO | Ca:O | 5:2
(d) NO₂ | N:O | 7:16
Proportions in compounds refer to the constant ratio by mass in which elements combine. This law of constant proportions ensures that every molecule of a substance is identical to every other one.
Teacher's Tip: These ratios are calculated using the atomic masses of the elements involved.
Exam Tip: Always write the ratio in its simplest whole-number form to avoid losing points.

Question 13: Name two compounds which dissolve in water.

Answer:
Two compounds which dissolve in water are sugar, table salt.
Solubility is a physical property where a compound breaks down into its individual molecules when mixed with water. Both salt and sugar are excellent examples because their molecules are attracted to water molecules.
Teacher's Tip: Table salt is scientifically known as Sodium Chloride (NaCl).
Exam Tip: Use scientific names like 'Sodium Chloride' along with common names like 'Table Salt' for a better impression.

EXERCISE-II
(ATOMS & MOLECULES)

Question 1: Define:
(a) Atom
(b) Molecule
(c) Atomicity
(d) Formula

Answer:
(a) Atom : An atom is the smallest indivisible unit of an element which exhibits all the properties of that element and may or may not have independent existence.
(b) Molecule : A molecule can be defined as the smallest unit of an element or a compound which exhibits all the properties of that element or compound and has independent existence. They are divisible into atoms.
(c) Atomicity : The number of atoms in a molecule of an element is called its atomicity.
(d) Formula : Formula is a short way of representing the molecule of an element or a compound.
These four terms are the basic language of chemistry that describe how matter is built. They help us differentiate between single particles and groups of particles that act as a unit.
Teacher's Tip: An atom is like a letter, a molecule is like a word, and a formula is how you write that word.
Exam Tip: When defining atomicity, always give an example like "Oxygen (O₂) has an atomicity of 2" to show you understand the concept.

Question 2: Why are symbols and formulae of substances important?

Answer:
Importance of symbols and Formulae :
Symbols and formulae of substance gives a lot of information like.
1. Types of elements present in the compound. E.g. (H₂O is made of two elements hydrogen and oxygen).
2. Number of each kind of atoms in one molecule. E.g. (water has 2 atoms of hydrogen combined with 1 atom of oxygen.)
3. Mass of one molecule of the compound. E.g. [H₂O has mass (1 x 2) + 16 = 18 g].
Using symbols acts as a universal language for scientists all over the world, regardless of their native tongue. It simplifies the study of complex chemical reactions by making them much shorter to write.
Teacher's Tip: Imagine trying to write a long paragraph every time you wanted to describe a cup of water—symbols save time!
Exam Tip: List at least three distinct points of importance to ensure you get full credit on a long-answer question.

Question 3: Mention three gaseous elements and write their molecular formulae.

Answer:
Three gaseous elements | molecular formula | Atoms in one molecule
Hydrogen | H₂ | 2
Oxygen | O₂ | 2
Chlorine | Cl₂ | 2
Gaseous elements often exist as diatomic molecules, meaning two atoms are bonded together. This makes the elements more stable when they are in their natural state in our atmosphere.
Teacher's Tip: Most gases you breathe (except noble gases) like to travel in pairs—think of them as "buddy" atoms.
Exam Tip: Don't forget the subscript '2' in H₂ and O₂; writing just 'O' represents an atom, not the gas molecule.

Question 4: State the informations obtained from the formula of a compound.

Answer:
A formula gives us the following information about a compound.
1. Types of elements present in the compound.
2. Number of each kind of atoms in one molecule of the compound.
3. Mass of one molecule of the compound.
Example:
A molecule of carbon dioxide gas is represented by CO₂ It indicates that a carbon dioxide molecule is formed by the combination of two elements i.e. carbon and oxygen. The number of carbon atom is one and that of oxygen atom is two. The mass of one molecule of carbon dioxide can be calculated by adding the mass of one atom of carbon and two atoms of oxygen.
A chemical formula is like a recipe that tells you exactly what goes into making a specific substance. It provides a quick summary of both the identity and the quantity of the components involved.
Teacher's Tip: Look at the small numbers (subscripts) in a formula; they tell you exactly how many of each atom you have.
Exam Tip: Always provide a concrete example like H₂O or CO₂ when explaining what information a formula provides.

Question 5: What is meant by
(a) 2H and H₂
(b) H₂O and 3H₂O ?

Answer:
(a) 2H is two atoms of hydrogen. H₂ is one molecule of hydrogen gas.
(b) H₂O represents one molecule of water. 3H₂O represents 3 molecules of water.
The number in front (coefficient) tells you the total count of independent pieces, while the small subscript tells you how atoms are glued together. This distinction is vital for balancing equations in later grades.
Teacher's Tip: Think of 2H as two separate LEGO bricks and H₂ as two bricks snapped together.
Exam Tip: Be very clear in your language—use the word "atom" for coefficients and "molecule" for subscripts.

Question 6: State the number of atoms of each kind, present in
(a) C₆H₁₂O₆
(b) H₂SO₄
(c) HNO₃
(d) CaCO₃
Also name these compounds.

Answer:
(a) C₆H₁₂O₆ has atoms of
Carbon: 6 atoms in number
Hydrogen: 12 atoms in number
Oxygen: 6 atoms in number
The name of the compound is Glucose.
(b) H₂SO₄
Hydrogen: 2 atoms in number
Sulphur: 1 atom in number
Oxygen: 4 atoms in number
The name of the compound is Sulphuric acid.
(c) HNO₃
Hydrogen: 1 atom in number
Nitrogen: 1 atom in number
Oxygen: 3 atoms in number
The name of the compound is Nitric acid.
(d) CaCO₃
Calcium: 1 atom in number
Carbon: 1 atom in number
Oxygen: 3 atoms in number
The name of the compound is Calcium Carbonate.
Counting atoms involves looking at the subscript following each element's symbol in the formula. If there is no number, it is understood that only one atom of that element is present.
Teacher's Tip: Practice naming these compounds; they are the most common ones you will see throughout middle school science.
Exam Tip: List each element on a new line and state the exact number of atoms clearly to make your answer easy to grade.

Question 7: Write the molecular formulae of compounds calcium oxide, hydrogen sulphide, carbon monoxide and lead sulphide.

Answer:
Compound Calcium oxide is formed of elements calcium (Ca) and oxygen (O)
Formula = CaO
Compound Hydrogen sulphide is formed of elements hydrogen (H), sulphide (S)
Formula is H₂S
Compound Carbon monoxide is compound of elements carbon (C) and oxygen (O)
Formula of carbon monoxide is CO
Formula of lead sulphide is PbS
Molecular formulas are written by balancing the combining powers, or valencies, of different atoms. This ensures that the compound formed is electrically neutral and stable.
Teacher's Tip: Use the "criss-cross" method for valency to find the correct number of atoms for each element.
Exam Tip: Always write the symbol of the metal (positive part) first and the non-metal (negative part) second.

EXERCISE-III

Question 1: Name:
(a) Three different forms of carbon.
(b) A form of carbon used as a gem.
(c) Two substances used to make electric wires.
(d) Two substances used to make jewellery.
(e) A substances used as an insulator.

Answer:
(a) 1. Diamond 2. Graphite 3. Coal
(b) Diamond is used as gem.
(c) 1. Copper 2. Aluminium as these are good conductors of electricity.
(d) 1. Gold 2. Silver as these are shining, lustrous, and ductile.
(e) Plastic is used as insulator as it is bad conductor of electricity.
These materials are chosen for specific tasks based on their inherent physical properties like conductivity or hardness. For instance, carbon can be as soft as graphite or as hard as diamond, making it incredibly versatile.
Teacher's Tip: Forms of the same element that look different are called 'allotropes'.
Exam Tip: When asked for insulators, you can also name 'Rubber' or 'Wood' if you forget 'Plastic'.

Question 2: Give one use of each of the following substances :
(a) Iron
(b) Brass
(c) Coal

Answer:
(a) Iron : To make machines tools and building material.
(b) Brass : To make water taps and utensils.
(c) Coal: Coal is used as fuel also used in thermal power plant to produce electricity.
The practical application of materials depends on their strength, durability, and cost. Iron's strength makes it perfect for construction, while coal's ability to burn makes it a primary source of energy.
Teacher's Tip: Iron is the backbone of the construction industry because it is both strong and abundant.
Exam Tip: Mention specific uses like "thermal power plants" for coal to show a higher level of understanding.

Question 3: Give reason:
(a) A frying pan is made up of steel but its handle is made up of wood.
(b) Graphite is used to make lead of the pencils.
(c) Argon is filled in electric bulbs.

Answer:
(a) Steel is good conductor of heat to cook food, pan is made of steel where as wood is insulator of heat and to hold, handle is made up of wood.
(b) Graphite leaves mark on the paper and makes it black.
(c) Argon is inert gas and protects the element of bulb from oxidation and burning. Hence increases bulb’s life.
Giving scientific reasons involves connecting the physical property of a material to its functional purpose. If we used metal for pan handles, we would burn our hands, so we utilize the insulating property of wood instead.
Teacher's Tip: "Inert" means it doesn't like to react with anything—that is why Argon is so safe inside a hot bulb.
Exam Tip: When answering "Give reason" questions, use terms like "conductor," "insulator," and "inert" for more professional answers.

Question 4: Answer the following questions:
(a) Why are copper and aluminium used to make electric wires?
(b) What do you understand by the statement: ‘metals are ductile and malleable’?
(c) Give the advantages of using symbols instead of names of elements or compounds.
(d) When iron is mixed with sulphur at room temperature, it does not form a compound. Why?
(e) Find the atomicity of the Following molecules:
1. calcium chloride
2. ammonium sulphide
3. acetic acid
4. dinitrogen oxide
5. carbon monoxide

Answer:
(a) Copper and aluminium are good conductors of heat and electricity. They can be drawn into wires and beaten into sheets. Therefore, they are used to make electric wires.
(b) Metals are ductile, i.e., they can be drawn or stretched into thin wires. They are malleable, i.e., they can be beaten into thin sheets.
(c) 1. Symbols increases scientific communication across the world. 2. Symbols helps to make equations and data shorter and concise. 3. Symbols are helpful for Scientists as it would take time and paper to do their job.
(d) When iron is mixed with sulphur at room temperature, it does not form a compound because the mixture of iron and sulphur requires heat to form a compound i.e. iron sulphide.
(e) Atomicity of
(i) Calcium chloride – CaCl₂ = (1 x 1) + (2 x 1) = 1 + 2 = 3
(ii) Ammonium sulphide – (NH₄)₂S = (1 + 4) x 2 + 1 x 1 = 2 + 8 + 1 = 11
(iii) Acetic acid – CH₃COOH = 1 + 1 x 3 + 1 + 1 + 1 + 1 = 5 + 3 = 8
(iv) Dinitrogen oxide – N₂O = 1 x 2 + 1 = 2 + 1 = 3
(v) Carbon monoxide – CO = 1 x 1 + 1 x 1 = 1 + 1 = 2
These answers explore the physical nature and symbolic representation of matter in detail. From the math of atomicity to the practical choice of wire materials, chemistry explains how the world is built.
Teacher's Tip: For part (d), remember that energy (heat) is usually the "trigger" for a chemical reaction to start.
Exam Tip: Practice the atomicity calculation for complex molecules like (NH₄)₂S by carefully multiplying everything inside the bracket by the subscript outside.

OBJECTIVE TYPE QUESTIONS

1. Fill in the blanks

(a) Atomicity refers to the number of atoms in the molecule of an element.
(b) The most abundant element in the earth’s crust is oxygen.
(c) A metal which is a liquid at room temperature is mercury.
(d) The most abundant element in the atmosphere is nitrogen.
(e) A metal which is a poor conductor of electricity is tungsten.
(f) A diatomic gaseous element is oxygen.
(g) A liquid non-metal is bromine.

2. Match the columns
Column A
(a) Metals
(b) Molecules
(c) Non-metals
(d) Noble gases
Column B
(i) Non-reactive
(ii) Brittle
(iii) Lustrous
(iv) Smallest unit of compound
Answer:
(a) Metals | (iii) Lustrous
(b) Molecules | (iv) Smallest unit of compound
(c) Non-metals | (ii) Brittle
(d) Noble gases | (i) Non-reactive

3. Indicate whether the following statements are true or false.

(a) A compound is made up of just one kind of atom.
Answer: False. Correct : A compound is made up of two or more elements is a fixed proportion by mass.

(b) Metals reflect light and are good conductors of electricity.
Answer: True.

(c) Metals can be polished.
Answer: True.

(d) Elements are made up of compounds.
Answer: False. Correct : Elements are made up of atoms.

(e) All elements are artificially prepared.
Answer: False. Correct: All elements are made up of a limited number of basic substances.

(f) Molecules can exist independently.
Answer: True.

(g) Molecules combine to form atoms.
Answer: False. Correct : Atoms combine to form molecule.

(h) Noble gases are highly reactive.
Answer: False. Correct: Noble gases are non-reactive.

(i) Ozone is a triatomic molecule.
Answer: True.

MULTIPLE CHOICE QUESTIONS

1. All pure substances have
1. the same physical state .
2. the same colour
3. the same composition
4. a definite set of properties
Answer: a definite set of properties

2. Sugar is a compound which consists of the elements
1. carbon and hydrogen
2. hydrogen and oxygen
3. carbon, hydrogen and oxygen
4. hydrogen, carbon and sulphur
Answer: carbon, hydrogen and oxygen

3. Atoms of different kinds combine to form molecules of
1. an element
2. a compound
3. a mixture
4. all of the above
Answer: a compound

4. Sulphur and carbon are
1. metals
2. non-metals
3. metalloids
4. noble gases
Answer: non-metals

5. Gold is used to make jewellery because
1. it is dull
2. lustrous and attractive
3. highly reactive
4. very cheap
Answer: lustrous and attractive

6. The most abundant elements in the universe are
1. neon and argon
2. hydrogen and helium
3. aluminium and copper
4. oxygen and nitrogen
Answer: hydrogen and helium

7. The compound used as common salt is
1. sodium chloride
2. calcium chloride
3. sodium oxide
4. hydrogen chloride
Answer: sodium chloride

8. Brass and bronze are
1. elements
2. mixtures
3. compounds
4. all of the above
Answer: mixtures

9. Sand is a compound of
1. silicon and nitrogen
2. silicon and oxygen
3. oxygen and sulphur
4. none of the above
Answer: silicon and oxygen

10. From the list given below select the correct substance which is most suitable to the statements given : (oxygen, diamond, zinc, graphite, gold)
1. A metal which is brittle. — Zinc
2. A non-metal which is a good conductor of electricity. — Graphite
3. The hardest naturally occurring substance. — Diamond
4. The most ductile metal. — Gold
5. A gaseous non-metal. — Oxygen

ADDITIONAL QUESTIONS FOR PRACTICE

Elements and Compounds

Question 1: Classify substances into pure and impure substances in the form of a chart or tabulation.

Answer:
SUBSTANCES
--- PURE SUBSTANCES
--- ELEMENTS (Metals: Iron, Non-Metals: Sulphur, Metalloids: Silicon, Noble Gases: Helium)
--- COMPOUNDS (Iron Sulphide)
--- IMPURE SUBSTANCES
--- MIXTURES
--- HOMOGENEOUS (Air)
--- HETEROGENEOUS (Oil & Water)
This chart organizes matter into logical categories based on purity and how components are combined. It shows that everything in the world fits into either a pure or an impure category.
Teacher's Tip: Use this chart as your master map for this whole chapter; it connects every term you have learned.
Exam Tip: Drawing a clean chart with arrows is much better than writing a long paragraph for classification questions.

Question 2: Differentiate between the terms – elements, compounds & mixtures.

Answer:
Element is a pure substance.
1. It is the basic unit of matter and cannot be broken down into two or more simpler substances by any means.
2. It is mainly classified into metals, non-metals, metalloids and noble gases.
Compound is a pure substance.
1. It is formed by combination of two or more elements.
2. The elements are combined together in a fixed ratio.
3. It can be broken down into its elements by chemical means.
Mixture is an impure substance.
1. It is formed by combination of two or more elements, compounds or both.
2. The substances are mechanically mixed together in any ratio.
The core difference lies in how substances are bonded and how easily they can be taken apart. While compounds are chemically bonded, mixtures are just tossed together like a salad.
Teacher's Tip: A compound is like a cake (ingredients are changed forever), while a mixture is like a fruit salad (you can still see the pieces).
Exam Tip: Always provide a point-by-point comparison when asked to differentiate between three different terms.

Question 3: The important physical properties of substances are colour, odour, nature, density & solubility in water. Name –
(a) two coloured gases (with their colours)
(b) a gas with a pungent, choking odour which is lighter than air
(c) a poisonous gas almost as heavy as air.

Answer:
(a) (i) Chlorine – greenish yellow ,(ii) Nitrogen dioxide – reddish brown.
(b) Ammonia.
(c) Carbon monoxide
Identifying gases based on their smell and color is a fundamental skill in a chemistry lab. These traits act as "fingerprints" for elements and compounds that we might not be able to see clearly otherwise.
Teacher's Tip: Never sniff a gas directly; always "waft" the air toward your nose gently with your hand.
Exam Tip: Ensure you provide the color name clearly next to the gas name for part (a) to get both points.

Question 4: Complete the statement – an element is a pure substance made up of (identical/different) atoms.

Answer: An element is a pure substance made up of identical atoms.
This means that every single atom in a piece of pure gold is exactly like every other atom in that same piece. This uniformity is what makes an element distinct from a mixture or a compound.
Teacher's Tip: If you find even one different atom, it's no longer a pure element; it's an impurity.
Exam Tip: "Identical" is the key word here—never use "similar" or "same-looking."

Question 5: Draw a labelled diagram of an atom including its nucleus, orbits & their contents.

Answer:
[Diagram description: A central Nucleus containing 9p (Protons) and 10n (Neutrons). Surrounding the nucleus are circular Orbits containing Electrons.]
The structure of an atom consists of a heavy core and a mostly empty space where tiny electrons zip around. Protons and neutrons stay locked in the center, determining the element's identity.
Teacher's Tip: Protons are Positive, Neutrons are Neutral, and Electrons are negative.
Exam Tip: Always use a compass or a circular object to draw neat orbits; messy circles can lose you marks.

Question 6: Elements are classified into – Metals – Non-metals – Metalloids – Noble gases.
State which of A, B, C, D is a –
1. Metallic element
2. Non-metallic element
3. Metalloid
4. Noble gas.
A : Is non-malleable, non-ductile & a poor conductor of electricity
B : Has lustre, is malleable and ductile & a good conductor of heat
C : Is unreactive and inert and present in traces in air
D : Shows properties of both metals and non-metals.

Answer:
A : Is non-malleable, non-ductile & a poor conductor of electricity — Non-metallic element
B : Has lustre, is malleable and ductile & a good conductor of heat — Metallic element
C : Is unreactive and inert and present in traces in air — Noble gas.
D : Shows properties of both metals and non-metals — Metalloid
Each of these categories represents a different group on the Periodic Table with unique ways of behaving. This classification system helps chemists predict how elements will react when mixed.
Teacher's Tip: Lustre (shine) and Malleability are the two biggest clues that you are looking at a metal.
Exam Tip: Match the description letters exactly as given in the question to avoid confusion.

Question 7: An atom of an element is denoted by a “symbol” Explain the meaning of the term ‘symbol’. State a reason for representing the following elements by their symbols.
(a) Hydrogen by ‘H’
(b) Helium by ‘He’,
(c) Copper by ‘Cu’ .

Answer:
Symbols : The short form or abbreviated name of the element (or radicals).
The reason for representing the following elements by their symbols are as follow :
(a) Hydrogen by ‘H’ – First letter of the name of element
(b) Helium by He – First two letters of the name of Noble gases.
(c) Copper by ‘Cu’ – Deriving symbols from their Latin names
Symbols serve as a shorthand that is recognized globally by all scientists. They are assigned based on a standard set of rules to prevent any confusion between different elements.
Teacher's Tip: If you see a symbol that doesn't match the English name, like Cu or Fe, it's almost always from Latin.
Exam Tip: Make sure to explicitly state "First letter" or "Latin name" as the specific reason to get full points.

Question 8: Match the metallic elements – with their correct symbols.
(a) Potassium (b) Sodium (c) Calcium (d) Magnesium (e) Zinc (f) Aluminium (g) Iron (h) Lead (i) Copper (j)Mercury (k) Silver (l) Platinum (m) Gold

Answer:
Metallic element | Symbol
a. Potassium | K
b. Sodium | Na
c. Calcium | Ca
d. Magnesium | Mg
e. Zinc | Zn
f. Aluminium | Al
g. Iron | Fe
h. Lead | Pb
i. Copper | Cu
j. Mercury | Hg
k. Silver | Ag
l. Platinum | Pt
m. Gold | Au
Learning these symbols is the very first step toward understanding chemical language. Many of these elements are used in our everyday lives, from the sodium in our salt to the iron in our buildings.
Teacher's Tip: Potassium is K because its Latin name is 'Kalium'—try to remember that specific one!
Exam Tip: Practice writing these symbols daily; many students accidentally write S for Sodium instead of Na.

Question 9: Match the non-metallic elements – with their correct symbols.
(a) Carbon (b) Chlorine (c) Oxygen (d) Phosphorus (e) Hydrogen (f) Nitrogen (g) Iodine (h) Bromine (i)Fluorine (j) Silicon (k) Sulphur

Answer:
Non-Metallic | Symbol
a. Carbon | C
b. Chlorine | Cl
c. Oxygen | O
d. Phosphorus | P
e. Hydrogen | H
f. Nitrogen | N
g. Iodine | I
h. Bromine | Br
i. Fluorine | F
j. Silicon | Si
k. Sulphur | S
Non-metals are essential components of air, water, and organic life. Their symbols are mostly the first letter of their English name, making them easier to memorize than some metals.
Teacher's Tip: Phosphorus is P and Potassium is K; don't get these two "P" elements mixed up!
Exam Tip: Always double-check if a symbol is one letter (O) or two letters (Si) to be precise.

Question 10: Match the noble gases – with their correct symbols
(a) Helium (b) Neon (c) Argon (d) Krypton (e) Xenon (f) Radon

Answer:
Noble gas | Symbols
(a) Helium | He
(b) Neon | Ne
(c) Argon | Ar
(d) Krypton | Kr
(e) Xenon | Xe
(f) Radon | Rn
Noble gases are unique because they don't naturally want to combine with other atoms. Their symbols are used to denote their presence as stable, independent atoms in the atmosphere.
Teacher's Tip: Noble gases are the "royalty" of the elements; they are so stable they don't need to bond with anyone else.
Exam Tip: Note that all noble gas symbols consist of two letters where the second letter is always lowercase.

Question 11: Give a reason why elements are tabulated in a table called the ‘Periodic table’.

Answer:
For arranging all the elements in a systematic and simple manner. The arrangement of elements was done in the form of a table called Periodic Table in which elements are arranged in increasing order of their atomic numbers. Elements in the Periodic Table are arranged in Horizontal rows called Periods and vertical columns called Groups. Metallic elements are placed on the left non-metallic on the right and noble gases on the extreme right of the Periodic table.
The Periodic Table is much more than a list; it is a map that predicts how an element will behave. Its structure allows scientists to find patterns in reactivity and size across different groups.
Teacher's Tip: Think of the Periodic Table as a library where books (elements) are sorted by genre (properties) and author (atomic number).
Exam Tip: Use keywords like "Periods," "Groups," and "Atomic Number" to show you understand the table's layout.

Question 12: Give the names and symbols of the first twenty elements of the periodic table. Differentiate them into metals, non-metals, metalloids and noble gases.

Answer:
Metals: Lithium-Li, Beryllium-Be, Sodium-Na, Magnesium-Mg, Aluminium-Al, Potassium-K, Calcium-Ca, Scandium-Sc, Titanium-Ti
Metalloids: Boron-B, Silicon-Si
Non-metals: Carbon-C, Nitrogen-N, Oxygen-O, Fluorine-F, Phosphorus-P, Sulphur-S, Chlorine-Cl
Noble gases: Helium-He, Neon-Ne, Argon-Ar
This grouping helps us see the diversity of elements found in nature and how they transition from metals to gases. Memorizing the first twenty elements is a standard milestone for any beginner chemistry student.
Teacher's Tip: Try a mnemonic song or rhyme to help you remember the first twenty elements in order.
Exam Tip: When asked for the first twenty, make sure you group them by type as shown to demonstrate your classification skills.

Question 13: Explain the term – molecules. Give three examples of atoms of the same element forming a molecule. State the atomicity of the same.

Answer:
Atoms of the same element or different elements combine to form a molecule.
Atoms of the same element forming a molecule 1. Oxygen 2. Nitrogen 3. Hydrogen.
Atomicity is the number of atoms present in one molecule of the element.
Element : Hydrogen H, Nitrogen N, Oxygen O
Molecule : H₂, N₂, O₂
Atomicity : 2, 2, 2
A molecule is the smallest piece of a substance that can exist on its own while keeping all its properties. When two atoms of the same type stick together, they form a simple diatomic molecule.
Teacher's Tip: Most common gaseous elements in the air come in pairs, which is why their atomicity is 2.
Exam Tip: Don't forget to distinguish between an atom (just 'H') and a molecule ('H₂') in your explanation.

Question 14: Give one example of (a) a triatomic molecule (b) a polyatomic molecule.

Answer:
(a) A Triatomic molecule
Element : Ozone
Molecule : O₃
Atomicity : 3
(b) A polyatomic molecule
Element : Phosphorus
Molecule : P₄
Atomicity : 4
Atomicity tells us exactly how many individual atoms are bonded together to form one stable unit of an element. While many gases are diatomic, others like ozone and phosphorus have more complex structures.
Teacher's Tip: "Tri" means three and "Poly" means many—easy prefixes to remember!
Exam Tip: S_8 (Sulphur) is another great example of a polyatomic molecule with an atomicity of 8.

Question 15: Explain the term compounds. Give the example of a compound containing
(a) hydrogen and oxygen
(b) carbon and oxygen
(c) nitrogen and oxygen
(d) calcium and oxygen.

Answer:
A compound is a pure substance made up of two or more different elements combined chemically in a fixed proportion.
Example of Atoms of different elements forming a compound
(a) 2 Atom of H and 1 atom of O = H₂O
(b) Carbon dioxide, 1 atom of carbon and 2 atom of O = CO₂
(c) Nitrogen dioxide, 1 atom of nitrogen 2 atom of O = NO₂
(d) Calcium oxide, 1 atom of calcium 1 atom of O = CaO
Compounds are entirely new substances created when atoms of different elements "click" together chemically. This combination is always in a specific ratio, which defines exactly what the compound is.
Teacher's Tip: If you change the ratio of atoms, you change the compound entirely—like H₂O is water, but H₂O₂ is peroxide!
Exam Tip: Always provide both the element names and the final chemical formula for a complete answer.

Question 16: State two characteristics of water which prove that it is a – compound.

Answer:
Characteristics of compounds
1. Elements in a compound are present in a definite proportion. Example 2 atoms of hydrogen combine with 1 atom of oxygen to give 1 molecule of water (compound) H₂ + O = H₂O (water)
2. Compounds have a definite set of properties example : The properties of the compound water are different from the properties of the elements hydrogen and oxygen in water.
These characteristics distinguish water from a simple mixture of hydrogen and oxygen gases. Because it has a fixed recipe and new properties, it must be classified as a chemical compound.
Teacher's Tip: A mixture of H and O gas would explode, but the compound water (H₂O) puts out fire—that's proof of a new set of properties!
Exam Tip: Focus on "fixed ratio" and "different properties" as your two main points to satisfy the examiner.

Question 17: Explain the terms ‘chemical formula’. State what a chemical formula denotes.

Answer:
Chemical formula is a representation of a substance either element or compound by means of symbols.
Chemical formula denotes the number of atoms of each element present in a compound.
Think of a chemical formula as a scientific abbreviation that tells a whole story in just a few letters. It identifies exactly which "building blocks" were used and how many of each are in the molecule.
Teacher's Tip: The subscript (small number) only applies to the element immediately before it.
Exam Tip: Use a clear example like H₂SO₄ to show how a formula denotes different atoms (2 Hydrogen, 1 Sulphur, 4 Oxygen).

Question 18: Give the symbols and the number of atoms of each element present in
(a) sodium chloride
(b) water
(c) carbon dioxide
(d) zinc chloride.

Answer:
(a) Sodium chloride
Elements: Sodium, Chlorine (Chloride) Symbols: Na, Cl
No. of atoms of each element present: 1 atom of Na, 1 atom of Cl
(b) Water (H₂O)
Elements: Hydrogen, Oxygen (oxide) Symbols: H, O
No. of atoms of each element present: 2 atoms of H, 1 atom of O
(c) carbon dioxide CO₂
Elements: Carbon, Oxygen (oxide) Symbols: C, O
No. of atoms of each element present: 1 atom of C, 2 atoms of O
(d) zinc chloride ZnCl₂
Elements: Zinc, Chlorine (chloride) Symbols: Zn, Cl
No. of atoms of each element present: 1 atom of Zn, 2 atoms of Cl
Breaking down compounds into their constituent atoms is a key part of chemical analysis. This step-by-step breakdown makes it clear exactly what a chemical formula is communicating.
Teacher's Tip: When an element name ends in "ide" (like chloride or oxide), it just means it is the non-metal part of a compound.
Exam Tip: Formatting your answer with "Elements," "Symbols," and "No. of Atoms" labeled clearly will help you get full marks.

Question 19: For writing a chemical formula – ‘symbols’ and combining capacity of an element with hydrogen i.e. ‘valency’ should be known. Explain the term – combining capacity of an element i.e. valency.

Answer:
The combining power of an element especially as measured by the number of hydrogen atoms can combine or displace with the element.
Valency is basically the "rulebook" that tells us how many bonds an atom can form. It determines the specific ratio in which elements will join together to create a stable compound.
Teacher's Tip: Think of valency as the number of "arms" an atom has to hold onto other atoms.
Exam Tip: Mentioning "hydrogen atoms" as the standard for measuring valency is the traditional and correct way to define it.

Question 20: State what are radicals. Give the names of the radicals –
(a) NO₃
(b) OH
(c) SO₄
(d) CO₃

Answer:
A radical is an atom or group of atom of same or different elements that behaves in the manner of positive or negative ions.
(a) NO₃ – nitrate
(b) OH – hydroxide
(c) SO₄ – sulphate
(d) CO₃ – carbonate
Radicals are special groups of atoms that like to stick together and act as a single unit during a chemical reaction. They often carry a charge, which allows them to bond with metals to form salts.
Teacher's Tip: Think of a radical as a "squad" of atoms that moves and reacts together as a team.
Exam Tip: Memorize the names and formulas of these four radicals perfectly, as they appear in many compound names.

Question 21: Match the symbols of metallic elements – with their correct combining power or capacity.

Answer:
(a) K — Valency-1 (C)
(b) Zn — Valency-2 (B)
(c) Al — Valency-3 (A)
(d) Na — Valency-1 (C)
(e) Ca — Valency-2 (B)
Valency helps us understand why some compounds have more of one element than another. For example, since Ca has a valency of 2 and Cl has 1, it takes two Cl atoms to satisfy one Ca atom.
Teacher's Tip: Most metals in group 1 (Na, K) always have a valency of 1.
Exam Tip: Grouping elements by their valency (like K and Na both being 1) is a smart way to memorize them.

Question 22: Match the symbols of non-metallic elements with their correct combining power or capacity.

Answer:
(a) O — Valency-2 (B)
(b) S — Valency-2 (B)
(c) Cl — Valency-1 (C)
Non-metals have negative valencies because they tend to "take" electrons during a chemical reaction. This makes them the perfect partners for metals, which like to "give" electrons.
Teacher's Tip: Oxygen almost always has a valency of 2 in school-level chemistry.
Exam Tip: When matching, look for elements in the same family (like Oxygen and Sulphur) because they usually share the same valency.

Question 23: Match the symbols of – radicals – with their correct combining power or capacity, (valency).

Answer:
(a) OH — Valency- 1 (C)
(b) SO₄ — Valency- 2 (B)
(c) NO₃ — Valency- 1 (C)
(d) CO₃ — Valency- 2 (B)
Radicals have their own specific valency just like single atoms do. This allows them to replace single atoms in chemical formulas without breaking their "group" structure.
Teacher's Tip: Hydroxide (OH) and Nitrate (NO₃) are your "1-valency" team; Sulphate and Carbonate are your "2-valency" team.
Exam Tip: Don't let the large number of atoms in a radical confuse you; look at the radical as one single block with one valency.

Question 24: Write the formula of the compound formed – given symbols and combining power or capacity (valency) of each element in the compound.
(a) K¹⁺ Cl^1- (b) Na¹⁺ Cl^1- (c) Ca²⁺ NO₃^1-

Answer:
(a) Formula = KCl
(b) Formula = NaCl
(c) Formula = Ca(NO₃)₂
Writing formulas involves the "Criss-Cross" method where the valency of one atom becomes the subscript of the other. This ensures that the total positive and negative charges in the compound perfectly cancel each other out.
Teacher's Tip: If you have more than one of a radical (like NO₃), you must put the whole radical in brackets before writing the subscript.
Exam Tip: Always simplify subscripts if they are the same (like K₁Cl₁ becomes KCl) for the final answer.

Question 25: Match the formulas of the following – gases – with their correct names :
Gases- (a) H₂ (b) N₂ (c) O₂ (d) Cl₂ (e) HCl (f) NH₃ (g) CO (h) CO₂ (i) SO₂ (j) NO (k) NO₂

Answer:
(a) H₂ — (4) Hydrogen
(b) N₂ — (5) Nitrogen
(c) O₂ — (3) Oxygen
(d) Cl₂ — (6) Chlorine
(e) HCl — (11) Hydrogen chloride
(f) NH₃ — (1) Ammonia
(g) CO — (7) Carbon monoxide
(h) CO₂ — (10) Carbon dioxide
(i) SO₂ — (8) Sulphur dioxide
(j) NO — (9) Nitrogen monoxide
(k) NO₂ — (2) Nitrogen dioxide
Chemical names often give a hint about the formula, such as "monoxide" meaning one oxygen and "dioxide" meaning two. This systematic naming allows scientists to know the formula just by hearing the name.
Teacher's Tip: "Di" means 2, so whenever you hear "dioxide," look for a small '2' next to the Oxygen.
Exam Tip: Be careful with Ammonia (NH₃); it's a common one to forget since the name doesn't sound like Nitrogen and Hydrogen.

Question 26: Match the formulas of the following – acids – with their correct names
(a) HCl (b) HNO₃ (c) H₂SO₄ (d) H₂CO₃

Answer:
(a) HCl — (3) Hydrochloric acid
(b) HNO₃ — (4) Nitric acid
(c) H₂SO₄ — (2) Sulphuric acid
(d) H₂CO₃ — (1) Carbonic acid
Acids are a specific group of compounds that almost always start with Hydrogen in their chemical formula. They play critical roles in everything from digestion in our stomachs to industrial manufacturing.
Teacher's Tip: If you see 'SO₄' it's Sulphuric, 'CO₃' is Carbonic, and 'NO₃' is Nitric—the radical name tells you the acid name.
Exam Tip: Learn these four common acids by heart; they are the "Big Four" of middle school chemistry.

Question 27: Match the formulas of the following – bases – with their correct names.
(a) NaOH (b) KOH (c) Ca(OH)₂ (d) Zn(OH)₂

Answer:
(1) Potassium hydroxide – (b) KOH
(2) Zinc hydroxide – (d) Zn(OH)₂
(3) Calcium hydroxide – (c) Ca(OH)₂
(4) Sodium hydroxide – (a) NaOH
Bases are the chemical opposites of acids and are usually identified by the presence of the Hydroxide (OH) radical. When a base and an acid meet, they neutralize each other to form salt and water.
Teacher's Tip: If a formula ends in 'OH', it's almost certainly a hydroxide base.
Exam Tip: Pay attention to the brackets in Ca(OH)₂; they indicate that the '2' applies to the whole hydroxide group.

Question 28: Complete the statements with the correct words.

Answer:
(a) Acids – are chemicals which are sour in taste and derived from plants and minerals.
(b) Bases – are chemicals which are hydroxide (or oxides) of metals eg. sodium hydroxide.
(c) Salts – are chemicals formed on reaction of a base with an acid giving salt and water.
These three categories—acids, bases, and salts—cover a huge portion of chemistry. Understanding their taste, origin, and how they interact is the foundation for studying chemical reactions.
Teacher's Tip: Remember the "ABC" of reactions: Acid + Base = Salt (and water).
Exam Tip: Use the word "neutralization" when describing how salts are formed to score extra points.

Question 29: In the chemical word equation – Zinc + Sulphuric acid -> Zinc sulphate + Hydrogen
(a) State the reactants and products of the above reaction. What does the arrow indicate.
(b) The molecular equation is : Zn + H₂SO₄ -> ZnSO₄ + H₂[g]. State what [g] represents.

Answer:
(a) Reactants: Zinc and Sulphuric acid. Products: Zinc sulphate and Hydrogen. The arrow indicates the direction of the reaction (from reactants to products).
(b) [g] represents gas.
A chemical equation is a shorthand way of showing exactly what happens during a chemical change. It tells you what you started with (reactants) and what you ended with (products).
Teacher's Tip: Reactants are always on the left of the arrow, and products are always on the right.
Exam Tip: Always explain the state symbols like [g] for gas, [l] for liquid, and [s] for solid if asked.

Objective Type Questions: Elements and Compounds

Q.I. Complete the statements given below by filling in the blank with the correct word/s.

1. An element is a pure substance which cannot be broken down by physical or chemical methods.
2. The basic unit of an element is an atom.
3. Atom contains nucleus, with positively charged protons.
4. Element silver has the symbol derived from its Latin name ‘argentum’.
5. From the elements – He, Br, Pt and O; the element which is metallic is Pt, an inert element is He, forms a triatomic molecule is O (as ozone), is liquid at room temperature is Br.
6. From the elements nitrogen, chlorine, bromine, the element present in the atmosphere is nitrogen.

Q.2. Match the statements in List I – 1 correct answers in List II – A to J.
List I
1. Fluorine and chlorine
2. A diatomic molecule
3. A molecule containing the elements hydrogen and oxygen
4. A metalloid - showing properties of both metals and non-metals
5. The term used for the substances which take part in the chemical reaction
6. The term which represents a substance by means of symbols
7. The term used for substances formed as a result of a chemical reaction
8. A compound containing one atom of zinc and two atoms of chlorine
9. The chemical name for nitrogen monoxide
10. A polyatomic molecule
List II
A : Nitrogen
B : Boron
C : Reactants
D : Chemical formula
E : Halogens
F : Water
G : Nitric oxide
H : Products
I : Phosphorus
J : Zinc chloride

Answer:
1. Fluorine and chlorine — E : Halogens
2. A diatomic molecule — A : Nitrogen
3. A molecule containing the elements hydrogen and oxygen — F : Water
4. A metalloid - showing properties of both metals and non-metals — B : Boron
5. The term used for the substances which take part in the chemical reaction — C : Reactants
6. The term which represents a substance by means of symbols — D : Chemical formula
7. The term used for substances formed as a result of a chemical reaction — H : Products
8. A compound containing one atom of zinc and two atoms of chlorine — J : Zinc chloride
9. The chemical name for nitrogen monoxide — G : Nitric oxide
10. A polyatomic molecule — I : Phosphorus
Matching these terms correctly shows that you understand both the names and the technical categories used in chemistry. It connects general descriptions to specific scientific names and roles.
Teacher's Tip: "Halogens" is the specific name for the group of elements that includes Fluorine and Chlorine.
Exam Tip: Draw straight lines or write the corresponding letter clearly to make matching questions easy for the teacher to grade.

Q.3. Select the correct answer from the choice in brackets.

Question 1: The symbol of – mercury [Mg / Hg / Ag]

Answer: Hg
Mercury's symbol is Hg, which comes from its ancient Latin name 'Hydrargyrum'. It is one of the most unique elements because it is a metal that stays liquid at room temperature.
Teacher's Tip: Mg is Magnesium and Ag is Silver; don't get these "M" and "g" symbols mixed up.
Exam Tip: Remember that Hg is the only liquid metal on the list, which helps you identify it quickly.

Question 2: The type of element-phosphorus [metallic / non-metallic / noble gas]

Answer: non-metallic
Phosphorus is a non-metal that is found in nature in several forms, including white and red phosphorus. It is vital for life and is an essential part of DNA and cell structures.
Teacher's Tip: Phosphorus is reactive and solid at room temperature, but it is not shiny or conductive like a metal.
Exam Tip: Phosphorus is located on the right side of the Periodic Table, which is the section for non-metals.

Question 3: The type of molecule- bromine [monoatomic / diatomic / triatomic]

Answer: diatomic
Bromine naturally exists as pairs of atoms, written as Br₂, which makes it a diatomic molecule. It is the only non-metal that is a liquid at room temperature.
Teacher's Tip: Think of "di-" as two, just like a bicycle has two wheels.
Exam Tip: Remember that most halogens (group 17) like bromine are diatomic.

Question 4: A compound [nitrogen / ozone/ zinc chloride]

Answer: zinc chloride
Zinc chloride is a compound because it is made of two different elements—zinc and chlorine—bonded together. Nitrogen and ozone are elements because they contain only one type of atom.
Teacher's Tip: A compound name almost always has two different parts, like "Zinc" and "Chloride."
Exam Tip: If the name contains more than one element name, it's a compound, not an element.

Question 5: The unreactive non-metallic element [chlorine / argon / sulphur]

Answer: argon
Argon is a noble gas, which means its outer electron shell is full and it has no reason to react with other elements. This stability makes it perfect for use in lightbulbs where other gases might burn up.
Teacher's Tip: "Inert" and "Unreactive" are just two ways of saying an element is "lazy" and won't bond.
Exam Tip: All noble gases are unreactive, so look for He, Ne, Ar, Kr, Xe, or Rn.

Question 6: The negatively charged particles in an atom [protons / neutrons / electrons]

Answer: electrons
Electrons are the tiny particles that orbit the nucleus and carry a negative charge. They are the primary particles involved in creating chemical bonds between atoms.
Teacher's Tip: Remember the "E" in Electron stands for "Empty of positive charge" (negative).
Exam Tip: Protons are Positive, Electrons are negative, and Neutrons are neutral—memorize this trio!

Question 7: The element which is malleable [sulphur / carbon / iron]

Answer: iron
Iron is a metal, and one of the defining traits of metals is malleability, or the ability to be hammered into sheets. Sulphur and carbon are non-metals and would crumble into powder if hammered.
Teacher's Tip: If you can make a sword or a car out of it, it's probably malleable.
Exam Tip: "Malleable" is a properties exclusively associated with metallic elements.

Question 8: An impure substance [element / mixture / compound]

Answer: mixture
A mixture is considered impure because its components are not chemically bonded and their ratio can change. Elements and compounds are pure because their chemical makeup is fixed and uniform.
Teacher's Tip: Think of sugar water; you can have a little sugar or a lot—that's a mixture.
Exam Tip: Always classify elements and compounds as pure and mixtures as impure for full marks.

Question 9: An element which is a poor conductor of heat [copper / carbon / aluminum]

Answer: carbon
Carbon is a non-metal, and most non-metals are poor conductors of heat and electricity. Copper and aluminum are metals known for being excellent conductors, which is why they are used in cookware and wiring.
Teacher's Tip: Non-metals act as insulators, meaning they block the flow of heat rather than helping it.
Exam Tip: Copper and Aluminum are the two most famous good conductors—rule them out first.

Question 10: A group of atoms of elements [ion / radical / combining power]

Answer: radical
A radical is a specific cluster of atoms that stays together as a single unit during chemical changes. Examples include Sulphate (SO₄) or Hydroxide (OH).
Teacher's Tip: A radical is a "group" while an ion can be a single charged atom.
Exam Tip: Use the definition from your textbook exactly: "a group of atoms behaving like a single unit."

Q.4 Match the compounds in List I – 1 to 20 with their correct formulas from in List II – A to T.

List I
1. Hydrochloric acid
2. Potassium hydroxide
3. Sulphuric acid
4. Zinc hydroxide
5. Sodium chloride
6. Nitric acid
7. Calcium hydroxide
8. Carbonic acid
9. Sodium hydroxide
10. Copper sulphate
11. Potassium chloride
12. Calcium carbonate
13. Magnesium sulphide
14. Zinc sulphate
15. Aluminium hydroxide
16. Sodium carbonate
17. Calcium sulphide
18. Magnesium oxide
19. Zinc oxide
20. Ammonium chloride.

List II
A. NaCl
B. NaOH
C. KCl
D. Ca(OH)₂
E. CaS
F. CuSO₄
G. CaCO₃
H. NH₄Cl
I. HCl
J. ZnO
K. HNO₃
L. H₂CO₃
M. MgS
N. MgO
O. KOH
P. ZnSO₄
Q. Na₂CO₃
R. H₂SO₄
S. Al(OH)₃
T. Zn(OH)₂

Answer:
1. Hydrochloric acid — (I) HCl
2. Potassium hydroxide — (O) KOH
3. Sulphuric acid — (R) H₂SO₄
4. Zinc hydroxide — (T) Zn(OH)₂
5. Sodium chloride — (A) NaCl
6. Nitric acid — (K) HNO₃
7. Calcium hydroxide — (D) Ca(OH)₂
8. Carbonic acid — (L) H₂CO₃
9. Sodium hydroxide — (B) NaOH
10. Copper sulphate — (F) CuSO₄
11. Potassium chloride — (C) KCl
12. Calcium carbonate — (G) CaCO₃
13. Magnesium sulphide — (M) MgS
14. Zinc sulphate — (P) ZnSO₄
15. Aluminium hydroxide — (S) Al(OH)₃
16. Sodium carbonate — (Q) Na₂CO₃
17. Calcium sulphide — (E) CaS
18. Magnesium oxide — (N) MgO
19. Zinc oxide — (J) ZnO
20. Ammonium chloride — (H) NH₄Cl
Matching these formulas requires recognizing both the metal symbol and the specific radical or non-metal attached. This comprehensive list covers most of the basic inorganic compounds you will encounter.
Teacher's Tip: Always look at the first word for the metal (like 'Sodium' = Na) and the second word for the rest.
Exam Tip: Double-check the subscripts, as many formulas look similar but have different numbers (like CaCO₃ vs Na₂CO₃).