Kerala Syllabus Class 8 Basic Science: Chapter 5 Elements and Compounds - Questions and Answers
Std 8: Chemistry: Chapter 4: Chemistry of Changes: Questions and Answers
♦ What is a molecule?
Molecules are the smallest, independent and stable particles that have all the fundamental properties of a pure substance.
♦ What do you know about molecules? Try to write it.
• Molecules have independent existence
• Molecules are stable
• New molecules are produced by chemical reactions.
• Molecules are divisible.
♦ Do you know that new molecules are formed through chemical reactions?
Experiment - 1
Burn a magnesium ribbon which is scrubbed clean.
Observation: It burns with shining flame. A white powder is formed.
Result: A new substance (molecule), magnesium oxide, is produced.
This is an example for the formation of new molecules when different types of molecules undergo chemical reactions.
♦ Let us see if we can dissociate molecules to make new molecules.
Experiment - 2
Take some sugar in a dry boiling tube. Cover the mouth of the boiling tube with a piece of cotton. Heat the boiling tube strongly for a few minutes. Observation: Sugar is converted into charcoal (Carbon). Water drops are formed on the side of the boiling tube.
Result: When sugar is strongly heated, it decomposes into Carbon (C) and water (H₂O).
Experiment - 3
Take some dilute hydrochloric acid in a test tube and add a small piece of magnesium to it. Introduce a burning incense stick to the mouth of the test tube.
Observation: A gas is bubbled from the test tube. When burning incense stick is introduced into the gas, the gas burns with a 'pop' sound. Also water drops are formed on the sides of the vessel/test tube.
Result: Magnesium reacts with dilute Hydrochloric acid to form hydrogen gas. Magnesium chloride is also formed as another product.
Magnesium + Hydrochloric acid → Magnesium chloride + Hydrogen
♦ Collect the hydrogen gas obtained in this experiment into another moisture-free test tube and introduce a burning incense stick to it. Record your observations.
When a burning incense stick is shown to hydrogen gas, the gas burns by itself (oxygen present in the air helps this burning). Water is formed.
Hydrogen + Oxygen → Water
In all these experiments, substances combine or decompose to produce new molecules.
♦ Under what conditions does a new molecule form?
a. When a chemical change occurs
b. When a physical change occurs
c. When it is converted into small particles
d. When heated strongly
Answer: a. When a chemical change occurs
♦ In which of the following are new molecules formed?
a. When naphthalene pellets sublimate.
b. When water evaporates.
c. When sugar becomes small particles.
d. When magnesium and hydrochloric acid react.
Answer: d. When magnesium and hydrochloric acid react.
♦ What are the new substances produced when sugar is heated?
Carbon and water
♦ Which gas is formed when magnesium and hydrochloric acid react?
Magnesium chloride and Hydrogen gas
♦ Henry Cavendish
Henry Cavendish proved that water is a substance made up of more than one element. His experiment of burning hydrogen gas in the presence of oxygen helped to know about the components of water.
♦ Sir Humphry Davy
In 1806, Sir Humphry Davy discovered that water could be split into hydrogen and oxygen by passing electricity through it.
♦ Who was the scientist who proved that water is a compound made of more than one element?
Henry Cavendish.
♦ Which gases did Cavendish use in his experiment to study the composition of water?
Hydrogen gas and Oxygen gas.
♦ What happened when Cavendish burned hydrogen in the presence of oxygen?
Water droplets were formed.
♦ What conclusion did Cavendish draw from his experiment?
Water is not an element but a compound made up of hydrogen and oxygen.
♦ Who discovered that water can be split into hydrogen and oxygen by passing electricity through it?
Sir Humphry Davy in 1806.
♦ What is the process called when electricity is passed through water to split it into hydrogen and oxygen?
Electrolysis.
♦ Which are the substances obtained when water is dissociated? Can hydrogen and oxygen be further split into components?
Water decomposes to form hydrogen and oxygen. But hydrogen or oxygen could not be further decomposed.
♦ Atom
Atoms are smaller particles got when molecules are divided.
♦ From which language does the word "atom" originate, and what does it mean?
The word "atom" comes from the Greek word atomos, which means indivisible.
♦ Who first proposed that all matter is made up of indivisible particles called atoms?
The Greek philosopher Democritus.
♦ What did Democritus call the indivisible particles of matter?
Atoms.
♦ Who proposed the Atomic Theory in 1807?
John Dalton.
♦ Why is Dalton’s Atomic Theory important in modern chemistry?
It provided a scientific explanation of the nature of atoms and laid the foundation for modern chemistry.
♦ John Dalton’s concepts of the Atomic Theory.
• All matter is made up of minute particles called atoms.
• An atom is the smallest particle that can participate in a chemical reaction.
• An atom cannot be divided during a chemical reaction.
• An atom cannot be created or destroyed.
• All atoms of an element are of the same size, property and mass.
• Atoms of different elements differ in their size, property and mass.
♦ Which is not the idea of Dalton's atomic theory?
a. All matter is made up of minute particles called atoms.
b.An atom is the smallest particle that can participate in a chemical reaction.
c. When a new atom is formed through a chemical reaction, its size decreases.
d. An atom cannot be divided during a chemical reaction.
Answer: c. When a new atom is formed through a chemical reaction, its size decreases.
♦ Which is not the idea of Dalton's atomic theory?
a. Atoms cannot be created or destroyed.
b. All atoms of an element are of the same size, property and mass.
c. Atoms of different elements differ in their size, property and mass.
d. The smallest atom of an element participates in a chemical reaction.
Answer: d. The smallest atom of an element participates in a chemical reaction.
♦ What is the name of the molecule formed by combining 2 hydrogen atoms and 1 oxygen atom?
Answer: Water
♦ Look at the table below.
| Molecules | Atoms in a molecule |
|---|---|
| Ammonia | One atom of nitrogen Three atoms of hydrogen |
| Oxygen | Two atoms of oxygen |
| Chlorine | Two atoms of chlorine |
| Methane | One atom of carbon Four atoms of hydrogen |
| Hydrogen | Two atoms of hydrogen |
| Carbon dioxide | One atom of carbon Two atoms of oxygen |
♦ Which molecules in the table consist of the same type of atoms?
Oxygen, Chlorine, Hydrogen
♦ Which of these molecules consists of different types of atoms?
Ammonia, Methane, Carbon dioxide
♦ What are elements?
Elements are pure substances made up of the same type of atoms. They cannot be further divided into components through chemical reactions.
♦ Compounds
Compounds are pure substances formed by the combination of atoms of two or more elements in a fixed ratio. These can be dissociated into constituent elements through chemical reactions.
♦ Molecules made up of different types of atoms are called --------.
Answer: compounds
♦ Categorise the following into elements and compounds.
Oxygen, water, salt, sugar, carbon, nitrogen, carbon dioxide, ammonia, magnesium oxide
| Elements | Compounds |
|---|---|
| • Oxygen • Carbon • Nitrogen | • Water • Salt • Sugar • Carbon dioxide • Ammonia • Magnesium oxide |
♦ We use many signs like the above ones in our day to day communication. Do you know what these signs represent ? Why are they used?
2. UNICEF
3. Hospital
4. Indian Rupee
5. Doctor
6. Advocate
7. Pedestrians
8. School premise
♦ Why are symbols used?
Symbols are used to represent concepts in a clear and simple manner. Symbols make communication and interpretation easier.
♦ What is used to represent elements in chemistry?
Signs known as chemical symbols.
♦ What does a chemical symbol represent?
A chemical symbol represents an atom of an element.
♦ Berzelius
The modern symbol system of representing elements by letters was introduced by a scientist named Berzelius.
♦ Who introduced the modern system of representing elements by letters?
Berzelius.
♦ Note the list of elements represented by the first letter of their name. Expand the list by adding more elements.
| Element | Symbol |
|---|---|
| Hydrogen | H |
| Carbon | C |
| Nitrogen | N |
| Sulphur | S |
| Oxygen | O |
| Boron | B |
| Phospherus | P |
| Fluorine | F |
| Iodine | I |
| Yitrium | Y |
| Uranium | U |
♦ The element Helium is represented by the symbol ‘He’. The first and second letters of the English name are used here. What could be the reason?
There may be more than one element that starts with the same letter in its English name. In such cases, along with the uppercase of the first letter, the lowercase of the second letter or any other prominent letter is used to make the symbol.
Thus, Helium → He ensures clarity and distinction from Hydrogen.
♦ Some elements are represented by another letter along with the first letter of the English name in upper case. See more examples of using two letters as symbols.
| Element | Symbol |
|---|---|
| Aluminium | Al |
| Chlorine | Cl |
| Curium | Cm |
| Calcium | Ca |
| Lithium | Li |
| Berylium | Be |
| Magnesium | Mg |
| Neon | Ne |
| Silicon | Si |
| Argon | Ar |
| Chromium | Cr |
♦ Write the symbols of the elements hydrogen, chlorine, carbon, helium, and magnesium.
• Hydrogen → H
• Chlorine → Cl
• Carbon → C
• Helium → He
• Magnesium → Mg
♦ Some elements have symbols based on their Latin names.
| Element | Latin name | Symbol |
|---|---|---|
| Sodium | Natrium | Na |
| Potassium | Kalium | K |
| Iron | Ferrum | Fe |
| Gold | Aurum | Au |
| Copper | Cuprum | Cu |
| Silver | Argentum | Ag |
| Mercury | Hydrargyrum | Hg |
| Lead | Plumbum | Pb |
| Tin | Stannum | Sn |
| Antimony | Stibium | Sb |
♦ Write the symbols of sodium, iron, copper, and gold, whose symbols are derived from their Latin names
• Sodium → Na (from Latin: Natrium)
• Iron → Fe (from Latin: Ferrum)
• Copper → Cu (from Latin: Cuprum)
• Gold → Au (from Latin: Aurum)
♦ Find out how the elements given below got their names.
• Chromium
• Iridium
• Neptunium
Chromium, Iridium, and Neptunium all have names linked to colour, mythology, and astronomy. Chromium comes from the Greek word chrōma, meaning “colour,” Iridium from the Greek goddess of the rainbow, and Neptunium from the planet Neptune, reflecting its position in the periodic table.
♦ Periodic Table
The Periodic Table is a table in which all the elements are categorised and arranged based on their properties.
♦ Which elements were added recently in the Periodic Table? Find out.
The most recently added elements to the Periodic Table are Nihonium (Nh), Moscovium (Mc), Tennessine (Ts), and Oganesson (Og).
♦ List the elements existing in solid, liquid and gaseous states with the help of the Periodic Table.
| Solid | Liquid | Gas |
|---|---|---|
| Sodium (Na) Iron (Fe) Copper (Cu) Gold (Au) Carbon (C) Sulphur (S) Phosphorus (P) Magnesium (Mg) Aluminium (Al) | Mercury (Hg) Bromine (Br) Cesium (Cs) Gallium (Ga) Rubidium (Rb) Francium (Fr) | Hydrogen (H₂) Nitrogen (N₂) Oxygen (O₂) Fluorine (F₂) Chlorine (Cl₂) Helium (He) |
In the periodic table given on Textbook page 75, Elements shown with a white background are solids. Elements shown with a blue background are liquids (Room Temperature). Elements shown with a brown background are gases.
♦ Find the synthetic elements with the help of the Periodic Table.
Synthetic elements are man-made and do not occur naturally on Earth.
Technetium, Rutherfordium, Dubnium, Seaborgium, Bohrium, Hassium, Meitnerium, Darmstadtium, Roentgenium, Copernicium, Nihonium, Flerovium, Moscovium, Livermorium, Tennessine, Oganesson.......etc.
♦ Who classified the elements into metals and nonmetals?
Antoine Lavoisier.
♦ Who classified the elements into groups called triads, each containing three elements?
Dobereiner
♦ What are triads in chemistry?
Groups of three elements with similar properties, classified by Döbereiner.
♦ What was Döbereiner’s observation about the atomic masses in a triad?
The atomic mass of the middle element is approximately the average of the atomic masses of the other two elements.
♦ Give an example of a Döbereiner’s triad.
Lithium (Li), Sodium (Na), and Potassium (K).
♦ Who proposed the Law of Octaves?
Newlands.
♦ What did Newlands observe when elements were arranged in ascending order of atomic mass?
Each eighth element was a repetition of the first in its properties. He compared this to the seven notes in music.
♦ Write Newlands’ Law of Octaves.
When elements are arranged in the ascending order of their atomic masses, each eighth element is a repetition of the first in its properties.
♦ Who first arranged the elements in a table? What was the basis of this arrangement?
The elements were first arranged in a table by Mendeleev.
The basis of this arrangement was the ascending order of their atomic masses.
♦ How many elements were known when Mendeleev prepared his table?
63 elements.
♦ Write Mendeleev's periodic law.
The physical and chemical properties of elements are the periodic functions of their atomic mass.
♦ In which year was Mendeleev’s Periodic Table formed?
1869.
♦ On what basis did Mendeleev arrange the elements in his table?
He arranged the 63 known elements in ascending order of their atomic masses.
♦ What is meant by group and period in a periodic table?
The horizontal rows in the table are called periods, and the vertical columns are called groups. Elements in a group show similar chemical properties.
♦ What are the characteristics of Mendeleev's periodic law table?
• Elements were arranged in the ascending order of atomic mass. Blank spaces were left for elements to be discovered and their properties were predicted.
• Elements with similar properties were grouped together to simplify the study of elements and their compounds.
• However, the order of atomic mass was not strictly followed in the case of all elements. Hydrogen could not be given an exact position.
• Elements with very different properties had to be placed in the same group.
♦ Who discovered that the properties of elements depend on their atomic number?
Henry Moseley
♦ What is the basis of the modern periodic table?
The modern periodic table is based on atomic number.
♦ Who discovered that the properties of elements depend on their atomic number?
Henry Moseley
♦ What is the difference between N and N₂, which are the two ways of representing nitrogen?
• If we write N, it refers to nitrogen and one atom of nitrogen.
• 2N - refers to two free nitrogen atoms.
• N₂ - refers to a nitrogen molecule formed by two atoms.
• 2N₂ - refers to two nitrogen molecules.
♦ What is the total number of atoms in 2N₂?
2N₂ means two nitrogen molecules.
Each molecule has 2 atoms, so: 2N₂ = 2 x 2 = 4 atoms
The total number of atoms in 2N₂ is 4 atoms of nitrogen.
♦ Classify the elements given in the table as monatomic elements, diatomic elements and polyatomic elements.
| Molecule | Number of atoms | Category |
|---|---|---|
| He | 1 | Monoatomic elements |
| O₂ | 2 | Diatomic elements |
| S₈ | 8 | Polyatomic elements |
| H₂ | 2 | Diatomic elements |
| P₄ | 4 | Polyatomic elements |
| Ne | 1 | Monoatomic elements |
| Cl₂ | 2 | Diatomic elements |
| Ar | 1 | Monoatomic elements |
| O₃ | 3 | Polyatomic elements |
♦ What does a chemical formula represent?
The chemical formula of the molecule represent one molecule of a substance.
♦ In monoatomic elements, what is considered the chemical formula?
The symbol itself is considered the chemical formula.
(Example: He → one helium atom)
♦ What does the number written on the left of the symbol in monoatomic molecules indicate?
It indicates the number of molecules, which is also the number of atoms.
(Example: 2He → 2 helium atoms / 2 helium molecules)
♦ What is atomicity?
Atomicity is the number of atoms present in a molecule.
(Example: O₂ has atomicity 2, P₄ has atomicity 4)
♦ How is atomicity represented in diatomic and polyatomic elements?
Atomicity is written as a subscript on the right of the symbol in the chemical formula.
(Example: O₂, Cl₂, P₄)
♦ How do we find the total number of atoms from a chemical formula?
We can find the number of molecules and the number of atoms from the chemical formula of the elements.
Multiply the number of molecules (left side) by the atomicity (subscript).
Example: 3O₂ → 3 oxygen molecules
Total number of atoms = 3×2 = 6 oxygen atoms
5Cl₂ → 5 chlorine molecules
Total number of atoms = 5×2 = 10 chlorine atoms.
♦ Give an example of a polyatomic element and its atomicity.
Phosphorus (P₄) → Atomicity = 4.
♦ Find the number of molecules and the total number of atoms in the following.
| Substance | No. of molecules | No. of atoms |
|---|---|---|
| H₂ | 1 | 1 x 2 = 2 |
| 5O₂ | 5 | 5 x 2 = 10 |
| 6N₂ | 6 | 6 x 2 = 12 |
| 4Cl₂ | 4 | 4 x 2 = 8 |
| S₈ | 1 | 1 x 8 = 8 |
♦ Chemical Formula of Compounds
• The formula of a compound is a short representation of the compound or one molecule of the compound.
• It gives the kind of different atoms and their number in a molecule of the compound.
• The chemical formula of a compound is made by using the symbols of different atoms present in the molecule and the number of each atom in a molecule.
Example:
• Water → H₂O → One molecule of water is composed of 2 Hydrogen atoms and 1 oxygen atom
• Ammonia → NH₃ → One molecule of ammonia contains 1 Nitrogen atom and 3 Hydrogen atoms.
• Carbon Tetrachloride → CCl₄ → One molecule contains 1 carbon atom and 4 chlorine atoms.
♦ Complete the table.
• To find the total number of atoms in a fixed number of molecules of a compound, multiply the number of molecules given with automicity of the compound (number of atoms in one molecule of the compound)
♦ Complete the table given below.
| Compound | No. of molecules | No. of atoms |
|---|---|---|
| 2HCl | 2 | H − 2 x 1 = 2 Cl − 2 x 1 = 2 4 |
| 4SO₂ | 4 | S = 4 x 1 = 4 O = 4 x 2 = 8 12 |
| 3C₂H₅OH | 3 | C = 3 x 2 = 6 H = 3 x 6 = 18 O = 3 x 1 = 3 27 |
| 2C₆H₁₂O₆ | 2 | C = 2 x 6 = 12 H = 2 x 12 = 24 O = 2 x 6 = 12 48 |
♦ Chemical Equations
A chemical equation is the representation of a chemical reaction which helps to understand it at a glance. It uses symbols/chemical formulae to represent the reactants and products.
Eg: Carbon reacts with oxygen to form carbon dioxide.
♦ What are the reactants in this reaction?
Carbon (C), Oxygen (O₂)
♦ What are the products?
Carbon dioxide (CO₂)
♦ How do you write the reactants using chemical formula?
C + O₂ → CO₂
♦ How about products?
The product is written as CO₂ using chemical formula.
♦ Calcium carbonate (CaCO₃) decomposes to form calcium oxide (CaO) and carbon dioxide (CO₂). How can this chemical reaction be represented?
Reactants: CaCO₃
Products: CaO, CO₂
Chemical formula: CaCO₃ → CaO + CO₂
♦ Write the chemical equations of some chemical reactions that you are familiar with.
• 2 H₂ + O₂ → 2 H₂O
• S + O₂ → SO₂
• Mg + 2HCl → MgCl₂ + H₂
• H₂ + Cl₂ → 2HCl
• NaOH + HCl → NaCl + H₂O
♦ Write the names of the scientists associated with the following terms.
i. Triads.
ii. Law of Octaves
iii. Elements are arranged in the ascending order of their atomic masses and prepared a table.
iv. The properties of elements depend on their atomic number.
Answer:
i. Döbereiner.
ii. Newlands.
iii. Mendeleev.
iv. Henry Moseley.
♦ Write two elements named after scientists.
Curium (Cm) – named after Marie Curie, and Einsteinium (Es) – named after Albert Einstein.
♦ Let's assess
1. Match the following.
A Element |
B Symbol | B Base |
|---|---|---|
| Francium | Fr | Country |
| Curium | Cm | Scientist |
| Rubidium | Rb | Colour |
| Neptunium | Np | Planet |
| Europium | Eu | Continent |
| Titanium | Ti | Satelite |
2. Cl is the symbol of the element chlorine. How can we represent two chlorine atoms and one chlorine molecule using this symbol?
• 2 Chlorine atoms = 2Cl
• 1 Clorine molecule = Cl₂
3. Find the number of atoms in each of the following and write which of them contains the most number of atoms.
5NH₃, 2H₂O, 5NO₂, 4CO₂
• 5NH₃ = 5 X 4 = 20
• 2H₂O = 2 X 3 = 6
• 5NO₂ = 5 X 3 = 15
• 4CO₂ = 4 X 3 = 12
4. Complete the table.
Element |
Basis of nomenclature | Symbol |
|---|---|---|
| Indium | Indigo - colour | In |
| Rutherfordium | Rutherford - Scientist | Rf |
| Germenium | Germany | Ge |
| Silver | Argentum - Latin name | Ag |
5. The names of some scientists are given.
(Dobereiner, Lavoisier, Newlands, Mendeleev)
Match the statements given below with the names given in brackets.
i. The chemical and physical properties of the elements are functions of their atomic masses.
ii. Elements were divided into groups containing three elements (triads) with similar properties.
iii. The elements were classified into metals and non-metals.
iv. When the elements were arranged in ascending order of atomic masses, it was found that every eighth element was a repetition of the first, in terms of its properties.
Answer:
i. Mendeleev
ii. Dobereiner
iii. Lavoisier
iv. Newlands
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