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Question 1:

H2 : Hydrogen ::______________________:
Nitrogen
(a) N
(b) N2
(c) N2
(d) n2

Answer:

(c) N2

Each molecule of nitrogen consists of two atoms joined together. This is a diatomic element.

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Question 2:

Which one of the following pairs has not been matched correctly?
(a) metal-gold
(b) non-metal−hydrogen
(c) compound−sugar
(d) monoatomic element−oxygen

Answer:

(d) monoatomic element–oxygen

Each molecule of oxygen consists of two bonded atoms. This is a diatomic element with formula O2.

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Question 3:

Study the table carefully. Which alphabet can be replaced by the word 'liquid'?
 

substance characteristic Example
solid molecules tightly packed Q
P molecules less tightly packed and can move a little R
gas molecules farther apart and are able to move freely S

a. P
b. Q
c. R
d. S

Answer:

a. P

Molecules of a liquid substance are less tightly packed and can move a little freely.
Example: Water.

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Question 4:

Study the chart below carefully:
graphic

Answer:

b. X- iron; Y- water

Each molecule of elements such as iron, carbon, copper, calcium, etc., contains only one atom. Therefore, molecule of iron is  monoatomic with symbol Fe.
Molecules of compounds contain atoms of different kind. Water is a compound and each of its molecule is made up of atoms of two elements, hydrogen and oxygen, with formula H2O.

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Question 5:

What is common to the following:
N2
CaO
H2O
NaCl

(a) Each of them represents one molecule of a different substance.
(b) Each of them represents the formula of a different substance.
(c) Each of them tells us the number of atoms of each kind present in one molecule of that       substance.
(d) All the above

Answer:

(d) All of the above

All these formulas represent a molecule of a different substance. Also, each of them represents the number of atoms of each kind present in one molecule of that substance.

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Question 6:

In which group do all the elements have a valency of one?
(a) hydrogen, oxygen, chlorine
(b) potassium, hydrogen, sulphur
(c) sodium, chlorine, hydrogen
(d) sodium, hydrogen, sulphur

Answer:

(c) sodium, chlorine, hydrogen

Hydrogen can donate or share only one electron. Thus, its valency is one.
When one atom of hydrogen combines with one atom of chlorine, it forms hydrogen chloride, HCl. Therefore, valency of chlorine is also one.
Similarly, when one atom of sodium combines with one atom of chlorine, it forms sodium chloride, NaCl. Therefore, valency of sodium is also one.

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Question 7:

Which of these is the symbol of zinc?
(a) Zn
(b) Z
(c) Zc
(d) Zi

Answer:

(a) Zn

 Zinc is represented by symbol Zn.

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Question 8:

Which of the following statements is true?
(a) Carbon dioxide is an element.
(b) The molecule is the smallest particle that can stand on its own.
(c) Every molecule contains two atoms.
(d) Calcium, nitrogen and oxygen are non-metals.

Answer:

(b) The molecule is the smallest particle that can stand on its own.

Molecule is the smallest particle of an element that can stand on its own. Molecules of elements contain one or more atoms of the same kind. For example, each molecule of metals such as sodium and calcium contains only one atom and are called monatomic. On the other hand, each molecule of non-metals such as oxygen, nitrogen and hydrogen consists of two atoms and are called diatomic.



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Question 9:

The three smaller particles present in an atom are
(a) elections, protons, neutrons
(b) electrons, portions, neutrons
(c) electrons, protons, neotrons
(d) electrons, protons, neutrons

Answer:

(d) electrons, protons, neutrons

An atom consists of three subatomic (smaller than an atom) particles. These are called protons, electrons and neutrons. Protons carry a positive electrical charge, electrons carry a negative electrical charge and neutrons carry no electrical charge.

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Question 10:

Which of these compounds does not contain the nitrogen atom?
(a) NaCl
(b) NH3
(c) NO2
(d) N2

Answer:

(a) NaCl

Nitrogen atom is represented by the symbol N. The compound NaCl contains the atoms of two elements, sodium (Na) and chlorine (Cl).

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Question 11:

Define the following terms: element, compound, molecule, symbol and formula.

Answer:

Element: Element is a pure chemical substance consisting of same type of atoms, and it cannot be broken down into simpler components by chemical means. Example: oxygen, calcium, iron, gold and mercury.

Compound: Compound is a pure chemical substance consisting of two or more different chemical elements. For example, water (H2O) is a compound made up of two elements, hydrogen and oxygen.

Molecule: Molecule is the smallest particle of a substance (element or compound) that retains the physical and chemical properties of that substance and can stand on its own. Molecule of an element is composed of one or more atoms of same kind. For example, H2is the molecule of hydrogen element formed from two hydrogen atoms. Molecule of a compound is composed of two or more atoms of different elements. For example, water is a molecule of compound composed of two atoms of hydrogen element and one atom of oxygen element.

Symbol: An element is represented by one or two letter code known as its chemical symbol usually derived from the first two letters of the name of the element, often in Latin, where only the first letter is capitalised. For example, symbol of aluminium is Al. Potassium has the symbol K derived from its Latin name kalium.

Formula: Formula represents the number of atoms of each kind of element that combine to make one molecule an element or a compound. For example, formula of sodium chloride is NaCl. It represents one molecule of sodium chloride containing one atom of sodium and one atom of chlorine. Similarly, formula of oxygen element is O2. It represents one molecule of oxygen formed from two atoms of oxygen.

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Question 12:

Separate these substances into elements, compounds and mixtures: air, copper, sea water, nitrogen, soil, mercury, lemon squash, sugar and vitamin C

Answer:

Elements: Nitrogen, mercury and copper
Compounds: Sugar and vitamin C
Mixtures: Air (mixture of gases), soil (mixture of liquids, minerals, organic matter, etc.), lemon squash (mixture of lemon juice, sugar and water) and sea water (mixture of water and salts)

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Question 13:

Classify the following as elements and compounds: MgO, H2, NaCl, Cl2, N2, Cl, Ca, H2O

Answer:

Elements
H2, Cl2, Cl ,N2, C and Ca

Compounds
MgO, NaCl and H2O

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Question 14:

Name the following elements: C, H, O, N, P, S, Na, Cl, Ca, Fe, Cu, Zn, Mg

Answer:

C: Carbon
H: Hydrogen
O: Oxygen
N: Nitrogen
P: Phosphorous
S: Sulphur
Na: Sodium
Cl: Chlorine
Ca: Calcium
Fe: Iron
Cu: Copper
Zn: Zinc
Mg: Magnesium

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Question 15:

Underline the word or phrase that completes each of the following sentences correctly.
(a) (Liquids/Solids/Gases) have a definite volume and a definite shape.
(b) (Fe/O2/FeO) is the formula of iron oxide.
(c) (H2O/CO2/H2) is the formula of an element.
(d) Salt water is a (Compound/mixture/element)
(e) (H2)/N2/Ca) is a monoatomic molecule.
(f) There are (2/3/4) atoms in a molecule of water.
(g) The symbol of potassium is (Po/P/K).

Answer:

(a) Solids have a definite volume and a definite shape.
(b) FeO is the formula of iron oxide.
(c) H2 is the formula of an element.
(d) Salt water is a mixture.
(e) Ca is a monatomic molecule.
(f) There are 3 atoms in a molecule of water.
(g) The symbol of potassium is K.

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Question 16:

Differentiate between an atom and a molecule. Can an atom of an element and its molecule be the same? If yes, give an example.

Answer:

Atom Molecule
Atoms may or may not exist freely depending upon their valency. All molecules can exist freely.
An atom is the smallest particle in an element that has all the properties of the element. It is not possible to further breakdown an atom through chemical means. Molecules may or may not be classified as smallest particles of matter depending upon whether they are monatomic or not. Molecules can be subdivided to individual atoms by chemical processes.
Atoms cannot be seen through naked eye or through high-resolution microscope. They can be observed only with an electron microscope. Molecules cannot be seen through naked eye but can be seen through high-resolution microscope.
Atoms are made up of electrons, protons and neurons. Molecules are made up of atoms of same or different elements.
Example: N, O, Fe, H Example: N2, H2, H2O, He
     
Yes, an atom of an element and its molecule be same. For example, an atom and a molecule of calcium metal (Ca) is same.

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Question 17:

What is the difference between O and O2?

Answer:

O represents one atom of oxygen element. It is the smallest individual particle of oxygen element and cannot stand on its own. 
O2 represents one molecule of oxygen element formed from two atoms of oxygen. It is the smallest part of elemental oxygen that can stand on its own.

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Question 18:

Using the kinetic theory of matter, explain why a gas completely fills any container.

Answer:

A gas is made up of molecules that are in a state of constant motion. The attractive force of cohesion between the molecules of a gas is negligible and their kinetic energy is very high. It is because of this reason the molecules are spread very far apart and are free to move in all directions. Therefore, a gas completely fills any container.

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Question 19:

The formulae of some compounds are given below. Write down the names of the elements that each one contains.
Co2, H2S, HgO, MgCl2, PbS

Answer:

CO2: It contains one atom of carbon and two atoms of oxygen.
H2S: It contains one atom of sulphur and two atoms of hydrogen.
HgO: It contains one atom of mercury and one atom of oxygen.
MgCl2: It contains one atom of magnesium and two atoms of chlorine. 
PbS: It contains one atom of lead and one atom of sulphur.

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Question 20:

State whether the statements given below are true or false. Rewrite the false statements correctly.
(a) All metals are solids at room temperature.
(b) Non-metals are good conductors of heat but poor conductors of electricity.
(c) A molecule of sodium is made of sodium atoms only, so it is an element.
(d) Compounds are made up of only one type of element.
(e) The symbol and formula of copper are the same.
(f) An atom consists of electrons, protons and neutrons.
(g) Some atoms can stand on their own.
(h) Water is called combination because it is formed by the combination of two or more substances.
(i) If two atom join together a compound is formed.
(j) The electron is a particle which is smaller than an atom.
(k) Zinc, copper, nitrogen and silver are all metals.
(l) An atom of iron can stand on its own.
(m) The molecule of a compound contains the same kind of atoms joined together.
(n) Oxygen, nitrogen and hydrogen are gases.
(o) The valency of hydrogen is one.

Answer:

(a) False. All metals except mercury are solids at room temperature.

(b) False. Non-metals are poor conductors of heat as well as electricity.

(c) True.

(d) False. Compounds may be made up of two or more types of elements.

(e) True.

(f) True.

(g) True.
 
(h) False. Water is called a compound formed from the combination of two atoms of hydrogen and one atom of oxygen.

(i) False. When two different atoms join together, a compound is formed.

(j) True.

(k) False. Zinc, copper and silver are metals while nitrogen is a non-metal.

(l) True.

(m) False. The molecule of a compound contains different kind of bonded atoms.

(n) True.

(o) True.



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Question 21:

Fill in the blanks choosing the correct word from those given below.
one, two, carbon, oxygen, compound(s), element(s), gas, symbol, formula, atom(s), molecule(s)
CO2 is the _______________ of a gas called carbon dioxide, It represents one ___________ of the _______________. This compound is made up of ______________and _______________. Each molecule contains one ______________ of carbon and _________________of _______________. The _____________ C represents one ______________ of while the _______________ O represents one ________________ of ______________.

Answer:

CO2 is the formula of a gas called carbon dioxide. It represents one molecule of the compound. This compound is made up of carbon and oxygen. Each molecule contains one atom of carbon and two of oxygen. The symbol C represents one atom of carbon while the symbol O represents one atom of oxygen.

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Question 22:

Name these compounds and write down the number of atoms of each kind that combine to form them. One example is done for you.
Example, FeO−Iron oxide−One atom of iron combined with one atom of oxygen.
MgO, CaO, CuS, MgS, PbS, CaS, CuO, ZnO, PbO, FeO, H2O, NaCl, CuCl2

Answer:

MgO–Magnesium oxide–One atom of magnesium combined with one atom of oxygen.
CaO–Calcium oxide–One atom of calcium combined with one atom of oxygen.
CuS–Copper sulphide–One atom of copper combined with one atom of sulphur.
MgS–Magnesium sulphide–One atom of magnesium combined with one atom of sulphur.
PbS–Lead sulphide–One atom of lead combined with one atom of sulphur.
CaS–Calcium sulphide–One atom of calcium combined with one atom of sulphur.
CuO–Copper oxide–One atom of copper combined with one atom of oxygen.
ZnO–Zinc oxide–One atom of zinc combined with one atom of oxygen.
PbO–Lead oxide–One atom of lead combined with one atom of oxygen.
FeO–Iron oxide–One atom of iron combined with one atom of oxygen.
H2O–Water–One atom of oxygen combined with two atoms of hydrogen.
NaCl–Sodium chloride–One atom of sodium combined with one atom of chlorine.
CuCl2–​Copper chloride–One atom of copper combined with two atoms of chlorine.

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Question 23:

Write down symbols or formulae for the following:
(a) an atom of sodium
(b) a molecule of nitrogen
(c) a molecule of oxygen
(d) a molecule of water
(e) a molecule of iron oxide
(f) a molecule of carbon dioxide
(g) a molecule of copper sulphide
(h) a molecule of calcium oxide
(i) a molecule of calcium chloride

Answer:

(a) an atom of sodium: Na
(b) a molecule of nitrogen: N2
(c) an atom of oxygen: O
(d) a molecule of water: H2O
(e) a molecule of iron oxide: FeO
(f) a molecule of carbon dioxide: CO2
(g) a molecule of copper sulphide: CuS
(h) a molecule of calcium oxide: CaO
(i) a molecule of calcium chloride: CaCl2

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Question 24:

Give one example each of:
(a) a monatomic element
(b) a diatomic element
(c) a particle that is smaller than an atom
(d) a metal
(e) a mixture of gases

Answer:

(a) a monatomic element: Helium is a monatomic element with symbol He.
(b) a diatomic element: Chlorine is an example of diatomic element with formula Cl2.
(c) a particle that is smaller than an atom: Proton is a particle that is smaller than an atom and is present in the nucleus of an atom.
(d) a metal: Aluminium (Al) is a metal.
(e) a mixture of gases: Air is a mixture of gases such as nitrogen, oxygen, water vapour and carbon dioxide.



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