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#### Question 1:

Atoms of elements

(a) can exist independently
(b) cannot exist independently
(c) may or may not exist independently
(d) can exist independently but don't like to!

​(c) may or may not exist independently
Atoms of some elements like hydrogen (H), oxygen (O) and nitrogen (N) do not exist independently, as they are unstable and readily form molecules H2, O2 and N(containing two or more atoms). But, the atoms of noble gases which are highly inert can exist independently. For example, Helium (He), Neon (Neon) and Argon (Ar)

#### Question 2:

A molecule of water

(a) cannot be broken down
(b) can be broken down by hammering it
(c) can be broken down by boiling the water
(d) can be broken down by passing an electric current through water.

(d) can be broken down by passing an electric current through water
A molecule of water is formed by 2 atoms of hydrogen and 1 atom of oxygen. Thus, it can be split into hydrogen and oxygen by passing an electric current through it. This process is called the electrolysis of water.

#### Question 3:

(a) elements
(b) molecules
(c) compounds
(d) mixtures

(a) elements
There are about 117 different elements in the periodic table. Many of them are available naturally, while the others are prepared by the scientists in a laboratory.

#### Question 4:

If you heat a liquid, it will ultimately change into

(a) a solid
(b) a gas
(c) either a solid or a gas depending on temperature
(d) neither solid nor gas−it will always remain a liquid

​​(b) a gas
When a liquid is heated, its particles start expanding after reaching the boiling point. Thus, the liquid turns into a gas.

#### Question 5:

When two elements are brought together, they

(a) always form a mixture
(b) always form a compound
(c) may form a mixture of a compound depending on the conditions
(d) may form a mixture or a compound depending on what they like.

(c) may form a mixture or a compound depending on the conditions
Not all elements react with each other, and in case they react, they will need a condition favouring such reaction. If the conditions are favourable for the elements to undergo a chemical reaction, they will form compounds. And, mixtures are formed when the conditions are not suitable for the reactions to take place.

#### Question 6:

Which of the following is to a mixture?

(a) air
(b) sea water
(c) pure water
(d) soil

(c) pure water
Pure water comprises 2 atoms of hydrogen and 1 atom of oxygen combined chemically in a fixed ratio. This hydrogen and oxygen can be later split by chemical means. Also, pure water has dissimilar properties from its constituents; thus, pure water is a compound and not a mixture.

#### Question 7:

The valency of which of these elements is zero?

(a) iron
(b) helium
(c) hydrogen
(d) oxygen

(b) helium
Helium is an inert element (noble gas) which will not combine with other elements. Hence, its valency is zero.

#### Question 1:

When will two elements on being mixed together form a compound?

A compound is only formed when two or more elements will chemically react with each other on mixing them together. In case the conditions to undergo a chemical reaction are not suitable, the elements will mix with each other to form a mixture.
Example :

Fe     +      S     $\stackrel{\mathrm{Heating}}{\to }$       FeS
Iron         Sulphur       Iron sulphide

#### Question 2:

Why are the properties of different compounds so different from each other?

Properties of one compound differ from that of the other because of the following reasons:
1. Each compound is formed by a different set of elements.
2. Even with the same set of elements, different compounds are formed by varying numbers of constituent atoms.
Example: Just varying the number of carbon and oxygen atoms, we get carbon monoxide or CO (toxic gas) and carbon dioxide or CO2 (non-toxic gas).

#### Question 3:

Why are most elements not found in the free state in nature?

Most of the elements are reactive and have a high affinity to combine with other elements to form a compound. Thus, most of the elements can not exist freely in nature.

#### Question 4:

How can you form iron sulphide from a mixture of iron and sulphur?

By heating the mixture of iron and sulphur we can produce iron sulphide. Therefore, heat which is the condition required for chemically reacting iron and sulphur, is satisfied.

Fe   +     S      FeS
Iron     Sulphur       Iron sulphide

#### Question 5:

What does a formula represent?

A formula represents the kind of elements present in a compound and the number of atoms and molecules of those elements in the compound.
Example: The formula $2{\mathrm{H}}_{2}\mathrm{O}$ represents 2 molecules of water, where each molecule of water contains 2 atoms of hydrogen and 1 atom of oxygen.

#### Question 6:

What are radicals? Give two examples each of radicals with valency of 1 and 2.

Two or more atoms of elements behaving as a group or unit are called radicals. These radicals have a common valency with a net charge as a whole.

The examples are given below:

â€‹  â€‹  â€‹  â€‹ Formula  â€‹  â€‹  â€‹  â€‹Valency
Hydroxide
â€‹   â€‹${\mathrm{OH}}^{-}$                   1
Nitrate
${\mathrm{NO}}_{3}^{-}$                   1
Sulphate        ${\mathrm{SO}}_{4}^{2-}$                  2
Carbonate     ${\mathrm{CO}}_{3}^{2-}$                  2
â€‹â€‹

#### Question 7:

Define valency in terms of hydrogen.

The combining capacity of an element or radical is called its valency. The valency of hydrogen is considered as 1, while the valency of other elements are determined. Therefore, valency in terms of hydrogen is defined as the combining or displacing capacity of a number of hydrogen atoms by one atom of an element or radical.

#### Question 1:

Explain the differences between an element and a compound. Give examples.

 Elements Compound Substance that contains only one type of atom and cannot be further divided chemically is called an element. Substance produced by combining two or more elements in a constant ratio is called a compound. The smallest particle of an element is an atom. The smallest particle of a compound is a molecule. Example: Silver (Ag) Example: Common salt (NaCl)

#### Question 2:

Explain the difference between the atom and the molecule of an element. Can a molecule of an element be the same as its atom? Give an example.

 Atom Molecule Atom is the smallest particle of an element which cannot be further divided chemically. Molecule is defined as the smallest particle of a compound which can be further divided in to atoms chemically. It may or may not exist freely in nature. A molecule bears the same property as its atom. Example: Hydrogen (H) Examples: Hydrogen gas (H2)

A molecule of an element can be same as its atom.
Example: A molecule of oxygen will have 2 atoms of oxygen.

#### Question 3:

Write the formulae of the following compounds, showing the steps involved.

(a) sodium oxide
(b) magnesium nitrate
(c) magnesium sulphate
(d) aluminium chloride

Let's write the formula of the given compounds along with the steps involved.
a. Sodium oxide
* The elements in sodium oxide are sodium and oxygen. The valency of sodium is 1 and oxygen is 2. Therefore, we write the symbols as Na1 and​ O2
* The common factor among the numbers 1 and 2 is nil.
* The valencies are exchanged and are written as subscripts to the elements. Thus, the formula of sodium oxide would be Na2O.

b. Magnesium nitrate
* Magnesium nitrate contains the element magnesium and nitrogen. The valency of magnesium is 2 and radical of nitrogen is 1. Therefore, we write the symbols as Mg2 and (NO3)1.
* The common factor among the numbers 1 and 2 is nil.
* The valencies are exchanged and written as superscripts to the elements. As the radical contains more than 2 atoms, the radical is written in brackets and the exchanged valency is written as its subscript. Thus, the formula of magnesium nitrate would be Mg (NO3)2.

c. Magnesium sulphate
* Magnesium sulphate contains the element magnesium and sulphur. The valency of magnesium is 2 and that of the radical of sulphur is also 2. Therefore, we write the symbols as Mg2 and ​(SO4)2.
* The common factor among the numbers 2 and 2 is one.
* As both the element and radical’s valencies are one, neither the radical will not be written in brackets nor the valency number will be written as a subscript. Thus, the formula of Magnesium sulphate would be MgSO4.

d. Aluminium chloride
* The elements in aluminium chloride are aluminium and chlorine. The valency of aluminium is 3 and chlorine is 1. Therefore, we write the symbols as Aland ​Cl1.
* The common factor among the numbers 1 and 3 is nil.
* The valencies are exchanged and are written as subscripts to the elements. Thus the formula of aluminium chloride would be AlCl3.

#### Question 8:

Which of these is the formula for calcium hydroxide?

(a) CaOH
(b) Ca2OH
(c) Ca(OH)3
(d) Ca(OH)2

(d) Ca(OH)2
Calcium hydroxide comprises of calcium and hydroxide as the radicals. Since, the valency of calcium is 2 and hydroxide radical is 1, the formula of calcium hydroxide will be Ca(OH)2 after interchanging their valencies.

#### Question 9:

The equation Na + H2O → NaOH + H2 is

(a) correct
(b) incorrect since it is not balanced
(c) incorrect since sodium does not react with water.
(d) incorrect since hydrogen should be written as H and not H2.

​(b) incorrect since it is not balanced

A chemical equation needs to be balanced. The balanced equation for the above reaction is,

#### Question 1:

What is the smallest particle of an element that can take part in a chemical reaction?

Atom is the smallest particle of an element that can take part in a chemical reaction.

#### Question 2:

The smallest particle of a compound is _________

The smallest particle of a compound is a molecule.

#### Question 3:

Name an element which exists in the free state in nature.

Neon is an element that exists freely in nature, as it is a noble gas.

#### Question 4:

Name an element whose atoms occur in pairs.

Oxygen, O2, is an element whose atoms occur in pairs.
Oxygen is a diatomic element made up of 2 atoms of oxygen, i.e., its atoms occur in pairs.

#### Question 5:

If the formula of an element is X3, its atomicity is _______

If the formula of an element is X3, its atomicity is ​3.

#### Question 6:

If two compounds that do not react chemically are mixed, what do we get?

If two compounds that do not react chemically are mixed, we will simply get a mixture of compounds.

#### Question 7:

From which of these two can elements be more easily separated−a mixture or a compound?

It is easy to separate the elements of a mixture by physical means. However, the elements of a compound are chemically bonded to each other, and can only be separated through chemical means.

#### Question 8:

Does a symbol represent an atom or a molecule? What are the symbols for each of the following?

(a) cobalt
(b) copper
(c) chlorine
(d) carbon

​Symbols represent atoms and not molecules.
The symbols of the following elements are:
a. Cobalt  - Co
b. Copper – Cu
c. Chlorine – Cl
d. Carbon – C

#### Question 9:

Write the formulae for

(a) sulphuric acid
(b) calcium hydroxide
(c) sugar
(d) common salt

​The formulae for the following are:
a. Sulphuric acid –H2SO4
b. Calcium hydroxide – Ca(OH)2
c. Sugar –C12H22O11
d. Common salt -
NaCl

#### Question 10:

If a certain compound has a formula H2X, what is the valency of X?

In the compound with the formula ${\mathrm{H}}_{2}\mathrm{X}$, the valency of X is 2.
This is because the valencies of the elements in a compound are interchanged while writing its formula. In this case, the valency of X is 2 and H is 1.

#### Question 11:

In a compound of two elements, the elements are combined in a certain fixed ratio that always remains the same. True of false?

Yes, it is true. The chemical composition or ratio of two or more elements forming a compound always remains the same. For example, no matter where we take water from, its elements will always be 2 atoms of hydrogen and 1 atom of oxygen, combined in the fixed ratio of 2:1.

#### Question 12:

MgO + HCI → MgCI2 + H2O is a balanced equation. True or false?

False, the equation is not balanced.
For a balanced equation, the number of atoms in the both side of the equation should be equal. Therefore, the balanced equation is,

Now, the number of atoms in the LHS and RHS of the reaction are equal.

#### Question 4:

What is a chemical equation? Why is it necessary to balance an equation?

​A chemical equation is the symbolic representation of a chemical reaction. With the help of symbols of elements and formulas of compounds, a chemical reaction is explained as a simple equation for better understanding.

One of the essential conditions of writing a chemical equation is that the total number of atoms of all the elements on both sides (LHS and RHS) of the chemical equation should be equal.

Example:

2H2     +   O2    $\to$    2H2O
Reactants           Product
LHS                  RHS

#### Question 5:

Balance the following equations.

(a) Mg + H2SO4 → MgSO4 + H2
(b) CaCO3 + HCl → CaCl2 + H2O + CO2
(c) CaO + H2O → Ca(OH)2
(d) Ag + HNO3 → AgNO3 + H2O + NO2

For balancing a chemical equation, the number of atoms should be equal on both the sides (LHS and RHS). Keeping this conditions in mind, let's balance the below given equations.

(a)

(b)

(c)

(d)

#### Question 6:

Using the example of calcium chloride explain how the formulae of compounds are derived (given that the valency of calcium is 2 and that of chlorine is 1).

The steps involved in writing the formula for a compound are as follows:
1. Note down the symbols of the elements or their radicals. Then note down the valencies as superscript on the symbols of the elements.
2. If there is a common factor in the valencies, then divide it to a simple one.
3. Exchange the valencies and write them as subscripts.
[Note: If the radicals contain more than 2 atoms, they will be written in brackets and the exchanged valence number is written as subscript.]

Let's use the above steps for writing the formula for calcium chloride.
* The elements in calcium chloride are calcium and chlorine. The valency of calcium is 2 and chlorine is 1. Therefore, we write the symbols as Caand​ Cl1.
* The common factor among the numbers 1 and 2 is nil.
* The valencies are exchanged and written as subscripts to the elements. Therefore, the formula of calcium chloride would be CaCl2.

#### Question 1:

The symbol of an element is X. Is it possible that the formula of the element will also be X?
Explain with examples.

Yes, it is possible that the symbol as well as the formula of an element can be the same (X). However, this is not applicable to all the elements.
For example, all mono-atomic (made up of only one atom) elements like C, Na, Ca, He, etc., have the symbol and formula.

#### Question 2:

Scientists can break atoms to get still smaller particles. In the light of what you have read in this chapter, do you think that a broken atom of say gold, will have the same properties as an atom of gold?

No, a broken atom of gold will not necessarily retain the same properties of its original atom. The smallest particle of an element is an atom; therefore, dividing it further will only destroy its properties.

#### Question 3:

Helium is the second most abundant element in the universe. However, there are no known compounds of helium. Why?