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Living Science Solutions for Class 7 Science Chapter 3 - Structure Of Matter

Living Science Solutions for Class 7 Science Chapter 3 Structure Of Matter are provided here with simple step-by-step explanations. These solutions for Structure Of Matter are extremely popular among class 7 students for Science Structure Of Matter Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the Living Science Book of class 7 Science Chapter 3 are provided here for you for free. You will also love the ad-free experience on Meritnation’s Living Science Solutions. All Living Science Solutions for class 7 Science are prepared by experts and are 100% accurate.

Page No 34:

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!

Answer:

​(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)

Page No 34:

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.

Answer:


(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.

Page No 34:

Question 3:

There are about 117 different

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

Answer:


(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. 

Page No 34:

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

Answer:


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

Page No 34:

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.

Answer:

(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.

Page No 34:

Question 6:

Which of the following is to a mixture?

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

Answer:

 (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. 

Page No 34:

Question 7:

The valency of which of these elements is zero?

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

Answer:

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



Page No 35:

Question 1:

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

Answer:


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

Page No 35:

Question 2:

The smallest particle of a compound is _________

Answer:

The smallest particle of a compound is a molecule.
Molecule is the smallest particle of a compound which usually exists freely in nature.

Page No 35:

Question 3:

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

Answer:

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

Page No 35:

Question 4:

Name an element whose atoms occur in pairs.

Answer:

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.

Page No 35:

Question 5:

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

Answer:

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

Page No 35:

Question 6:

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

Answer:

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

Page No 35:

Question 7:

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

Answer:

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.

Page No 35:

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

Answer:

​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

Page No 35:

Question 9:

Write the formulae for

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

Answer:

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

Page No 35:

Question 10:

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

Answer:

In the compound with the formula H2X, 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.

Page No 35:

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?

Answer:

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.

Page No 35:

Question 12:

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

Answer:

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,

MgO + 2HCl  MgCl2 + H2O
Now, the number of atoms in the LHS and RHS of the reaction are equal.



Page No 36:

Question 4:

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

Answer:

​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  -----> 2H2O
   Reactants              Product
    LHS                        RHS
 

Page No 36:

Question 5:

Balance the following equations.

(a) Mg + H2SO4 → MgSO4 + H2
(b) CaCO3 + HCI → CaCI2 + H2O + CO2
(c) CaO + H2O → Ca(OH)2
(d) Ag + HNO3 → AgNO3 + H2O + NO2

Answer:

​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) Mg + H2SO4       MgSO4 + H2

(b) CaCO3 + 2HCl  CaCl2 + H2O + CO2
 
(c) CaO + H2O  Ca(OH)2

(d) Ag + 2HNO3 AgNO3 + H2O + NO2

Page No 36:

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).

Answer:

Formula is the symbolic representation of compounds. If we know the symbols, radicals and valencies of different elements, we can write the formula of any compound easily.
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 aluminium chloride would be CaCl2.

Page No 36:

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.

Answer:

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.

Page No 36:

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?

Answer:

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.

Page No 36:

Question 3:

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

Answer:

Helium is an inert element (noble gas). It is highly non-reactive in nature, and can hardly combine with any other element to form any compound. Therefore, even if helium is abundantly present in the universe there are no known compounds of it.



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