Living Science 2020 2021 for Class 7 Science Chapter 3 - Structure Of Matter

Answer:

The smallest particle of an element is an atom whereas the smallest particle of a compound is a molecule. The difference between an atom and a molecule is that the atom is the smallest unit of an element, which may or may not exist independently whereas a molecule is formed when two or more similar or different types of atoms are joined chemically. e.g. 'O', 'H', and 'C', etc are atoms but O₂, H₂, and CO_2, etc are molecules.


 

Answer:

No, I do not agree. An atom is the smallest particle of an element that can take part in a chemical reaction but may or may not exist independently. e.g. atoms of oxygen, chlorine and a few other elements are unstable independently, so they join to form a molecule which is more stable and independent in nature. That's why elements like oxygen and chlorine are generally found as O₂ and Cl₂. Hence, for all elements, we can not say that the smallest particle 'atom' will always exist independently. 

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No, only a few elements are found in the free state in nature. Most of the elements have the tendency to combine with each other to form compounds. These elements are, therefore, not found in the free state in nature.

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No, the finding was not correct. In compounds, constituent elements are present in a fixed ratio.
 

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Yes, this time the finding was correct. e.g. Air is considered as a mixture of various elements like oxygen, nitrogen, carbon dioxide, etc. Now, we take one sample of air from a normal residential area and another from a high peak mountain. It is sure that both the samples of the mixture(Air) won't contain the same elements in a similar ratio. It's because on high altitudes i.e. on mountains the level of oxygen in the air is very low as compared to a normal residential area. So, the ratio of oxygen with other elements will be different in both samples.

Hence, it is possible that two samples of a mixture of the same elements contain the elements, mixed in a different ratio.

Answer:

The Symbol of oxygen is 'O' and the formula for oxygen is 'O₂'.  The difference between the symbol and formula is that the symbol 'O' is a free oxygen atom whereas O₂ is two oxygen atoms chemically bound to form an oxygen molecule.
 

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'The number of hydrogen atoms that can combine with or be displaced by one atom of an element' is known as the valency of an atom. No, this is not a modern definition. 

Answer:

Let the valency of X in XH₃ is x.

Valency of H = 1 (Given)

As, NH₃ is a neutral molecule, so,

$$\begin{gathered} (\mathrm{Valency} \mathrm{of} \mathrm{X})+(3\times \mathrm{Valency} \mathrm{of} \mathrm{H}) = 0 \\ \Rightarrow \left(\mathrm{x}\right) +(3\times 1)=0 \\ \Rightarrow \mathrm{x}=-3 \end{gathered}$$

So, the valency of X is $-$3.

Let's assume the valency of Y in YX₂ = y

Valency of X = $-$3

As, YX₂ is a neutral molecule, so,

$$\begin{gathered} (\mathrm{Valency} \mathrm{of} \mathrm{Y})+(2\times \mathrm{valency} \mathrm{of} \mathrm{X})=0 \\ \Rightarrow \left(\mathrm{y}\right)+(2\times (-3\left)\right)=0 \\ \Rightarrow \mathrm{y}=6 \end{gathered}$$

So, the valency of Y in YX₂ is 6.

Hence, the valency of X is $-$3 and valency of Y is 6.
 

Answer:

Given reaction: $\mathrm{Na}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{NaOH}+{\mathrm{H}}_2$

In the given reaction, the atoms are not balanced individually on both sides of the reaction. On the reactant side, there are only two hydrogen atoms while on the product side, there are three hydrogen atoms present in total. As, the number of hydrogen atoms is not balanced in the reaction, so the given reaction is an unbalanced reaction.

The balanced chemical equation will be: $2\mathrm{Na}+2{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{NaOH}+{\mathrm{H}}_2$
 

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 N₂ (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)

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.

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(a) elements.
There are 118 elements in the periodic table.

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

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(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.

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. 

Answer:

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

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 (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. 

Answer:

 ​(b) incorrect since it is not balanced

A chemical equation needs to be balanced. The balanced equation for the above reaction is, 
$2 \mathrm{Na} + 2 {\mathrm{H}}_2\mathrm{O} \to 2 \mathrm{NaOH} + {\mathrm{H}}_2$

Answer:

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

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The smallest particle of a compound is a molecule.

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

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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.

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

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

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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.

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​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

Answer:

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

Answer:

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.

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.

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,

$\mathrm{MgO} + 2\mathrm{HCl} \underset{}{\overset{}{\to }} {\mathrm{MgCl}}_{2 }+ {\mathrm{H}}_2\mathrm{O}$
Now, the number of atoms in the LHS and RHS of the reaction are equal.

Answer:

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

Answer:

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). 

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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. 

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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   $\underset{}{\overset{ \mathrm{Heating} }{\to }}$   FeS
 Iron     Sulphur       Iron sulphide

Answer:

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.

Answer:

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.

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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 Al₃ and ​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.

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    $\to$    2H₂O
       Reactants           Product
           LHS                  RHS
 

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) $\mathrm{Mg} + {\mathrm{H}}_2{\mathrm{SO}}_{4 } \underset{}{\to } {\mathrm{MgSO}}_{4 }+ {\mathrm{H}}_2$

(b) ${\mathrm{CaCO}}_3 + 2\mathrm{HCl} \underset{}{\overset{}{\to }} {\mathrm{CaCl}}_{2 }+ {\mathrm{H}}_2\mathrm{O} + {\mathrm{CO}}_2$
 
(c) $\mathrm{Ca} + {\mathrm{H}}_2\mathrm{O} \underset{}{\to } \mathrm{Ca}{\left(\mathrm{OH}\right)}_2$

(d) $\mathrm{Ag} + 2{\mathrm{HNO}}_3 \underset{}{\to {\mathrm{AgNO}}_3 + {\mathrm{H}}_2\mathrm{O} + {\mathrm{NO}}_2}$

Answer:

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 Ca² and​ 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.

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.

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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.

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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.

Answer:

There are two reactants in the given reaction. The first reactant contains one carbon atom and 4 hydrogen atoms, so it is a methane molecule(CH₄). The other reactant contains 2 molecules each containing two atoms of oxygen, so it is oxygen(i.e. 2O₂). On the product side, the first product contains one carbon and two oxygen, so it is carbon dioxide(i.e. CO₂). The other product contains 2 molecules each containing 2 atoms of oxygen and one atom of hydrogen, so it is water(i.e. 2H₂O).

Following reaction is the example of the combustion reaction, the equation for the same can be written as,

${\mathrm{CH}}_4\left(\mathrm{g}\right)+2{\mathrm{O}}_2\left(\mathrm{g}\right)\to {\mathrm{CO}}_2\left(\mathrm{g}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{l}\right)$