Elements And Compounds Around Us
Class 8th Science Term 1 Tamilnadu Board Solution
- Water is made up of two elements, hydrogen and oxygen. Water is a liquid, whereas hydrogen…
- Using the elements enclosed in the bottle, frame formulae for some compounds.…
- Find the valency of the underlined element in the given formulae. i) H2O ii) KCl iii)…
- Write the chemical name of the given formulae. i) MgO ii) HCl iii) NH3 iv) ZnO v) NaCl…
- Write the names of the planets (Greek God) after which these elements are named. a)…
Evaluation
Question 1.Water is made up of two elements, hydrogen and oxygen. Water is a liquid, whereas hydrogen and oxygen are gases. Hydrogen catches fire easily. Oxygen helps in burning. Water is used to put off the fire.
From the above information, answer the following questions.
a) What are the elements present in water?
b) In which state do these elements exist?
c) Write the property of hydrogen.
d) Write the property of oxygen.
e) Do the properties of water differ from hydrogen and oxygen?
Answer:
a) Water is made up of two elements i.e. hydrogen & oxygen.
Water is a compound of two hydrogen atoms and one oxygen atom in the ratio 2:1 by volume.
2H2 + O2→ 2H2O
b) Hydrogen & oxygen exists in gases state.
Hydrogen & oxygen exists in gases state at standard temperature and pressure. This is because the intermolecular forces of attraction between the particles are weak.
c) Hydrogen is colourless, odourless gas and it is lighter than air. It is highly flammable it readily catches fire.
d) Oxygen is colourless, odourless & tasteless gas. Oxygen is not flammable; it makes other things ignite at low temperature. But oxygen itself does not catch fire.
e) Yes, the properties of water are different from its constituent particles, i.e. hydrogen & oxygen.
Water is made up of two elements hydrogen & oxygen and is called a compound. Usually the properties of compounds are different from the properties of their constituent elements. Water exists in a liquid state & it acts as a fire extinguisher whereas hydrogen is highly inflammable gas and oxygen supports combustion.
Question 2.
Using the elements enclosed in the bottle, frame formulae for some compounds.
Answer:
1. Components= Hydrogen & Oxygen
Formulae= H2O
Explanation: Water is a compound of two hydrogen atoms and one oxygen atom in the ratio 2:1 by volume.
2H2 + O2→ 2H2O
2. Components = Sodium & chlorine
Formulae = NaCl
Explanation: One sodium atom combines with one chlorine atom to form sodium chloride.
Na + Cl → NaCl
3. Components = Carbon, hydrogen & oxygen
Formulae = C12H22O11
Explanation: Sugar is a compound which is made up of 12 carbon atoms, 22 hydrogen atoms & 11 oxygen atoms.
4. Components = Sodium, carbon, hydrogen & oxygen
Formulae = NaHCO3
Explanation: Baking soda is sodium hydrogen carbonate
Na2CO3 + CO2 + H2O → 2NaHCO3
5. Components = Calcium, oxygen & chlorine
Formulae = CaOCl2
Explanation: Bleaching powder is formed by passing chlorine gas over slaked lime
Ca(OH)2 + Cl2 → CaOCl2 + H2O
6. Components = Calcium & oxygen
Formulae = CaO
Explanation: Calcium oxide is formed by thermal decomposition of calcium carbonate.
CaCO3→ CaO + CO2
7. Components = Calcium, oxygen & hydrogen
Formulae = Ca(OH)2
Explanation: It is obtained by combining calcium oxide with water.
CaO + H2O → Ca(OH)2
8. Components = Calcium, carbon & oxygen
Formulae = CaCO3
Explanation: Calcium carbonate is formed by passing carbon dioxide into a solution of calcium hydroxide.
Ca(OH)2 + CO2→ CaCO3 + H2O
9. Components = Magnesium & chlorine
Formulae = MgCl2
Explanation: It indicates two atoms of chlorine chemically combined with magnesium to form magnesium chloride.
Mg + Cl2→ MgCl2
10. Components = Magnesium & oxygen
Formulae = MgO
Explanation: It indicates one oxygen molecule chemically combined with magnesium to form magnesium oxide.
2Mg + O2→ MgO
11. Components = Potassium & chlorine
Formulae = KCl
Explanation: One potassium atom combines with one chlorine atom to form potassium chloride.
K + Cl → KCl
12. Components = Hydrogen & chlorine
Formulae = HCl
Explanation: One hydrogen atom combines with one chlorine atom to form hydrogen chloride.
H + Cl → HCl
13. Components = Calcium & Chlorine
Formulae = CaCl2
Explanation: One calcium atom combines with two chlorine atom to form calcium chloride.
Ca + Cl2→ CaCl2
Question 3.
Find the valency of the underlined element in the given formulae.
i) H2O ii) KCl iii) Al2O3 iv) Fe2O3 v) CH4
Answer:
i) H2O
Valency of oxygen is 2
Let the valency of oxygen is x
We know that,
Charge on hydrogen is +1
Therefore, 2⨉1 + x = 0
2+x=0
x= -2
As the picture shows,
ii) KCl
Valency of potassium is 1
Let the valency of potassium is x
We know that,
Charge on chlorine is -1
Therefore, x + (-1) = 0
x – 1 =0
x= 1
As the picture shows,
iii) Al2O3
Valency of aluminium is 3
Let the valency of aluminium is x
We know that,
Charge on oxygen is -2
Therefore, 2⨉x + 3⨉(-2) = 0
2x – 6 =0
2x= 6
x = 
x = 3
iv) Fe2O3
Valency of iron is 3
Let the valency of iron is x
We know that,
Charge on oxygen is -2
Therefore, 2⨉x + 3⨉(-2) = 0
2x – 6 =0
2x= 6
x =
x = 3
v) CH4
Valency of carbon is 4
Let the valency of carbon is x
We know that,
Charge on hydrogen is +1
Therefore, x+4⨉1 = 0
x+4=0
x= -4
Question 4.
Write the chemical name of the given formulae.
i) MgO ii) HCl iii) NH3 iv) ZnO v) NaCl
Answer:
i) MgO
Magnesium oxide
Magnesium combines with oxygen molecule to form magnesium oxide.
2Mg + O2→ 2MgO
Magnesium + Oxygen → Magnesium oxide
ii) HCl
Hydrochloric Acid
Hydrogen combines with chlorine to form hydrochloric acid.
H + Cl → HCl
Hydrogen + Chlorine → Hydrochloric Acid
iii) NH3
Ammonia
One nitrogen molecule combines with three hydrogen molecules to form ammonia.
N2 + 3H2→ NH3
Nitrogen + Hydrogen → Ammonia
iv) ZnO
Zinc oxide
Zinc combines with oxygen to form zinc oxide.
2Zn + O2→ 2ZnO
Zinc + Oxygen → Zinc oxide
v) NaCl
Sodium chloride
Sodium combines with chlorine to form Sodium chloride.
Na + Cl → NaCl
Sodium + Chlorine → Sodium chloride
Question 5.
Write the names of the planets (Greek God) after which these elements are named.
a) Plutonium b) Neptunium c) Uranium
Answer:
The following diagram shows the solar system:
