Chapter 2 Element Compound And Chemical Reaction Nature Of Matter Short Answer Type Questions
Question 1. What do you mean by a substance? Substances are those which can be sensed by our sense organs.
Answer:
Substance:
They have mass and occupy a certain volume and have inertia.
Question 2. What do you mean by change of state?
Answer:
Change of state:
The transformation of matter from one physical state to another is known as a change of state. For example during melting, a solid is transformed into liquid.
Question 3. What do you mean by sublimation?
Answer:
Sublimation:
When heated, solid melts and is transformed into liquid. Further heating converts liquid into vapour.
But there are some substances, which when heated, are directly transformed into vapour without passing through the liquid state.
The transformation of a solid directly into vapour on being heated is called sublimation.
Question 4. What do you mean by the physical property of a substance?
Answer:
Physical property of a substance:
The property of a substance, which gives us an idea about the nature and external condition of the substance is called its physical property.
It cannot provide us with an idea about its internal structure (or “molecular-level information”). Example of physical properties is – physical state, colour, odour, melting point, boiling point, magnetic property, solubility in a particular solvent, etc.
Question 5. What do you mean by the chemical property of a substance?
Answer:
Chemical property of a substance:
The property of a substance which decides the tendency and capacity of that substance to react with other substances is called its chemical properties.
For example, the sulphur burns in the air to produce sulphur dioxide (SO2); zinc reacts with dilute sulphuric acid to produce hydrogen gas.
Read And Learn More WBBSE Solutions For Class 8 School Science Short Answer Type Questions
Question 6. Among three jars, one contains a piece of zinc, one contains petrol and the other contains nitrogen. How can you identify them?
Answer:
Zinc is solid, petrol is liquid and nitrogen is gas. So from their physical state, each of them can be identified.
Question 7. Among two jars, one contains camphor and the other contains naphthalene. How can you identify them?
Answer:
Camphor and naphthalene, both are volatile solids. But each of them has its characteristics odour. By this odour, they can be identified.
Question 8. Among three containers, one contains mustard oil, the other coconut oil and the third contains diesel. How can you identify them?
Answer:
Mustard oil, coconut oil and diesel, all have their characteristic odours. So by their odour, they can be individually identified.
Question 9. An element X forms an oxide X20. An aqueous solution of X20 turns red litmus paper blue. What is the nature of the oxide?
Answer:
An element X forms an oxide X20. An aqueous solution of X20 turns red litmus paper blue.
State whether element X is a metal or a non-metal. Since the aqueous solution of X2O turns red litmus paper blue, X2O is a basic oxide.
Non-metallic oxides are generally either acidic or neutral oxides. Since X2O is a basic oxide, hence element X must be a metal since metals form basic oxides.
Question 10. Four containers containing the tip of a pencil, glycerine, a piece of iron wire and water are supplied to you. How can you identify each of them?
Answer:
Four containers containing the tip of a pencil, glycerine, a piece of iron wire and water are supplied to you.
The tip of the pencil is made of graphite. Graphite and iron wire are both solid while water and glycerine are liquid.
Graphite is soft and slippery but iron wire is hard and not slippery. So they can be distinguished from one another.
Glycerine is more viscous and slippery, but water is not slippery. So they can be distinguished from each other.
Question 11. You are provided with a gas jar containing nitric oxide. What will happen if the gas jar is opened?
Answer:
You are provided with a gas jar containing nitric oxide.
When the gas jar containing nitric oxide is opened, brown fumes are formed. This is because nitric oxide reacts with oxygen present in the air, and brown-coloured nitrogen dioxide gas is formed.
⇒ \(2 \mathrm{NO}+\mathrm{O}_2 \rightarrow 2 \mathrm{NO}_2\)
Question 12. Take two glasses filled with water. In one glass some sugar is added and in another glass, calcium oxide is added. What will you observe?
Answer:
Take two glasses filled with water. In one glass some sugar is added and in another glass, calcium oxide is added.
When sugar is added to a glass of water and stirred well, sugar dissolves in water. The temperature of the glass of water remains the same after the dissolution of sugar.
When calcium oxide is added to water, the temperature of the glass of water increases. This happens due to an exothermic reaction between calcium oxide and water.
⇒ \(\mathrm{CaO}+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Ca}(\mathrm{OH})_2+\text { heat }\)
Question 13. Some amounts of common salt and sugar are taken on a spoon and are separately heated. What will you observe?
Answer:
Some amounts of common salt and sugar are taken on a spoon and are separately heated.
When some amount of common salt (i.e., sodium chloride) is taken on a spoon and is strongly heated, no visible change (of colour or odour or physical state etc.) is observed.
When some sugar is taken on a spoon and is strongly heated, the sugar first turns brown and then becomes black. In the case of sugar, water is first liberated from it and ultimately only black carbon is left on the spoon.
⇒ \(\mathrm{C}_{12} \mathrm{H}_{22} \mathrm{O}_{11} \rightarrow 12 \mathrm{C}+11 \mathrm{H}_2 \mathrm{O}\)
Question 14. How do sodium, potassium and calcium react with cold water?
Answer:
Sodium and potassium react vigorously with cold water and form hydrogen gas. The gas catches fire and burns with blue flame by the heat produced during the reaction.
⇒ \(\begin{gathered}
2 \mathrm{Na}+2 \mathrm{H}_2 \mathrm{O} \rightarrow 2 \mathrm{NaOH}+\mathrm{H}_2 \uparrow \\
2 \mathrm{~K}+2 \mathrm{H}_2 \mathrm{O} \rightarrow 2 \mathrm{KOH}+\mathrm{H}_2 \uparrow
\end{gathered}\)
Calcium reacts less vigorously with cold water.
⇒ \(\mathrm{Ca}+2 \mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Ca}(\mathrm{OH})_2+\mathrm{H}_2 \uparrow\)
Question 15. How do iron, aluminium and zinc react with water?
Answer:
Iron, aluminium and zinc do not react with either cold or hot water. They react with steam to produce metallic oxide and hydrogen gas.
⇒ \(\begin{aligned}
2 \mathrm{Fe}+3 \mathrm{H}_2 \mathrm{O} & \rightarrow \mathrm{Fe}_2 \mathrm{O}_3+3 \mathrm{H}_2 \uparrow \\
2 \mathrm{Al}+3 \mathrm{H}_2 \mathrm{O} & \rightarrow \mathrm{Al}_2 \mathrm{O}_3+3 \mathrm{H}_2 \uparrow \\
\mathrm{Zn}+\mathrm{H}_2 \mathrm{O} & \rightarrow \mathrm{ZnO}+\mathrm{H}_2 \uparrow
\end{aligned}\)
Question 16. If you are supplied with some amount of iron powder and some amount of zinc powder, how can you distinguish them?
Answer:
Iron powder and zinc powder – both are solids. But iron is a magnetic material while zinc is not. So, when a magnet is brought near iron powder, they are attracted towards it.
But when a magnet is brought near the powder, they are not at all attracted by the magnet.
Question 17. Mention the role of iron in the oxidation-reduction of tissues and cells. Name two states of matter which are fluid.
Answer:
Several proteins are required for the release of energy in mitochondria by using oxygen inhaled through breathing. Iron is required to make these proteins. Liquids and gases are fluids.
Question 18. Why gases do not have definite volume and shape but solids do?
Answer:
The molecules of solids are very closely packed with negligible intermolecular space. The intermolecular force of attraction among solid molecules is maximum.
Hence solids possess definite shapes and volumes. Gas molecules are loosely packed with the negligible intermolecular force of attraction. The intermolecular space in gas molecules is the maximum among the three states of matter. Hence gases neither have a definite volume nor shape of their own.
Question 19. Why is sodium kept immersed in kerosene oil?
Answer:
Sodium is a highly reactive metal which reacts vigorously with cold water to form sodium hydroxide and hydrogen gas.
⇒ \(2 \mathrm{Na}+2 \mathrm{H}_2 \mathrm{O} \rightarrow 2 \mathrm{NaOH}+\mathrm{H}_2\)
It also reacts with oxygen in the air at room temperature to form sodium oxide.
⇒ \(4 \mathrm{Na}+\mathrm{O}_2 \rightarrow 2 \mathrm{Na}_2 \mathrm{O}\)
Hence, sodium is stored under kerosene oil to prevent its reaction with oxygen, moisture and carbon dioxide in the air.
Question 20. Give two examples of metalloids and show that they exhibit properties of both metals and non-metals
Answer:
| Metalloid | Metallic property | non- metallic property |
| Arsenic | Fair conductor of electricity | Arsenic hydride (AsH3) is a weak base |
| Selenium | Good conductor of electricity at 200°C. | It exists in one non-metallic allotropic form. |
Question 21. When calcium metal is added to water, the gas evolved does not catch fire but the same gas evolved on adding sodium metal to water catches fire-why?
Answer:
Sodium metal reacts vigorously with cold water and produces hydrogen gas. This is a highly exothermic reaction and the gas catches fire by the heat evolved in the reaction.
Calcium reacts less violently with cold water and less heat is evolved during the reaction. This cannot make the hydrogen gas burn.
Question 22. Which metal foil is used for packing some of the medicine tablets? Both physical and chemical changes occur when ammonium chloride is heated-explain.
Answer:
Aluminium foil is used for packing medicine tablets.
| Process | Physical change involved | Chemical change involved |
| Heating of ammonium chloride (NH4CI) | Ammonium chloride sublimes | Decomposition into ammonia and hydrogen chloride. |