CBSE Class 9 Science Advanced Chemistry Matter in Our Surrounding Short Answer Questions Set 01

Short Answer Type

Question 1. A student heats a beaker containing ice and water. He measures the temperature of the content of the beaker as a function of time. Which of the following Fig. would correctly represent the result? Justify your choice.

Answer:
The correct graph is (d).
The beaker contains ice and water together, so the temperature is 0 °C at the start.
The temperature stays at 0 °C while the ice melts, because the heat goes into the latent heat of fusion.
After all the ice has melted, the temperature of the water rises. Graph (d) shows this: a flat part at 0 °C followed by a rise.
Graph (a) wrongly rises at once, (b) and (c) wrongly show cooling.
In simple words: Temperature stays at 0 °C till all ice melts and then rises, as in (d).

Teacher's Note:
1. Mixture of ice and water means start at 0 °C.
2. A flat line at 0 °C is the sign of melting.

Question 2. It is a hot summer day, Priyanshi and Ali are wearing cotton and nylon clothes respectively. Who do you think would be more comfortable and why?
Answer:
Priyanshi, who is wearing cotton, would be more comfortable.
In summer we sweat to cool the body, and the sweat must evaporate to take away heat. Cotton absorbs sweat and exposes it to the air, so it evaporates quickly and cools the body.
Nylon does not absorb sweat, so the sweat stays on the skin and Ali feels sticky and hot.
In simple words: Cotton soaks up sweat, which evaporates and cools the body.

Teacher's Note:
1. Evaporation takes latent heat from the body.
2. Say the word absorb when writing about cotton.

Question 3. What are the general characteristics of particle of matter ?
Answer:
1. Particles of matter are very, very small.
2. There are spaces between the particles.
3. The particles are constantly moving, and move faster as the temperature rises.
4. The particles attract each other with forces that are strong in solids, weaker in liquids and very weak in gases.
In simple words: Matter has tiny, moving particles with gaps between them that attract each other.

Teacher's Note:
1. Learn the four characteristics as a list.
2. Each one has a simple experiment: KMnO₄ dilution, sugar in water, diffusion and breaking chalk, ice and iron.

Question 4. Why do gases have neither a fixed shape nor a fixed volume ?
Answer:
The forces of attraction between gas particles are almost negligible and the spaces between them are very large.
The particles move about randomly at high speed in all directions, so they spread out and fill whatever container they are in.
So a gas takes the shape and the volume of its container.
In simple words: Gas particles move freely, so the gas fills the whole container.

Teacher's Note:
1. Link the answer to weak forces and large gaps.
2. A gas always occupies the whole of its container.

Question 5. A gas exerts pressure on the walls of the containers. How it is possible ?
Answer:
Gas particles move randomly at high speed.
They keep colliding with one another and with the walls of the container.
Each collision gives a small push to the wall, and the large number of such collisions per second gives a steady force on each unit of area of the wall. This force per unit area is the gas pressure.
In simple words: Millions of particles hit the walls every second and push on them.

Teacher's Note:
1. Pressure means force per unit area.
2. Heating makes particles hit harder and more often, so pressure rises.

Question 6. What do you understand by the term latent heat.
Answer:
Latent heat is the heat absorbed or given out by a substance during a change of state, without any change in temperature.
Latent heat of fusion: heat needed to change 1 kg of solid into liquid at its melting point (ice: 3.34 × 10⁵ J/kg).
Latent heat of vaporisation: heat needed to change 1 kg of liquid into vapour at its boiling point (water: about 22.6 × 10⁵ J/kg).
In simple words: It is the hidden heat that changes the state but not the temperature.

Teacher's Note:
1. Latent means hidden, because the thermometer shows no rise.
2. Write the unit J/kg.

Question 7. The freezing point of a substance is the same as the melting point. Justify this statement by a simple example. Is the freezing point of sea water is same as that of rain water ?
Answer:
Example: ice melts at 0 °C to give water, and water freezes at the same 0 °C to give ice. So the melting point of ice and the freezing point of water are the same.
No, the freezing point of sea water is not the same as that of rain water. Sea water has salts dissolved in it, which lower the freezing point, so sea water freezes below 0 °C (about −2 °C), while rain water, which is nearly pure, freezes at about 0 °C.
In simple words: A substance melts and freezes at the same temperature, but impurities such as salt lower it.

Teacher's Note:
1. Use the ice and water example.
2. Impurities lower the freezing point.

Teacher's Note:
1. Use the table form in an exam.
2. Mention surface versus bulk first.

Question 9. Why should wet clothes be spread while drying ?
Answer:
Evaporation is a surface phenomenon, and its rate increases with the surface area.
Spreading the clothes exposes a larger area to the air, and also lets the wind carry away the water vapour, so the clothes dry faster.
In simple words: More surface exposed means faster evaporation.

Teacher's Note:
1. Surface area is one of the factors of evaporation.
2. Hanging them in the sun and wind helps more.

Question 10. Why do we wear cotton clothes in summer ?
Answer:
In summer we sweat more. Sweat evaporates by taking latent heat from the body, which cools us.
Cotton is a good absorber of water, so it absorbs the sweat and exposes it to the air for quick evaporation, which keeps the body cool.
In simple words: Cotton absorbs sweat, so cooling by evaporation is quick.

Teacher's Note:
1. Evaporation cools the surface it leaves.
2. Synthetic cloth does not absorb sweat well.

Question 11. Why water droplets appear in a glass containing ice cold water ?
Answer:
The air around us contains water vapour.
On touching the cold outer surface of the glass, the vapour loses energy and condenses into tiny drops of liquid water, which we see as droplets.
In simple words: Water vapour in the air cools on the cold glass and turns to liquid.

Teacher's Note:
1. The water does not come out of the glass.
2. This is condensation, the opposite of evaporation.

Question 12. Why do gases diffuse very fast ?
Answer:
The particles of a gas move very fast, as they have high kinetic energy.
The spaces between gas particles are very large and the forces between them are very small, so they mix easily with the particles of another gas.
In simple words: Fast-moving particles with large gaps and weak forces mix quickly.

Teacher's Note:
1. Compare with solids, where diffusion is very slow.
2. Heating makes diffusion faster.

Question 13. Solids are generally very heavy while gases are light. Explain.
Answer:
In a solid the particles are packed very closely, so a large mass is present in a small volume, giving a high density.
In a gas the particles are very far apart, so a small mass occupies a large volume, giving an extremely small density.
So solids feel heavy and gases feel light.
In simple words: Closely packed particles give high density; widely spaced particles give low density.

Teacher's Note:
1. Density means mass per unit volume.
2. Say 'for the same volume' while comparing.

Question 14. Define diffusion ? Give one example each of diffusion of (i) a solid into a liquid, (ii) a liquid into another liquid, (iii) a gas into a liquid and (iv) a gas into another gas
Answer:
Diffusion is the intermixing of particles of two different types of matter on their own, from a region of higher concentration to a region of lower concentration.
(i) Solid into liquid: a crystal of potassium permanganate (or copper sulphate) spreading its colour in water.
(ii) Liquid into liquid: spirit (ethanol) mixing with water.
(iii) Gas into liquid: oxygen or carbon dioxide dissolving in water.
(iv) Gas into gas: the smell of perfume (or of cooking) spreading through the air of a room.
In simple words: Diffusion is the self-mixing of two substances.

Teacher's Note:
1. Give four different examples, one for each part.
2. Diffusion is fastest in gases.

Question 15. When a solid melts, its temperature remains the same, so where does the heat energy go ?
Answer:
The heat supplied is used to overcome the forces of attraction between the particles, so that they leave their fixed positions and move freely as a liquid.
It does not raise the kinetic energy of the particles, so the temperature does not rise.
This heat, which is hidden, is the latent heat of fusion.
In simple words: The heat goes into breaking the solid's structure, not into raising the temperature.

Teacher's Note:
1. Latent heat of fusion of ice is 3.34 × 10⁵ J/kg.
2. Water at 0 °C has more energy than ice at 0 °C.

Question 16. Discuss the significance of the boiling point of a liquid.
Answer:
The boiling point is a fixed temperature at which a liquid changes to vapour throughout its bulk, at a given pressure.
1. It is a characteristic property, so it is used to identify a liquid and to check its purity: a pure liquid boils at a sharp fixed temperature.
2. It tells us how volatile a liquid is: a low boiling point means a very volatile liquid.
3. It shows the temperature that cooking water cannot exceed in an open vessel, and changing the pressure (pressure cooker, mountains) changes it.
In simple words: The boiling point identifies a liquid and shows how easily it turns into vapour.

Teacher's Note:
1. Mention purity checking.
2. Boiling point rises with pressure and falls with altitude.

Question 17. Sometimes for performing minor surgery surgeon uses ether, why?
Answer:
Ether is a very volatile liquid, and it evaporates quickly when sprayed on the skin.
Evaporation takes latent heat from the skin, which cools it quickly and makes the area numb, so a minor surgery is possible with little pain.
In simple words: Ether evaporates fast, chills the skin and numbs it.

Teacher's Note:
1. Volatile means it evaporates easily.
2. The same cooling is felt when acetone is poured on the palm.

Question 18. What is the purpose of sipping coffee from a saucer instead of sipping from a glass or cup ?
Answer:
A saucer is wide and shallow, so the hot coffee has a larger surface area exposed to air.
Evaporation is faster from a larger surface, taking heat away, so the coffee cools quickly and can be sipped soon.
In simple words: A wide surface lets the coffee evaporate and cool faster.

Teacher's Note:
1. Rate of evaporation increases with surface area.
2. Blowing on the coffee carries vapour away and also cools it.

Question 19. How is water kept in an earthen pot (matka) become cool during summer ?
Answer:
The earthen pot has many tiny pores in its wall.
Water seeps out through them and evaporates from the outer surface. The evaporation takes latent heat from the water inside the pot, so the water becomes cool.
In simple words: Water evaporating from the pot's surface cools the water inside.

Teacher's Note:
1. Hot, dry weather makes it work better.
2. A pot kept in the shade also cools faster in the breeze.

Question 20. Why a desert cooler is more effective in summer ?
Answer:
In summer the air is hot and dry (low humidity), so water evaporates rapidly from the wet pads of the cooler.
Evaporation takes latent heat from the air passing through, so the air coming out loses its temperature and feels cold.
In simple words: Hot, dry air makes the water in the cooler evaporate quickly, which cools the air.

Teacher's Note:
1. Cooling by evaporation works best in low humidity.
2. In humid weather a desert cooler is less effective.

Question 21. Give two reasons why the rate of diffusion of liquid in another liquid is greater than the rate of diffusion of a solid in a liquid ?
Answer:
1. The kinetic energy of the particles of a liquid is greater than that of the particles of a solid.
2. The interparticle space in a liquid is greater than in a solid, so the particles can move through each other more easily.
In simple words: Liquid particles are more energetic and have larger gaps than solid particles.

Teacher's Note:
1. Compare liquid with solid on kinetic energy and spaces.
2. This question also appears as a solved example in the chapter.