Class 9 Science Chapter 1 Question Answer | Matter in Our Surroundings | English Medium | ASSEB

Matter in Our Surroundings

Welcome to HSLC Guru! This study guide covers Class 9 Science Chapter 1 — Matter in Our Surroundings for the ASSEB (Assam State School Education Board) syllabus. Here you will find a clear chapter summary, complete textbook question-answers, additional MCQs, fill in the blanks, true/false items, and a glossary of key terms — designed to help students master the chapter and score better in examinations.

Summary

Anything that occupies space and has mass is called matter. The air we breathe, the water we drink, the food we eat, stones, clouds, plants, and even our own bodies — all are forms of matter. Early Indian philosophers classified matter into five basic elements known as the Panch Tatva — air, earth, fire, sky, and water. Modern scientists, however, classify matter on the basis of its physical state (solid, liquid, gas) and chemical nature (elements, compounds, mixtures).

Matter is made up of extremely tiny particles. These particles are very small in size, have spaces between them, are in continuous random motion, attract each other with a force called the force of attraction, and the kinetic energy of the particles increases with rise in temperature. The mixing of particles of two different types of matter on their own is called diffusion. Diffusion becomes faster on heating because particles gain kinetic energy. Matter has mass (the amount of matter contained in a body) and volume (the space it occupies).

Matter exists in three common physical states — solid (definite shape and volume, high density, incompressible, particles tightly packed), liquid (definite volume but no fixed shape, takes the shape of its container, less dense than solids, slightly compressible), and gas (neither definite shape nor definite volume, highly compressible, very low density, particles move randomly at high speeds). The states of matter can be inter-converted by changing temperature or pressure. On heating, a solid changes to a liquid at its melting point (the process is called fusion), and the liquid changes to gas at its boiling point (called vaporisation). On cooling, the reverse changes — condensation and freezing — take place.

The amount of heat energy required to change 1 kg of a solid into liquid at its melting point at atmospheric pressure is called the latent heat of fusion, while the amount needed to convert 1 kg of liquid into vapour at its boiling point is the latent heat of vaporisation. The change of state directly from solid to gas without passing through the liquid state (e.g., camphor, naphthalene balls, dry ice) is known as sublimation. The phenomenon of change of a liquid into vapour at any temperature below its boiling point is called evaporation. Evaporation depends on the surface area, temperature, humidity and wind speed. Evaporation causes cooling because particles absorb energy from the surroundings as latent heat to overcome the forces of attraction. Apart from the three common states, two more states exist — plasma (super-energetic ionised gas, found in stars, fluorescent tubes, neon signs) and Bose-Einstein Condensate (BEC) (formed by cooling a gas of extremely low density to super-low temperatures).

Textbook Questions and Answers

1-Mark Questions

Q1. What is matter?

Answer: Anything that occupies space and has mass is called matter.

Q2. Name the five basic elements of matter according to early Indian philosophers.

Answer: Air, earth, fire, sky and water — collectively called the Panch Tatva.

Q3. Define diffusion.

Answer: The intermixing of particles of two different types of matter on their own is called diffusion.

Q4. What is the melting point of ice?

Answer: The melting point of ice is 273.15 K (0°C) at atmospheric pressure.

Q5. What is the boiling point of water?

Answer: The boiling point of water is 373.15 K (100°C) at one atmospheric pressure.

Q6. Give one example of a substance that sublimes.

Answer: Camphor (other examples — naphthalene balls, ammonium chloride, dry ice).

Q7. Convert 25°C to Kelvin.

Answer: K = °C + 273.15 = 25 + 273.15 = 298.15 K.

Q8. Why do we feel cool after sweating?

Answer: Sweat evaporates from our skin, taking heat (latent heat of vaporisation) from the body, so we feel cool.

Q9. What is the SI unit of temperature?

Answer: The SI unit of temperature is the Kelvin (K).

Q10. Name the state of matter present inside a fluorescent tube.

Answer: Plasma.

2-3 Mark Questions

Q1. List the characteristics of particles of matter.

Answer: The main characteristics of particles of matter are: (i) particles of matter are extremely small in size; (ii) they have spaces between them; (iii) they are continuously moving (possess kinetic energy); (iv) they attract each other with a force of attraction; (v) the kinetic energy of particles increases with rise in temperature.

Q2. Differentiate between a solid, a liquid and a gas on the basis of shape, volume, density and compressibility.

Answer: Solid: definite shape and volume; high density; almost incompressible. Liquid: no fixed shape but definite volume; medium density; slightly compressible. Gas: neither fixed shape nor fixed volume; very low density; highly compressible.

Q3. Why do gases diffuse very fast?

Answer: In gases, particles are far apart with very large spaces between them and the force of attraction between particles is negligible. They also possess very high kinetic energy and move randomly at high speeds, so they spread (diffuse) into another gas extremely quickly.

Q4. What is meant by latent heat of fusion and latent heat of vaporisation?

Answer: Latent heat of fusion is the amount of heat energy required to change 1 kg of a solid into liquid at its melting point at atmospheric pressure without any change in temperature. Latent heat of vaporisation is the amount of heat energy required to change 1 kg of a liquid into vapour at its boiling point at atmospheric pressure without any change in temperature.

Q5. List four factors that affect the rate of evaporation.

Answer: The four factors are: (i) Surface area — the larger the surface area, the faster the evaporation. (ii) Temperature — evaporation increases with rise in temperature because more particles get enough kinetic energy to escape. (iii) Humidity — when the air contains less moisture, evaporation is faster. (iv) Wind speed — higher wind speed carries away water particles, increasing evaporation.

Q6. Why does a desert cooler cool better on a hot dry day?

Answer: A desert cooler works on the principle of evaporation. On a hot dry day, the temperature is high and the humidity (water content) of the air is low. Both of these factors increase the rate of evaporation of water from the cooler pads. As the water evaporates, it absorbs latent heat from the surrounding air, cooling it down effectively.

5-6 Mark Questions

Q1. Explain the three states of matter on the basis of arrangement of particles, intermolecular forces and movement.

Answer: Matter exists in three common states — solid, liquid and gas — depending on how its particles are arranged.

(a) Solid state: Particles are very closely packed in a regular pattern. Spaces between particles are minimum and the force of attraction between them is very strong. Particles can only vibrate about their fixed positions and cannot move freely. As a result, solids have a definite shape, definite volume, high density, and are almost incompressible.

(b) Liquid state: Particles are close to each other but the arrangement is not regular. Spaces between particles are larger than in solids, and the force of attraction is weaker than in solids but stronger than in gases. Particles can slide past one another, so liquids flow and take the shape of the container. They have a definite volume but no definite shape, and they are slightly compressible.

(c) Gaseous state: Particles are very far apart, the spaces between them are very large, and the force of attraction is negligible. Particles move randomly in all directions at high speeds, colliding with one another and with the walls of the container. Hence gases have neither a fixed shape nor a fixed volume, are highly compressible, and have very low density.

Q2. Describe an activity to show that the particles of matter are very small and have spaces between them.

Answer: Activity 1 — particles are small: Take 2-3 crystals of potassium permanganate and dissolve them in 100 mL of water. Take out 10 mL of this solution and add it to another 90 mL of water. Continue diluting in the same way 5-6 times. Even after several dilutions, the water remains coloured. This shows that there are millions of tiny particles in just one crystal of potassium permanganate, proving that particles of matter are extremely small.

Activity 2 — particles have spaces: Take a 100 mL beaker and fill half of it with water. Mark the level of water. Now dissolve some salt or sugar in the water and stir well. Observe the water level — it does not rise much, although salt has been added. This shows that the particles of salt occupy the spaces between the particles of water, proving that there are spaces between particles of matter.

Q3. Explain the inter-conversion of states of matter with the effect of temperature and pressure. Draw the schematic flow.

Answer: The three states of matter can be inter-converted by changing temperature or pressure.

Effect of temperature: When a solid is heated, its particles gain kinetic energy and start vibrating faster. At a certain temperature called the melting point, the particles overcome the forces of attraction and the solid changes into a liquid. This is called fusion or melting. On further heating, the kinetic energy of liquid particles increases until they overcome the remaining forces of attraction at the boiling point and the liquid changes to gas (vaporisation). On cooling, the reverse processes occur — gas → liquid (condensation) and liquid → solid (freezing/solidification).

Effect of pressure: By applying high pressure and reducing temperature, gases can be liquefied. For example, LPG (liquefied petroleum gas) and CNG (compressed natural gas) are stored under high pressure. Compressing brings the particles closer, increasing the force of attraction, and the gas turns into liquid.

Schematic flow: Solid ⇌ Liquid ⇌ Gas (heat in forward direction; cool in reverse). Solid ⇌ Gas directly is sublimation/deposition.

Q4. Define evaporation. Why does evaporation cause cooling? Mention any four factors affecting evaporation.

Answer: Evaporation is the phenomenon of change of a liquid into vapour at any temperature below its boiling point. The particles at the surface of a liquid that have higher than average kinetic energy escape into the air as vapour.

Cooling caused by evaporation: During evaporation, the liquid particles need energy to overcome the force of attraction between them. They take this energy (the latent heat of vaporisation) from the surroundings. As a result, the surroundings lose heat and become cool. This is why we feel cool when we apply spirit on our hand or when sweat evaporates from our body.

Factors affecting evaporation: (i) Surface area — increase in surface area increases evaporation, that is why clothes are spread out to dry. (ii) Temperature — higher temperature gives more particles enough energy to escape, so evaporation is faster. (iii) Humidity — if the surrounding air already contains a lot of water vapour (high humidity), evaporation slows down. (iv) Wind speed — higher wind speed carries away the water particles from above the liquid, increasing the rate of evaporation.

Q5. Write short notes on (a) Sublimation (b) Plasma (c) Bose-Einstein Condensate (BEC).

Answer: (a) Sublimation: The change of state directly from solid to gas without passing through the liquid state on heating, and the reverse process (gas → solid) on cooling, is called sublimation. Examples — camphor, naphthalene balls, ammonium chloride, dry ice (solid CO₂) and iodine.

(b) Plasma: Plasma is the fourth state of matter consisting of super-energetic and super-excited ionised particles. It is found in the Sun and stars, in fluorescent tubes (helium gas), and in neon sign bulbs (neon gas). The particles emit characteristic colours of light when energised.

(c) Bose-Einstein Condensate (BEC): BEC is the fifth state of matter, formed by cooling a gas of extremely low density to super-low temperatures (close to absolute zero, 0 K). It was theoretically predicted by Indian physicist Satyendra Nath Bose and Albert Einstein. The 2001 Nobel Prize in Physics was awarded to Eric A. Cornell, Wolfgang Ketterle and Carl E. Wieman for achieving BEC experimentally in 1995.

Additional Multiple Choice Questions

Q1. Which of the following has the highest force of attraction between particles?

(a) Solid (b) Liquid (c) Gas (d) Plasma

Answer: (a) Solid.

Q2. The temperature at which a solid melts to become a liquid at atmospheric pressure is called its —

(a) boiling point (b) melting point (c) freezing point (d) condensation point

Answer: (b) melting point.

Q3. 0°C in Kelvin scale is —

(a) 273.15 K (b) 0 K (c) 100 K (d) 373.15 K

Answer: (a) 273.15 K.

Q4. Which of the following does NOT undergo sublimation?

(a) Camphor (b) Naphthalene (c) Common salt (d) Ammonium chloride

Answer: (c) Common salt.

Q5. Which factor does NOT affect evaporation?

(a) Temperature (b) Humidity (c) Wind speed (d) Colour of liquid

Answer: (d) Colour of liquid.

Q6. The fifth state of matter is —

(a) Plasma (b) BEC (c) Liquid crystal (d) Vapour

Answer: (b) BEC (Bose-Einstein Condensate).

Q7. Dry ice is the solid form of —

(a) Water (b) Carbon dioxide (c) Oxygen (d) Nitrogen

Answer: (b) Carbon dioxide.

Q8. Which state of matter is found inside a neon sign bulb?

(a) Solid (b) Liquid (c) Gas (d) Plasma

Answer: (d) Plasma.

Q9. The amount of water vapour present in air is called —

(a) Density (b) Humidity (c) Pressure (d) Volume

Answer: (b) Humidity.

Q10. A gas can be liquefied by —

(a) increasing temperature (b) decreasing pressure (c) decreasing temperature and increasing pressure (d) none of these

Answer: (c) decreasing temperature and increasing pressure.

Fill in the Blanks

Q1. Anything that has mass and occupies space is called __________.

Answer: matter.

Q2. The intermixing of particles of two different types of matter on their own is called __________.

Answer: diffusion.

Q3. The change of state directly from solid to gas is known as __________.

Answer: sublimation.

Q4. The SI unit of temperature is __________.

Answer: Kelvin (K).

Q5. Evaporation causes __________ of the surroundings.

Answer: cooling.

True or False

Q1. Particles of matter are stationary.

Answer: False — they are in continuous random motion.

Q2. Gases can be compressed easily.

Answer: True.

Q3. The boiling point of water is 273.15 K.

Answer: False — it is 373.15 K (100°C).

Q4. Plasma is found in fluorescent tubes and neon signs.