This chapter, 'Matter in Our Surroundings,' for Class 9 Science, delves into the fundamental concept of matter. It defines matter as anything occupying space and possessing mass, rooted in the ancient Indian concept of Panch tatva. The notes explore the characteristics of matter's particles, including their tiny size, the presence of vacant spaces, continuous motion, and the forces of attraction holding them together. The chapter then elaborates on the five states of matter: solid, liquid, gas, plasma, and Bose-Einstein condensate, detailing their unique properties and microscopic explanations. It also covers the interconversion of states of matter, the effects of temperature and pressure, sublimation, latent heat, evaporation, and boiling, along with the Kelvin and Celsius scales for temperature measurement.
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Anything that occupies space and has mass, and is felt by senses, is called matter. Matter is the form of five basic elements, the Panch tatva – air, earth, fire, sky, and water.
The states of matter can be classified based on:
SOLID |
 |
| Fixed shape and definite volume. |
| Inter-particle distances are the smallest. |
| Incompressible. |
| High density and do not diffuse easily. |
| Inter-particle forces of attraction are the strongest. |
| Constituent particles are very closely packed. |
LIQUID |
 |
| Not a fixed shape but a fixed volume. |
| Inter-particle distances are larger than in solids. |
| Almost incompressible. |
| Density is lower than solids and they can diffuse. |
| Inter-particle forces of attraction are weaker than in solids. |
| Constituent particles are less closely packed than in solids. |
GAS |
 |
| Neither a fixed shape nor a fixed volume. |
| Inter-particle distances are the largest. |
| Highly compressible. |
| Density is the least and they diffuse readily. |
| Inter-particle forces of attraction are the weakest. |
| Constituent particles are free to move about. |
A plasma is an ionized gas. It is a very good conductor of electricity and is affected by magnetic fields. Like gases, plasma has an indefinite shape and an indefinite volume. Example: Ionized gas in fluorescent tubes or stars.
A Bose-Einstein condensate (BEC) is a state of matter that can arise at very low temperatures, close to absolute zero. Scientists who worked with BEC received a Nobel Prize in 1995. In a BEC, atoms cooled to near absolute zero lose all individual identity and behave as a single quantum entity.
1. Solids have a definite shape and a definite volume because the particles are locked into fixed positions.
2. Solids do not flow easily because the particles cannot move or slide past one another.
3. Solids are not easily compressible because there is very little free space between particles.
1. Liquids have a definite volume because there is little free space between particles, making them almost incompressible.
2. Liquids have an indefinite shape because the particles can slide past one another.
3. Liquids flow easily because the particles can move/slide past one another.
1. Gases are easily compressible because there is a great deal of free space between particles.
2. Gases flow very easily because the particles randomly move past one another.
3. Gases have an indefinite shape and an indefinite volume because the particles can move past one another with very weak forces of attraction.
1. Plasmas have an indefinite shape and an indefinite volume because the charged particles can move past one another.
2. Plasmas are easily compressible due to the large spaces between ionized particles.
3. Plasmas are good conductors of electricity and are affected by magnetic fields because they are composed of ions.
1. Particles in a BEC are less energetic than in solids because they exist at extremely low temperatures.
2. Particles in a BEC are indistinguishable as they are cooled to such low temperatures that they occupy the same quantum state.
3. BEC exhibits superfluidity because particles can flow without friction.
Matter can change its state. For example, water can exist in three states: solid (ice), liquid (water), and gas (water vapour).
Sublimation is the changing of a solid directly into vapours on heating, and vapours directly into solid on cooling. Examples include ammonium chloride, camphor, and iodine.
On heating a solid, the kinetic energy of its particles increases. This overcomes the forces of attraction between particles, causing the solid to melt and convert into a liquid. The temperature at which a solid melts to become a liquid at atmospheric pressure is called its melting point. The melting point of ice is 273.16 K. The process of melting is also known as fusion.
Increasing or decreasing the pressure can change the state of matter. Applying pressure and reducing temperature can liquefy gases. Solid carbon dioxide (CO2) is stored under high pressure. On decreasing the pressure to 1 atmosphere, solid CO2 gets converted directly to gaseous state without becoming liquid. This is why solid carbon dioxide is also known as dry ice.
Latent heat is the hidden heat that breaks the forces of attraction between molecules during a change of state.
| Fusion | Vaporisation |
| Heat energy required to change 1 kg of solid into liquid at its melting point. | Heat energy required to change 1 kg of liquid to gas at atmospheric pressure at its boiling point. |
Particles of matter are always moving and never at rest. At any given temperature, particles in a gas, liquid, or solid have different amounts of kinetic energy. In liquids, a small fraction of particles at the surface, having higher kinetic energy, can escape the forces of attraction and convert into vapour. This phenomenon of changing a liquid into vapours at any temperature below its boiling point is called evaporation.
The particles of the liquid absorb energy from the surroundings to regain the energy lost during evaporation, which results in cooling.
Boiling is a bulk phenomenon where particles from the entire body of the liquid change into the vapour state at a specific temperature. Evaporation is a surface phenomenon where particles from the surface gain enough energy to overcome intermolecular forces and change into the vapour state at any temperature.
Kelvin (K) is the SI unit of temperature. The relationship between Celsius (°C) and Kelvin (K) is: T(K) = T(°C) + 273.16. For most calculations, T(K) = T(°C) + 273 is used. The Kelvin scale is preferred as it always has positive values.
Atmosphere (atm) is a unit of pressure. The SI unit of pressure is Pascal (Pa). 1 atmosphere = 1.01 × 105 Pa. The pressure exerted by the atmosphere is called atmospheric pressure. Normal atmospheric pressure at sea level is 1 atmosphere.
Q.1 Define matter. Q.2 What happens if you put copper sulphate crystals in water? Q.3 A substance has a definite volume but no definite shape. State whether this substance is a solid, a liquid, or a gas. Q.4 Arrange the following substances in increasing order of force of attraction between the particles: (a) Milk (b) Salt (c) Oxygen. Q.5 A substance has neither a fixed shape nor a fixed volume. State whether it is a solid, a liquid, or a gas. Q.6 The melting point of a substance is below the room temperature. Predict its physical state. Q.7 What is vapour? Q.8 Name the temperature at which the solid and liquid states of a substance can coexist. Q.9 What is the effect of pressure on the boiling point? Q.10 Name any two substances which sublime. Q.11 Define Condensation. Q.12 For any substance, why does the temperature remain constant during the change of state? Q.13 Which is the slower process, Evaporation or Boiling? Q.14 State the effect of surface area on the rate of evaporation. Q.15 Why are we able to sip hot tea faster from a saucer rather than from a cup? Q.16 What is the SI unit of temperature? Q.17 Why is the Kelvin scale of temperature regarded as a better scale than Celsius?
1. Pressure on the surface of a gas is increased. What will happen to the inter-particle forces? 2. Name the three basic states of matter. 3. What happens when a liquid is heated? 4. A gas can exert pressure on the walls of the container. Assign a reason. 5. Convert the following temperatures to the Kelvin Scale: (a) 100°C (b) 37°C 6. What is meant by density? 7. Give the characteristics of the particles of matter. 8. Water droplets seen on the outer surface of a glass containing ice-cold water are due to _______________. (Condensation) 9. Change of gaseous state directly to solid state without going through the liquid state is called _______________. (Deposition/Desublimation) 10. _______________ is a surface phenomenon. (Evaporation)
1. Define Latent heat of vaporisation. 2. Explain why the temperature remains constant during the change of state of any substance. 3. Define Sublimation with examples. 4. Do we sweat more on a dry day or a humid day? Justify your reason. 5. Why do we see water droplets on the outer surface of a glass containing ice-cold water? 6. Convert the following temperatures to the Kelvin scale: (a) 25°C (b) 373°C 7. List two properties that liquids have in common with solids. 8. List two properties that liquids have in common with gases. 9. What will happen to the melting point temperature of ice if some common salt is added to it? Justify your answer. 10. How will you show that air has maximum compressibility?
1. Define the terms: (a) Latent heat of fusion (b) Latent heat of vaporization. 2. State the effect of (i) surface area (ii) nature of the liquid on the rate of evaporation. 3. Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Why? 4. What is the physical state of water at 250°C, 100°C, and 0°C? 5. Give reasons: i) A sponge can be pressed easily; still, it is called a solid. ii) Water vapour has more energy than water at the same temperature. 6. What are intermolecular forces? How are these related to the three states of matter? 7. Is it possible to liquefy atmospheric gases? If yes, suggest a method.
1. a) What is meant by evaporation? What are the factors on which the rate of evaporation depends? b) How does evaporation cause cooling? 2. State the properties of all the five states of matter. 3. Define: Melting point, Freezing point, & Boiling point.
Matter is anything that occupies space and has mass. It is composed of particles and can be felt by our senses. In ancient Indian philosophy, matter was described as being made up of five basic elements: air, earth, fire, sky, and water.
The particles of matter are very tiny, have vacant spaces between them, are in continuous motion, and are held together by forces of attraction.
The five states of matter are solid, liquid, gas, plasma, and Bose-Einstein condensate (BEC).
Boiling is a bulk phenomenon where particles from the entire liquid change into vapor at a specific temperature (boiling point). Evaporation is a surface phenomenon where particles from the surface gain enough energy to change into vapor at any temperature below the boiling point.
During evaporation, particles absorb heat energy from the surroundings to regain the energy lost, which leads to a cooling effect in the remaining liquid.
Sublimation is the process where a solid directly changes into vapors on heating and vapors directly change into solid on cooling, without passing through the liquid state. Examples include ammonium chloride, camphor, and iodine.
Latent heat is the heat energy absorbed or released during a change of state without any change in temperature. It is required to overcome the forces of attraction between the particles.
The Kelvin scale is the SI unit of temperature. The relationship is T(K) = T(°C) + 273.16. For practical purposes, 0°C is often taken as 273 K.
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