Ch 1 Chemistry Class 9

 Chapter -1  Matter In Our Surroundings.

States of Matter

• Matter can be classified as solid, liquid and gas on the basis of interparticle forces and the arrangement of particles.
• These three forms of matter are interconvertible by increasing or decreasing pressure and temperature.  For example, ice can be converted from solid to a liquid by increasing the temperature.

Property	Solid	Liquid	Gas
Shape and volume	Fixed shape and volume	No fixed shape but has volume	Neither definite shape nor volume
Energy	Lowest	Medium	Highest
Compressibility	Difficult	Nearly difficult	Easy
Arrangement of molecules	Regular and closely arranged	Random and little sparsely arranged	Random and more sparsely arranged
Fluidity	Cannot flow	Flows from higher to lower level	Flows in all directions
Movement	Negligible	Depends on interparticle attraction	Free, constant and random
Interparticle space	Very less	More	Large
Interparticle attraction	Maximum	Medium	Minimum
Density	Maximum	Medium	Minimum
Rate of diffusion	Negligible	It depends on interparticle attraction.	Maximum

Atomic view of the three states of matter

Solid

Liquid

Gas

Evaporation

The phenomenon by which molecules in liquid state undergo a spontaneous transition to the gaseous phase at any temperature below its boiling point is called evaporation.

• For example, the gradual drying of damp clothes is caused by the evaporation of water to water vapour.

Factors affecting evaporation

• Temperature: The rate of evaporation increases with an increase in temperature.
• Surface area: The rate of evaporation increases with an increase in surface area.
• Humidity: The rate of evaporation decreases with an increase in humidity.
• Wind speed: The rate of evaporation increases with an increase in wind speed.

Cooling due to evaporation

During evaporation, the particles of a liquid absorb energy from the surroundings to overcome the inter-particle forces of attraction and undergo the phase change. The absorption of heat from the surrounding makes the surrounding cool.
For example, sweating cools down our body.

Physical Nature of Matter

• A physical property is that aspect of the matter that can be observed or measured without changing its nature or composition.
• It is independent of the amount of matter present.
• Physical properties include appearance, colour, odour, density, texture, melting point, boiling point, solubility, etc.

Characteristics of Particles of Matter

Matter

Matter is anything that has mass and occupies space.

• Everything that we can touch, see, hear, taste and also smell is matter.
• It is made up of really tiny particles which cannot be seen through the eye.

• The particles of which the matter is comprised influence its state and properties (physical and chemical).

  1. Particles of matter have spaces between them

  • This characteristic is one of the concepts behind the solubility of a substance in other substances. For example, on dissolving sugar in water, there is no rise in water level because the particles of sugar get into the interparticle spaces between the water particles.

  2. Particles of matter are always in motion

  • Particles of the matter show continuous random movements due to the kinetic energy they possess.
  • A rise in temperature increases the kinetic energy of the particles, making them move more vigorously.

  3. Particles of matter attract each other
  In every substance, there is an interparticle force of attraction acting between the particles. To break a substance we need to overcome this force. The strength of the force differs from one substance to another.

  
  

  Diffusion

  When the particles of matter intermix on their own with each other, the phenomenon is called diffusion. For example, spreading of ink in water.

  • During diffusion, the particles occupy the interparticle spaces.
  • The rate of diffusion increases with increase in the temperature, due to increase in kinetic energy of the particles.

  
  

  Can Matter Change Its State?

  Effect of change of temperature on state of matter

  On increasing temperature, the kinetic energy of the particles of the matter increases and they begin to vibrate with a higher energy. Therefore, the interparticle force of attraction between the particles reduces and particles get detached from their position and begin to move freely.

  • As a result, the state of matter begins to change.
  • Solids undergo a phase change to form liquids.
  • Similarly, liquids also undergo a phase change to form gases.

Temperature: Common and SI units

→ Common unit: Degree Celsius (^oC)

→ SI unit: Kelvin (K)

→ Relation between common unit and SI unit of temperature:

                                    0 ^oC = 273K

Change of State of Matter:
Physical states of matter can be interconverted into each other by following two ways:

1. By changing the temperature

2. By changing the pressure

1. Effect of Change of Temperature:

Solid to liquid:

→ On increasing the temperature of solids, the kinetic energy of the particles increases which overcomes the forces of attraction between the particles thereby solid gets converted to a liquid.

→ Melting: Change of solid state of a substance into liquid is called melting.

→ Melting point: The temperature at which a solid melts to become a liquid at the atmospheric pressure is called its melting point.

→ Melting point of ice is 0^oc.

(a) Liquid to gas:

→ On heating a liquid like water, the kinetic energy of its particles increases as high as in a gas, thus causing the liquid to change to a gas.

→ Boiling: The change of a liquid substance into gas on heating is called boiling.

→ Boiling point: The temperature at which a liquid boils and changes rapidly into a gas at the atmospheric pressure is called its boiling point.

→ Boiling point if water is 100^oC.

(b) Gas to liquid:

→ On cooling a gas like steam (or water vapour), the kinetic energy of its particles is lowered down, causing them to move slowly and bringing them closer, forming a liquid.

→ Condensation: The process, in which a gas, on cooling, turns into a liquid at a specific temperature is called condensation or liquefaction.

(c) Liquid to solid:

→ When a liquid is cooled down by lowering its temperature, its particles lose the kinetic energy and come to a stationary position, causing the liquid to turn to solid.

Freezing: The change of a liquid substance into solid by lowering its temperature is called freezing.

Freezing point: The temperature at which the state of a substance changes from a liquid to a solid is called the freezing point of that substance.

 Fusion: The process of melting, that is, change of solid state into liquid state is also known as fusion.

Latent heat: The heat energy that is required to change the state of a substance without causing any ruse in the temperature of the substance is called latent heat. Since, the heat energy is hidden in the bulk of the matter, it is called latent heat.

Latent heat of fusion: The heat energy required to convert 1 kilogram of a solid into liquid at atmospheric pressure, at its melting point, is known as the latent heat of fusion

Latent heat of vaporisation: The heat energy required to convert 1 kilogram of liquid into gas, at atmospheric pressure, at its boiling point, is known as the latent heat of vaporisation

Note: Water vapour at 373 K have more energy than water at the same temperature because
particles in steam have absorbed extra energy in the form of latent heat of vaporisation.

Sublimation: The change of state of a substance directly from a solid to gas or gas to solid, without changing into the liquid state, is called sublimation. 

2. Effect of change of pressure

→ Gas to liquid: Gases can be liquefied by applying pressure and reducing the temperature. When a high pressure is applied to a gas, it gets compressed and if the temperature is lowered, the gas is liquefied.

→ Solid CO₂ gets converted directly to gaseous state on decrease of pressure to 1 atmosphere without coming into liquid state. This is the reason that solid carbon dioxide is also known as dry ice. 

Evaporation:
The process of conversion of a substance from the liquid state to the gaseous state at any temperature below its boiling point is called evaporation or vaporisation.

Factors affecting the rate of evaporation:

→ Surface area: The rate of evaporation increases on increasing the surface area of the liquid.

→ Temperature: The rate of evaporation increases with an increase in temperature.

→ Humidity: Decrease in the humidity increases the rate of evaporation.

→ Wind speed: An increase in the wind speed increases the rate of evaporation.

Evaporation causes cooling:
During the process of evaporation, the particles of liquid absorb energy or latent heat of vaporisation from the surrounding to get converted to gaseous state. This absorption of energy from the surroundings make the surroundings cold.

For example: The perspiration or sweating in our body keep the body temperature constant by taking away the extra heat from body as the latent heat of vaporisation.

Some important physical quantities and their SI units:

Melting point

The melting point of a solid is defined as the temperature at which solid melts to become liquid at the atmospheric pressure.

• At melting point, these two phases, i.e., solid and liquid are in equilibrium, i.e., at this point both solid state and liquid state exist simultaneously.

Boiling point

The boiling point of a liquid is defined as the temperature at which the vapour pressure of the liquid is equal to the atmospheric pressure.

Latent heat of fusion

It is the amount of heat energy that is required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point.

Latent heat of vaporisation

It is the amount of heat energy that is required to change 1 kg of a liquid into gas at atmospheric pressure at its boiling point.

Sublimation

The transition of a substance directly from its solid phase to gaseous phase without changing into the liquid phase (or vice versa) is called sublimation.

Sublimation – Solid to Gas Phase Transformation

Effect of change in pressure on state of matter

By applying pressure, the interparticle spaces between particles of matter decreases. Thus, by applying pressure and reducing temperature we can convert a solid to liquid and a liquid to gas.

Flowchart for inter-conversion of the three states of matter