Matter in our surroundings

• Matter –

Anything which has mass and occupies space is called matter.

• Atom –

An atom is the smallest unit of an element which may or may not have an independent existence.

• Molecule –

A molecule is the smallest unit of a substance which always exists independently. It is made up of atoms of either the same element or of different elements.

For example, H₂O is a molecule of water which is made up of 2 atoms of hydrogen and 1 atom of oxygen. H₂ is a molecule of hydrogen which consists of 2 atoms of hydrogen.

• Difference between a monoatomic molecule and an atom.

An atom either exists independently or combine with atoms of other element by an ionic or covalent bond to gain stability, whereas a monoatomic molecule is a neutral atom which means it does not have valence electrons and they exist independently in nature.

For example, noble gas elements are monoatomic molecules.

• Inter-molecular space –

The space between the molecules in a matter is known as inter-molecular space.

• Intermolecular force of attraction –

The force of attraction between the molecules in a matter is known as the intermolecular force of attraction.

• Force of cohesion –

The force of attraction between similar kinds of molecules is known as the force of cohesion.

• Force of adhesion –

The force of attraction between different kinds of molecules is known as the force of adhesion.

For example, in a glass of water, the force of attraction between the water molecules is the force of cohesion and the force of attraction between the water molecules and the surface of the glass is the force of adhesion.

• State of matter –

Generally, there are three states of matter, namely – solid, liquid and gas.

But now we consider there are five states of matter, namely – Solid, Liquid, Gas, Plasma and the Bose–Einstein Condensate(BEC).

• Plasma is the state that consists of super energetic and super excited particles. These particles are in the form of ionised gases. The fluorescent tube and neon sign bulbs consist of plasma. The Sun and the stars glow because of the presence of plasma in them. The plasma is created in stars because of very high temperatures.
• The Bose–Einstein Condensate (BEC) is formed by cooling a gas of extremely low density, about one-hundred-thousandth the density of normal air, to super low temperatures.
• Melting or Fusion –

The process of changing of solid into liquid state at a fixed temperature is known as melting or fusion.

• Melting point –

The temperature at which a solid starts melting at the atmospheric pressure is known as its melting point.

The melting point of ice is 273.15 K.

• Latent heat of Fusion –

The amount of heat energy that is required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point is known as the latent heat of fusion.

• Freezing –

The process of changing of solid into liquid state at a fixed temperature is known as melting or fusion.

• Freezing point –

The fixed temperature at which a liquid starts to freeze at the atmospheric pressure is called the freezing point.

The freezing point of water is 0°C or 273.15 K

• Boling –

The process of changing liquid into gaseous state at a fixed temperature is known as boiling.

• Boiling point –

The temperature at which a liquid starts boiling at the atmospheric pressure is known as its boiling point.

The boiling point of water is 100°C.

• Condensation –

The process of changing a gas to its liquid state at a fixed temperature is known as condensation.

• Evaporation –

The phenomenon of change of a liquid into vapours at any temperature below its boiling point is called evaporation.

• Sublimation –

A change of state directly from solid to gas without changing into liquid state is called sublimation

• Deposition –

The direct change of gas to solid without changing into liquid is called deposition.

• Factors affecting the rate of evaporation –

1. Temperature – directly proportional
2. Surface area – directly proportional
3. Wind speed – directly proportional
4. Humidity – inversely proportional

• The amount of heat energy that is required to change 1 kg of a solid into liquid at atmospheric pressure at its melting point is known as the latent heat of fusion.
• Kelvin is the SI unit of temperature.
• Other standard units of temperature are Celsius and Fahrenheit.
• 0°C =273.15 K and
• Dry ice – Solid carbon dioxide is known as dry ice.

It is stored under high pressure. Solid CO2 gets converted directly to a 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.

• The SI unit of pressure is Pascal (Pa) or Nm^-2.
• 1 atmospheric pressure = 1.01 × 105 Pa.
• The pressure of air in the atmosphere is called atmospheric pressure.
• The atmospheric pressure at sea level is 1 atmosphere and is taken as the normal atmospheric pressure.
• The maximum density of water is 1 g/cm³ at 4°C.

NCERT QUESTIONS:

INTEXT:

1) The mass per unit volume of a substance is called density. (density = mass/volume). Arrange the following in order of increasing density – air, exhaust from chimneys, honey, water, chalk, cotton and iron.

Answer –

Air, Exhaust from chimney, cotton, water, honey, chalk, iron

2)  (a) Tabulate the differences in the characteristics of states of matter.

Answer –

(b) Comment upon the following:

Fluidity – The ability of a matter to flow is known as fluidity.

Kinetic energy –

The energy possessed by the moving body is known as the Kinetic energy(K.E.).

Thus, kinetic energy is directly proportional to the mass of the object and directly proportional to the square of the velocity of the moving body.

Density –

The mass per unit volume of a substance is called density.

Density = Mass/ Volume.

The S.I. unit of density is kg/m³.

3) Give reasons –

1. A gas fills completely the vessel in which it is kept.

– Gas has a negligible inter-molecular force of attraction, so the molecule of the gas can move freely and can acquire space as much available to them.

2. A gas exerts pressure on the walls of the container.

– In the gaseous state, the particles move about randomly at high speed. Due to this random movement, the particles hit each other and also the walls of the container. The pressure exerted by the gas is because of this force exerted by gas particles per unit area on the walls of the container.

4) Liquids generally have lower density as compared to solids. But you must have observed that ice floats on water. Find out why.

Answer –

We know that, generally on decreasing the temperature of a substance, the substance contracts but in the case of water from 4°C to 0°C it expands i.e. the volume of water increases. Due to this its density decreases as the density of a substance is inversely proportional to the volume.

Thus, the density of the ice is lower than the water, due to which it floats on water.

5) Convert the following temperature to Celsius scale:

1. 300 K = (300 – 273) °C = 27°C
2. 573 K = (573 – 273) °C = 300°C

6) What is the physical state of water at:

1. 250°C – Gaseous state as water starts to change into vapour at 100°C.
2. 100°C – At 100°C water will be in both the liquid and gaseous state.

7) For any substance, why does the temperature remain constant during the change of state?

Answer –

During the change of state, the heat supplied to it gets used up in changing the state by overcoming the forces of attraction between the particles. So the temperature remains constant during the change of state.

8) Suggest a method to liquefy atmospheric gases.

Answer –

Applying high pressure and reducing temperature can liquefy gases.

9) Why does a desert cooler cool better on a hot dry day?

Answer –

Both the factors i.e. high temperature and less humidity increase the rate of evaporation which causes the cooling of the surrounding air. So, the desert cooler cools better on a hot dry day.

10) How does the water kept in an earthen pot (matka) become cool during summer?

Answer –

When water is kept in an earthen pot, its surface gets wet by water and evaporation starts from its surface for which it takes the heat from the water present inside the pot. So, during summer the water kept inside the earthen pot become cool.

11) Why does our palm feel cold when we put some acetone or petrol or perfume on it?

Answer –

Because the particles of the acetone or petrol or perfume gain energy from our palm to evaporate.

12) Why are we able to sip hot tea or milk faster from a saucer rather than a cup?

Answer –

Because the saucer has the larger surface area than the cup and greater the surface area faster will be the cooling rate.

13) What type of clothes should we wear in summer?

Answer –

During summer, we perspire more because of the mechanism of our body which keeps us cool. We know that during evaporation, the particles at the surface of the liquid gain energy from the surroundings or body surface and change into vapour. The heat energy equal to the latent heat of vaporisation is absorbed from the body leaving the body cool. Cotton, being a good absorber of water helps in absorbing sweat and exposing it to the atmosphere for easy evaporation.