Topic covered : Introduction of matter, Characteristics of matter, Sublimation, Scales of temperature, Evaporation, and much more….. Class 9 Science Notes Chapter 1 Matter in our surroundings

Introduction of matter

In our surrounding, we see large things with different shapes, sizes and textures. Everything in this universe is made up of material have named “matter”. The air we breathe, the food we eat, stones, clouds, stars, plants and animals, even a small drop of water or a particle of sand – everything is matter. They have MASS and VOLUME. 

Matter may be defined as anything that occupies space possesses mass and the presence of which can be felt by any one or more of our five senses.

• It is important to note that heat, light, sound, electricity, magnetism, etc. Are not considered as matter because they are massless and do not occupy space. Similarly, vacuum is not a matter since it neither occupies space nor has mass.

According to ancient Indian philosophers, all matter whether living or non-living is made up of five basic constituents commonly called panchtatvas, i. e, air, earth, fire, sky and water. A similar classification of matter was also evolved by ancient Greek philosophers.

Scientists have classified matter into the following two types on the basis of their properties:

1. Physical classification- On the basis of physical properties, matter has been classified as solids, liquids and gases.
2. Chemical classification- On the basis of chemical properties, matter has been classified as elements, compounds and mixtures.

Characteristics of particle of Matter

Particles in matter are in a state of continuous motion not stationary, but have a tendency to acquire motion. The rate of movement of the particles is directly proportional to the thermal energy of the particles.

Particles in matter attract one another: This attraction is inversely proportional to the distance between the particles. However, the magnitude of these inter-particle forces differs from one substance to another.

Particles in matter have spaces between them: Empty spaces called voids. This separates the particles from one another. The distance between them ranges from 10^-8 cm to 10^-5 cm. Due to these voids matter is able to disperse into one another bringing about diffusion

State of matter

The classification of matter into three different states, namely solid, liquid and gaseous state is termed as the physical classification of matter. Most properties of solid, liquid and gases that can be observed with our sense organs are called as ‘macroscopic’ properties.

The description of the behaviour of the three states of matter in terms of atomic theory is called ‘microscopic’ description of matter. From the study of the observable properties of different states of matter one can understand the microscopic nature of matter in terms of the behaviour of constituent particles.

Solid

The molecules in a solid are very closely packed. The inter-molecular force of attraction is very strong in solids. Hence solids have a definite size and shape. When the solid is heated, molecules gain more kinetic energy and vibrate more, inter-molecular distance increases and the solid expands. Example: Iron, wood, stone, sand, gold, ice, pencil, book, needle, piece of thread, etc

PROPERTIES of Solids STATE

Some of the common properties of solids, which distinguish them from other two states of matter, are:

• Solids are rigid and have definite shapes.
• Solids have definite volume irrespective of the size or shape of the container in which they are placed.
• Solids are almost incompressible.
• Many solids have a crystalline appearance and have definite pattern of angles and planes.
• Solids diffuse very slowly as compared to liquids and gases. Constituent particles are very closely packed in solids permitting very little space for their movement.
• Solids have a much higher density (mass to volume ratio) than that of gases and liquids.
• Most solids become liquids when heated. Some undergo sublimation on heating. The temperature at which a solid changes into liquid is called the melting point and the process is called as melting. Due to the varying natures of solids their melting temperatures vary considerably.

Liquid

Molecules in liquids have vibratory motion as well as strong translatory motion. Thus the molecules of a liquid can move freely within the liquid. As a result, liquids don’t have a fixed shape. It takes up the shape of the container. Example: Water, milk, petrol, kerosene oil, alcohol, cooking oil, cold drink, etc

PROPERTY OF Liquid State

• Due to weak intermolecular forces, the molecules are in constant random motion.
• The average kinetic energy of molecules in a given sample is proportional to the absolute temperature.
• Liquids do not have a fixed shape but have a fixed volume
• Liquids are not rigid but have the property to flow.
• Liquids possess the property of diffusion

Gas

The molecules of a gas are far apart. The inter-molecular force of attraction is negligible and they have no fixed freedom of motion. Hence gases have neither fixed shape nor volume.

Example: Air, oxygen, nitrogen, hydrogen, ammonia, carbon dioxide, compressed natural gas (CNG), etc.

Properties OF Gaseous State

• Gases do not have fixed shape
• Gases have minimum fluidity and least rigidity:
• Gases do not keep their volume and are highly compressible
• Gases are generally very light
• Kinetic Energy of a particle in the gaseous state is very high
• Gases exert pressure
• Gases diffuse rapidly (The rate of diffusion of a gas is inversely proportional to the square root of its density. This is called Graham’s Law of diffusion.)

Plasma  

Plasma is much like gases except that their constituent particles have become electrically charged, and their behaviour in consequence depends strongly upon electromagnetic forces. Most of the matter in the universe is in the plasma state.

Example: A fluorescent tube contains plasma (When electricity is passed, the gas gets ionized and glowing plasma is formed. The colour of this glowing plasma depends upon the nature of the gas). The sun and the stars glow due to the presence of plasma. The plasma in stars is formed due to high temperature.

Temperature

Temperature is defined as the measure of average heat. On the other hand it is defined as the thermal energy of the particles in a substance. Temperature is independent of the number of particles or size and shape of the object. The water boiling temperature is same for all type of containers.

• By adding or removing heat, the temperature of a substance can be increased or decreased.
• The device which is used to define the measure of temperature of an object is Thermometer.

SCALES OF MEASURING TEMPERATURE

There are three scales on which the temperature of a system can be measured. These are called Celsius scale, Fahrenheit scale and the Kelvin scale (k) or absolute scale.

Celsius scale

The Celsius scale was discovered by Andrews Celsius at 1743. This Celsius scale is to define the measure of temperature.

On the Celsius scale, the freezing point of water is taken as 0⁰C while its boiling point is taken as 100⁰C. The interval is divided into 100 divisions all are at equal distance. Every division being denoted as one degree Celsius (⁰C).

• The Celsius scale is also called as centigrade scale because the range of temperature is divided into 100 equal divisions.

Kelvin scale

Another type of scale which is used to define the measure of temperature is Kelvin scale. The Kelvin scale is also known as absolute scale of temperature. The lowest fixed point is taken from the lowest temperature to which a substance to be cooled such as -273.15⁰C.

• As a substance is cooled continuously causes the decrease of temperature. But there is a limit to the lowest temperature to which a substance to be cooled.

Absolute zero

In the Kelvin scale, the lowest possible temperature is taken as zero. This temperature is called as absolute zero. At the point absolute zero there is no molecular motion and there is no heat energy. At absolute zero all atomic and molecular motions stop. Hence the absolute zero is the lowest possible temperature which is denoted by 0K or -273.15⁰C.

• Kelvin scale (K): Celsius Scale (⁰C) + 273
• Celsius scale (⁰C): Kelvin Scale (K) – 273
• In this Kelvin scale the freezing point of water is 273 K and the boiling point of water is 373 K. In this scale the division is divided into 100 equal divisions.

Fahrenheit Scale

This scale was invented by Fahrenheit. It is used for clinical and meteorological purposes. On this scale of temperature, the freezing point of water is taken as32 degree Fahrenheit (written as 32⁰ F) and the boiling-point is taken as 212⁰ F. The interval between the two fixed points is divided into 180 equal parts; each part corresponds to a difference of temperature of 1⁰ F. In order to convert the Centigrade Scale into Fahrenheit Scale we use the formula:

C = 5/9(F – 32)

Change of state of matter

When we provide heat to water, at a certain temperature it starts getting vaporised. This takes place at constant temperature that known as the boiling point. It is nearly 100⁰C at normal pressure. But water can also be vaporised at a lower temperature by lowering pressure.

• Ice can be converted into water at a fixed temperature known as melting point which is 0⁰C at ordinary pressure. If the pressure is increased ice can melt at a lower temperature than 0⁰C. However, at a given pressure the temperature of the substance remains constant until the change of state is complete.
• Similarly steam can be condensed into water and water can be freeze into ice at the respective temperatures.

Melting point

It is the process of changing a solid into its liquid state. The temperature at which a solid changes to the liquid state is known as melting point.

Boiling point

When a liquid is heated, the molecules gain more kinetic energy and the temperature starts rising. After it has reached a specific point though heat is being supplied, the temperature becomes steady. At this temperature, the energy supplied gets utilized in breaking the inter-molecular force of attraction.

Effect of change of temperature on states of matter

When we heat a solid, we add energy to the system increasing the vibration of the particles. Eventually these particles break free from their binding forces and start to melt.

The normal melting point of a crystal is the temperature at which it melts under one atmospheric pressure. Increase in pressures usually raises the melting point. Most solids when heated eventually melt but few undergo sublimation. In this phase change, the solid goes directly into a gaseous phase. For example: iodine and camphor

1. Change the state from solid to liquid-Melting

The temperature at which a solid melts to become a liquid at atmospheric pressure is called its melting point. This process of melting change of solid state into liquid state is also called 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 called latent heat of fusion.

• Change of state from liquid to solid—freezing.

The temperature at which a liquid freezes to become a solid at atmospheric pressure is called the freezing point.

• Change the state from liquid to gas—boiling.

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

To demonstrate that temperature remains constant during boiling or during conversion of liquid into vapours.

The amount of heat energy that is required to change 1 kg of a liquid into vapours at atmospheric pressure at its boiling point is called latent heat of vaporization.

• Steam is more effective than boiling water for heating purposes.
• Impurities Increase the boiling points of liquids.

Effect of pressure on the boiling point of a liquid

• The boiling point of a liquid depends upon the pressure acting on it. It increases, if the pressure is increased or decreases if the pressure acting on it is decreased.

Change of gaseous to liquid state-Condensation

• The process of changing a gas (or vapour) to a liquid by cooling is called condensation.

Sublimation

• Sublimation involves direct conversion of a solid into the gaseous state on heating and vice-versa on cooling with no temperature change without passing through the intervening liquid state.

Example of some other substance which undergo sublimation are; ammonium chloride, camphor, naphthalene, benzoic acid, iodine, etc.

Aplications of sublimation

• In very cold places, the snow does not melt but sublimes directly to vapours.
• In frost-free refrigerators, ice on the walls of the freezer sublimes when warm air is circulated through the compartment during the defrost cycle.

Sublimation depicted at sub-microscopic level

On heating a liquid, the energy of the particles increases. They overcome the inter-particle force of attraction and turn into a gas or vapour at its boiling point. Boiling point of the liquid is a temperature at which the vapour pressure of the liquid is equal to the atmospheric pressure. Changing the atmospheric pressure can change boiling point of a liquid.

• For instance water boils on a mountain top at a much lower temperature than at sea level. Atmospheric pressure being less requires lesser thermal energy to get vapour pressure equal to atmospheric pressure.

Changing of solid to liquid state (melting)

When a solid is heated it starts melting at a certain fixed temperature (melting point). At this stage even when the heating is continued, the temperature does not change until the whole of solid is converted into liquid. The state when solid and liquid phases of a substance coexist is called solid-liquid equilibrium. Solid-liquid equilibrium is described as,

• If no heat is exchanged with the surroundings, then the temperature and the mass of the two phases (solid and liquid) remain constant.

For example, if we place ice and water at 273 K (0°C) under normal atmospheric pressure in a perfectly insulated thermos flask, since the flask is insulated, there will be no exchange of heat between its contents and the surroundings. We notice that

• The temperature of both the phases remains constant, i.e., temperature of the system does not change.
• Mass of each phase (ice and water) does not change with time.

Rate of transfer of molecules from ice to water = Rate of transfer of molecules from water to ice

Changing of solid to gaseous state (Sublimation)

Sublimation is a direct change from the solid state to gaseous state, without passing through the liquid state. If a mixture contains two solids, one sublimable, and the other non-sublimable, they can be separated by the process of sublimation.                                                                                                                    The solid that sublimes, escapes in the form of vapours. When these vapours are collected on a cold surface, they condense to form the sublimable solid, known as the sublimate.

Some solid substances when heated get converted directly to the gaseous or vapour state without first passing through the liquid state. When this vapour or gas is cooled, it gets converted directly into a solid, without passing through the liquid state. Such change of state of a solid directly into gas or vapour and back to the solid state, without passing through the liquid state, is called as sublimation. When a sublimable solid substance is heated, it is said to ‘sublime’ into a gaseous state; and when sublimable substances are cooled from their vapour state, the solid obtained is called the ‘sublimate’. Some sublimable substances are: iodine, camphor, naphthalene, dry ice carbon dioxide) etc.

Importance

The technique of sublimation is used for separating such solid substances which sublime on heating from the non volatile substances. e.g. ammonium chloride can be separated from the sodium chloride.

Changing liquid to gaseous state (boiling)

The constant temperature at which a pure liquid is completely transformed into its vapour under normal atmospheric pressure is called its boiling point and this phenomenon is called boiling.

In other words, boiling point is that constant temperature at which the vapour pressure of a liquid is exactly equal to the atmospheric pressure.

A pure liquid has a constant boiling point at a constant pressure, which is a characteristic property of the liquid. Impure liquids boil at a higher temperature than the pure liquid. Hence, the determination of this physical constant not only helps in judging the purity of the liquid, but also to a certain extent in the identification of a liquid sample.

Changing of gaseous state to liquid state (condensation)

Condensation is the change of water from its gaseous form (water vapour) into liquid water. Condensation generally occurs in the atmosphere when warm air raises, cools and loses its capacity to hold water vapour. As a result, excess water vapour condenses to form cloud droplets.

Changing of liquid to solid state (freezing)

Freezing or solidification is a phase change in which a liquid turns into a solid when its temperature is lowered below its freezing point. The reverse process is melting.

All known liquids, except liquid helium, freeze when the temperature is lowered enough. Liquid helium remains liquid at atmospheric pressure even at absolute zero, and can be solidified only under pressure. For example, agar displays a hysteresis in its melting and freezing temperatures. It melts at 85 °C (185 °F) and solidifies from 31°C to 40 °C (89.6 °F to 104 °F).

Freezing is a common method of food preservation that slows both food decay and the growth of micro-organisms. Besides the effect of lower temperatures on reaction rates, freezing makes water less available for bacterial growth.

Latent heat of fusion

• Latent heat is the heat released or absorbed by a chemical substance or a thermodynamic system during a change of state that occurs without a change in temperature. The term was introduced around 1750 by Joseph Black.

Latent Heat of fusion is the amount of heat required to change the phase from solid to liquid state, or the heat released when there is a phase change from liquid to solid, without a temperature change. Heat of fusion means “hidden heat which causes melting/freezing”.

It is defined as the amount of heat required for the 1 mole of ice to bring a change in its state, that is, from solid state to liquid state. It is also known as enthalpy of fusion. The particular temperature at which there is a change in the state of the ice is known as the melting point of ice.  The heat of fusion of ice is found to be 333.55 KJ/ kg.

Latent heat of vaporization

Latent heat of vaporization of a substance is the amount of heat required to change one unit mass of that substance from liquid to gaseous state. During this process, the temperature of the substance remains constant. The heat absorbed in the process used to change the state of that substance from liquid to gaseous state. The heat energy increases the internal energy of the substance in form of change of state with no rise in temperature.

Liquefaction of Gases

When the temperature of the gas is lowered, both the volume of the gas and the kinetic energy of the molecules decrease. The molecular motion becomes slow and molecules become sluggish. The progressive decrease of temperature brings the molecules closer and closer because they are unable to resist the attractive force that starts operating between them.

This process of liquefaction by bringing gas molecules closer can also be achieved by increasing the pressure of the gas: this also decreases the volume of the gas. For example, sulphur dioxide can be liquefied at 265 K if pressure is 760 mm of Hg. It can also be liquefied at 293K if the pressure is increased to 2470 mm of Hg.

Thus, liquefaction of gases can be achieved by either decrease of temperature or by increase of pressure.

Evaporation

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

• Evaporation requires energy. A liquid draws heat energy from the surrounding thereby cooling the surrounding.

Water placed in a porous pot becomes very cool after some time. This is because water molecules draw energy from the water itself for evaporation and hence, the temperature of water in the post falls.The process, by which a soluble solid can be obtained from a solution by allowing the solvent to vaporize, is called evaporation.

Factors affecting the evaporation

1. Increase in temperature:

We know from our everyday experience that evaporation of water becomes fast if the atmospheric temperature is high. In other words, evaporation of a liquid occurs at a faster rate in summer than in winter. This is due to the reason that as the temperature increases; the kinetic energy of the particles of the liquid also increases.

• Decrease in humidity:

By humidity, we mean the amount of water vapours present in air. It may be noted that air around us can hold only a certain definite amount of water vapours at a particular temperature. In case the humidity of air is already high, the amount of water vapours in the air is already high, it can hold only a little more amount of water vapours to reach that optimum level, therefore, the rate of evaporation decreases.

• Intermolecular forces of the liquid:

When the intermolecular forces in a liquid are weak, escape of molecules is easier from the surface resulting in an increase in the rate of evaporation. At high intermolecular forces between molecules in a liquid, only those molecules escape, which have sufficient energy to overcome the intermolecular attraction?

• Surface area:

The area that is exposed to evaporation directly influences the rate of evaporation. Larger the surface area, greater is the rate of evaporation of the liquid.

• Nature of the liquid:

The liquid that contains large charged molecules result in a slow evaporation process with the requirement of additional energy for overcoming the electromagnetic interactions that allow the molecules to escape.

• Rate of airflow:

Movement of fresh air over the substance increases the concentration of the substance in the air resulting in an increased rate of evaporation.

Evaporation cause cooling

We have learnt above that during evaporation, only the liquid particles having high kinetic energy leave the surface of the liquid and get converted into vapours. As a result, the average kinetic energy of the remaining particles of the liquid decreases and hence the temperature falls. Thus, evaporation causes cooling.

If we place some water in an open vessel, it keeps on evaporating. For evaporation to occur, heat energy is taken from the water. The particles of water, in turn, absorb energy from the surroundings to regain the energy lost during evaporation. This absorption of energy from the surrounding makes the surroundings cold.

Some examples of cooling caused by evaporation

1. Why do we sprinkle water on the roof or open ground in summer?

During the hot summer evenings, people often sprinkle water on the roof of the house or open ground in from of their house. The water evaporates by absorbing the large latent heat of vaporisation of water from the ground and the surrounding air. By losing heat, the ground becomes cool and comfortable. Similarly, on hot sunny days, water evaporating from the leaves of the trees keeps the surroundings cool.

• Pouring of acetone on palms.

If you pour some acetone (nail polish remover) on your palm, you feel cool. The reason being that the energy needed for evaporation is taken from the palm. By losing heat, palm feels cooling.

• Why do we wear cotton clothes in summer?

During summer, we prefer to wear cotton clothes to keep ourselves cool. The reason being that during summer, we sweat or perspire a lot. Now cotton is a good absorber of water. Therefore, it absorbs wet from the body and exposes it to the air (atmosphere). Due to increase in surface area, sweat evaporates by taking large latent heat of vaporization of water from the body and the cotton clothes. As a result, our body feels cool and comfortable. On the other hand, synthetic clothes made up of polyester, nylon, etc. do not absorb much of sweat and hence fail to cool our body in summer.

• Perspiration keeps our body cold.

On a hot summer day of after doing heavy exercise, the temperature of our body tends to rise. Due to increase in temperature, our body gives out sweat. When the sweat evaporates, it takes heat equal to large latent heat of vaporization of water from our body. By losing heat, our body feels cool.

• Why do we see water droplets on the outer surface of a glass tumbler containing ice-cold water?

Take some ice-cold water in a glass tumbler. Soon you will notice water droplets on the outer surface of the tumbler. This is due to the reason that the water vapours present in air, on coming in contact with the cool surface of the glass, lose energy and get condensed or get converted into the liquid state, which we see as water droplets.

• Why is ice rubbed on a burnt part of the skin?

When a finger or some other part of our body gets burnt, we rub the burnt portion with an ice cube. The reason being that due to burning, the temperature of the injured skin increases. When ice is rubbed, the excess heat from the skin is taken by large latent heat of fusion of water. As a result, the temperature of the injured skin decreases and we feel less pain.

Brownian motion

The movement of particle (random) suspended in a fluid is called Brownian motion. Brownian motion’s mathematical model has several applications in real world. One of which is the stock market fluctuations.