Concept of matter

Matter is anything that has mass and occupies space. Therefore, anything around us provided it has mass and can occupy the space, is termed as matter. There are many kinds of matter. 

Can you mention some? 



The word matter is used to cover all the substances and materials from which the earth and universe is composed of. These include all materials around us such as water, soil, plants, animals, air, clothes, etc.

Any particular kind of matter is called a substance. Substances include elements and compounds. An element is a substance which is the limit of chemical analysis. When two or more elements are combined chemically, a compound is formed. Matter is made up of atoms, ions or molecules. You will learn more about this later.


States of matter

Any chemical substance we study exists in any of the three forms (or physical states). The three different states of matter are
1. solid state
2. liquid state and
3. gaseous states



So, each of the many millions of substances around us can be classified as a solid, a liquid or gas. Look around you and name substances that are solids, liquids and gases. The state in which any matter exists depends on temperature and sometimes pressure conditions. 

One substance may exist as a solid in one condition and as a liquid or gas under a different condition. Water is an example of such substances. This change is called a change in the state of matter.

The three physical states of matter differ in the way they respond to temperature and pressure. All three states can increase in volume (expansion) when the temperature is increased. They decrease in volume (contraction) when the temperature is decreased. 

Gases are easily compressed. Liquids are only slightly compressible. Solids are incompressible. They are not affected by change in pressure.


Differences in properties of the three states of matter


One State of Matter to Another

We have seen that matter exists in three different states - solids, liquids and gases. We can use the kinetic theory of matter to explain how a substance changes from one state to another. Basically, changes from one state to another are caused by alterations in temperature and pressure. Normally molecules, ions or atoms of a substance move faster when the temperature is increased.


Melting

Melting is a change from solid to liquid state. When solids are heated, their constituent particles (atoms, molecules or ions) get energy and vibrate more violently. Vibrations of these particles overcome (exceed) their binding forces. The particles become mobile. The crystalline structure of solid is destroyed. A liquid state is reached and the particles are free to move. The temperature at which this happens is called melting point of the solid.

The melting point of a solid tells us something about the strength of forces holding its constituent particles together. Substances with high melting points have strong forces between their particles. Those with low melting points have weak forces between their particles.



Freezing

Freezing is a change from liquid to solid state. Freezing is the opposite of melting. The process is reversed at the same temperature if a liquid is cooled. The temperature at which a substance turns to a solid is called freezing point. The melting point and freezing point of any given substance are both the same. For example, the melting and freezing of pure water takes place at 0°C. Melting is not affected by any changes in atmospheric pressure.


Evaporation and boiling



Boiling is a change from liquid to vapor state at a particular temperature. 

Evaporation is the change from liquid to vapor state at any given temperature. 

If a liquid is exposed to open air, it evaporates. Splashes of water evaporate at room temperature. After rain, small pools of water dry up. When a liquid changes into a gas at any temperature, the process is called evaporation. Evaporation takes places from the surface of the liquid. The larger the surface area, the faster the liquid evaporates. The warmer the liquid is, the faster it evaporates. Thus, surface area and temperature affects the rate of evaporation of a liquid.

When a liquid is heated, its molecules get more energy and move faster. They knock into each other violently and bounce further apart. As the heating goes on, its molecules vibrate even faster. Bubbles of gas (due to air dissolved in water) appear inside the liquid. The whole process is called boiling. The temperature at which a liquid boils is called boiling point.

The molecules at the surface of the liquid gain enough energy to overcome the forces holding them together. They break away from the liquid and from a gas (vapor). As more of the liquid molecules escape to form a gas, a liquid is said to evaporate. This occurs at the boiling point of a liquid.

The temperature at which a liquid boils explains how strong the forces holding its particles (molecules) together are. Liquids with high boiling points have strong forces of attraction between their molecules than those liquids with low boiling points.

The boiling point of a liquid can change if the surrounding pressure changes. If the surrounding pressure falls, the boiling point also falls. The boiling point of water at standard pressure (760 mmHg) is 100°C. On a high mountain, where pressure is low, it is lower than 100°C. If the surrounding pressure is increased, the boiling point rises. The same behavior is experienced by a gas when the pressure is either increased or decreased.


The melting and boiling points of some common chemical substances at standard temperature and pressure (s.t.p)

 

Physical state at room temperature

 

 

Substance

 

(20°C)

Melting point (°C)

Boiling point(°C)

 

 

 

 

 

Oxygen

 

gas

-219

-183

 

 

 

 

 

Nitrogen

 

gas

-210

-196

 

 

 

 

 

Ethanol (alcohol)

 

liquid

-117

78

 

 

 

 

 

Water

 

liquid

0

100

 

 

 

 

 

Sulphur

 

solid

115

444

 

 

 

 

 

Common   salt

(sodium

 

 

 

chloride)

 

solid

801

1465

 

 

 

 

 

Copper

 

solid

1083

2600

 

 

 

 

 

Carbon dioxide

 

gas

sublimation point (°C): -78

 

 

 

 

 


From the above explanation, obvious differences between evaporation and boiling can be detected. See table below

Evaporation

Boiling

 

 

1.Occurs at all temperatures

Occurs at one particular temperature (boiling point)

 

 

2.Occurs on the surface of the liquid

Occurs both inside and on the surface of the liquid

 

 

3.Takes place slowly

Takes place faster

 

 

4.Bubbles are not necessarily formed

Bubbles are formed

 

 


Therefore, the two terms can be defined as follows: 

Evaporation is a change in state of a substance from liquid to gas (vapor) state at any temperature.

Boiling is a change in state of a substance from liquid to gas at a particular temperature and pressure.


Condensation and solidification

The reverse of evaporation is condensation. This is brought about by cooling. When a gas is cooled down, its particles lose energy. They move more and more slowly. When they knock into each other, they do not have enough energy to bounce away again. They stay close together and a liquid forms. This process is called condensation. 

When the liquid is cooled further, the movement of the particles slows down even more. Eventually, they stop moving and a solid forms. This is called solidification.
Condensation can be defined as a change in state of a substance from gas (vapor) to liquid. Solidification is a change from liquid to solid state of a substance. Solidification is the same as freezing.


Sublimation

A few solids do not melt when they are heated. Instead, they change directly from the solid to gaseous state without passing through the liquid state. This change in state is called sublimation. When a solid changes directly into gas, it is said to sublime. Iodine, solid carbon dioxide ("dry ice") and ammonium chloride are examples of solids that sublime. Like melting, sublimation also occurs at one particular temperature for each pure solid.