7. Refrigerator

As you know, whenever a gas turns into a liquid, heat is invariably developed. Similarly, heat is required to turn a liquid into a gas. Whenever a liquid is vapourised, heat is taken from the immediate surroundings. When ether evapo­rates on your finger, it turns from a liquid to a gas and takes heat from your finger in this process. If there were enough ether on your finger and a constant supply of heat could be. kept, the rapid evaporation would cool your finger so much' that it would become frozen. This is the principle the modern refrigerator is based upon.

Some gases are easily liquefied and when liquid, they are easily turned back into the gaseous state. Sulphur dioxide is pumped under pressure until it liquefies. The liquid sul­phur dioxide is pumped jnto a coil of small pipes which sur­rounds a number of trays filled with water. As soon as the liquid enters this coil, it passes through a nozzle which creates a very fine ,spray, .causing it to expand. In expanding this liquid becomes a gas again and when expanding, it takes up a considerable amount of heat from the immediate surround­ings. The water in the trays has to supply this heat, and finally it loses enough heat to freeze to ice. The gas thus formed is led back to the compressor where it is compressed again and sent out in a liquid form. The same cycle takes place all over again and again. This continuous changing from a liquid to a gas and from a gas back to a liquid is going on as long as the compressor runs. The compressor thus really supplies the energy required for making the ice in the trays and producing the cold air which circulates throughout the refrigerator. This energy, however, undergoes several interest­ing transformations in the process of refrigeration as we have just seen.

_{8.- BOILING}

If we heat some water in an open glass container, we can see that evaporation goes on from the top surface. This evap­oration is in^ifcated, by the clouds forming where the vapour mixes with the colder air and condenses. We find that the temperature of water gradually rises until the thermometer registers 100°C. A little before this point is reached, bubbles appear on the sides of the container. They consist partly of gases driven from liquid and partly of water-vapour, for evap­oration is directed into the bubbles. Water is said to boil' when vapour is formed both at the bottom of the container and at the top of it. The motion of the boiling water is caused by the bubbles of vapour rising irirofqjh the water. The tem­perature of the boiling water is constant. This temperature is known as the boiling point of the liquid.

The boiljng point of a liquid is the temperature at which it boils under some given pressure. When this point has been reached, further heating does not increase the temperature of the liquid but only changes it into steam.

When water boils in a container, we say that we see steam coming out of it. In fact, what we see is not steam at all but fine water particles. Steam itself is invisible. It is the con­densed steam in the form of fine particles of water that we see.

As liquids always jncrease in volume when passing into the vapour state, an increase in pressure always produces an increase in the boiling point. _ ._y , r|£^

% Just as solids may under certain conditions be cooled below their melting points without fr&ziilg^so liquids may be heated above their boiling points without boiling.

The principal law&'of boiling are as follow^f

1. When a liquid is heated, it begins to boil at a definite temperature, known as the boiling point, and on further heating the temperature remains constant at this value until the whole of the liquid is converted into vapour.

2. This temperature is constant for a given liquid if the pressure is constant.

3. The boiling point of a liquid increases if the pressure upon it is increased. ''^;" ^s%

4. A definite quantity of heat is required to convert the unit mass of the liquid into vapour at the same temperature. This is known as the latent heat of evaporation.

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_{9. STRUCTURE OF THE ATOM}

The atom is the basic partide of all matter. All solids, gases, and liquids are composed of atoms.

For a time the atom was considered to be indivisible but then it has been found that the atom in its turn can be divid- ed into many different components. I

In dividing the atom, man releases forces of great magni­tude. These are forces that bind the central core of the atom. This central core— the nucleus — is extremely small in di­ameter.

The nucleus of the atom is of a highly complex structure. It is the three main components of the atom that we shall deal with below. These are called protons, neutrons and elec­trons.

The proton carries a positive charge of electricity, the number of protons in the nucleus determining the element tti#t the atom forms. _f

For example, if the nucleus has a single proton, then it will form the gas hydrogen. If 92 protons are present, the element will be uranium and so on. In short, if the number of protons in the nucleus is known, the element can be found out at once..,

As mentioned above, the proton carries a charge of posi­tive electricity. We know the bodies charged with the same kind of electricity to repel one another. When two protons are brought close together they repel one another with great force.

The second of these basic components of the nucleus is the neutron. The neutron does not carry a definite electric charge. The sub-p&rt'icles' that form the neutron do ca_rrr^ , charges but the charge of one balances that of another, leav­ing the neutron neutral. It is from this state that it gets its name.

\ The third component of the atom is the electron. The elec­trons revolve around the nucleus. Each electron carries a

negative charge, of electricity that is equal to the positive charge of a proton in the nucleus.

As the charge of the electron i§ negative and that of the proton positive,, it might be thought mat ,the proton would attract the lighter electron and draw¹ it into the nucleus. This would happen if the electron were not revolving around the nucleus. „ .■

The speed of the electron establishes sufficient centrif­

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ugal force so that it counteracts the neutral attraction. Thus -the higher the speed of the revolving electron, i. e. tire.greater

its energy, the farther from the nucleus it will revolve.

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