Matter and its Properties
Matter consists of atoms and molecules. The innumerable substances that occur in nature-needles, men, mountains, stars, everything we can think of consists of atoms. Atoms combine to form molecules. For example, two atoms of hydrogen combine with one atom of oxygen to form a water molecule. A molecule can be as simple as a hydrogen molecule, which consists of two hydrogen atoms, or as complicated as a DNA (deoxyribonucleic acid) molecule, which consists of millions of atoms of hydrogen, carbon, nitrogen, oxygen and phosphorous. By definition, a molecule is the smallest particle of a substance that has all its chemical and physical properties. A molecule can be subdivided into atoms that have chemical properties of their own.
● States of Matter
Matter commonly exists in three states: the solid state, the liquid state and the gaseous state. In all states the molecules are perpetually moving. In a solid, molecules vibrate about fixed positions. Molecules in a liquid also vibrate but simultaneously they move freely throughout the material. In a gas the molecules are much further apart than in solids and liquids and move at high velocities.
All substances can be transformed from one state to another by changing temperature. Ice, water and steam are different states of the same substance.
There is fourth state of matter-the ionised state called plasma. Though this state is less common on the earth, the sun and other stars are in this state.
Molecular Properties of Matter
(i) Diffusion: Diffusion is the mixing up of molecules of different gases, liquids and even solids. When a bottle of perfume is opened in one corner of a room, its molecules mix with molecules of air and smell soon spreads even to the far corner of the room. The diffusion in liquids is not as fast as in gases. Sugar crystals placed at the bottom of a bottle containing water, will diffuse into water to make a uniform solution in a couple of weeks. Diffusion also occurs in solids though at an extremely slow speed.
(ii) Surface Tension : An insect called 'pond skater' can easily walk on the surface of the water. A slight depression of the surface is produced by the legs of the insect, showing that the surface acts like an elastic "skin".
If a needle is placed on small piece of blotting paper, which is then placed on the surface of the water, the paper sinks in a few seconds leaving the needle floating on water. A close examination reveals that the needle rests in a slight depression as if lying on an elastic skin.
Thus the surface of a liquid behaves like an elastic membrane and, therefore, has a tendency to contract. This property of a liquid is called surface tension. Surface tension is caused by molecular attractions..
When a paint brush is dipped in water all its hair spread out but when it is taken out it is covered with a thin film of water which contracts due to surface tension and pulls the hair together.
Liquid drops, such as raindrops, oildrops, drops of molten metals, dewdrops etc. are all spherical because their surface tend to contract in order to have minimum surface area. For a given volume, a sphere has the minimum surface area.
Soaps and detergents lower the surface tension of water. This increases the wetting power of water or its ability to detach dirt particles from clothes and utensils.
Mosquitoes breed on stagnant water. Their larvae keep floating on water due to surface tension. When oil is sprinkled on the stagnant water its surface tension is lowered resulting the drowning and death of the larvae.
(iii) Capillarity : If a clean glass tube having a small inside diameter (called a capillary tube) is dipped in water, the water rises in the tube. This phenomenon is called capillarity. Water rises in the capillary tube because water molecules are attracted to glass more than to each other. If the same capillary tube is dipped in mercury, the level of mercury in the tube is lower than the level outside because mercury molecules are less attracted to glass than to each other.
The force of attraction between unlike molecules is called adhesion and that between like molecules cohesion.
The melted wax of a candle is drawn up into the wick by capillary action. Oil rises up a lampwick for the same reason.
If one end of a sugar cube is dipped into tea, the entire cube is quickly wet on account of capillary action. The fine pores of a blotting paper act as tiny capillary tubes. The ink rises into the blotting paper through these pores.
The capillary action in soils is important in bringing water to the roots of plants.
Bricks are porous and, therefore, subsoil water can seep up them by capillary action. To avoid dampness in a building, a layer of nonporous material, such as slate, is necessary in its foundation.
A drop of water spreads on a glass plate because the adhesion of water molecules for glass is greater than their cohesion. A drop of mercury remains almost spherical because the cohesion of mercury molecules is greater than their adhesion to glass.
(iv) Viscosity : Viscosity is another molecular property of fluids (gases and liquids). It is measured in terms of the coefficient of viscosity, n. Its SI unit is pa s (Pascal second). Liquids have higher coefficients of viscosity than gases. Some liquids are more viscous than others. For example, honey is more viscous than water.
The viscosity of liquids, in general, falls rapidly with temperature. For water the coefficient of viscosity (n) at 80°C is one-third of its value at 10°C. The coefficient of viscosity of liquids, except water, rises with pressure. For water, n decreases with increasing pressure.
Viscosity plays the same role in fluids as does friction in solids. Like, the motion of a block sliding on a table is opposed by the friction between the two surfaces. the motion of an object moving through a fluid is also opposed by fluid friction or viscosity.
The viscous force F acting on an object falling through a fluid of coefficient of viscosity n depends on its size r (in case of a ball r is its radius) and its velocity v.
This is Stokes' law.
As the falling object gains velocity (due to the downward force of gravity acting on it), the opposing viscous force also increases. A stage comes when the viscous force equals the gravitational force and, therefore, the net force on the falling object becomes zero. The object then stops accelerating and falls with constant velocity, known as the terminal velocity. It is obvious that the terminal velocity is more for heavier objects.
Terminal velocity also depends on the size, it is more for smaller objects. If a metal sphere and a wooden sphere of equal mass (due to much lower density of wood, the wooden sphere will have much bigger radius) are dropped simultaneously from a high altitude, the metal sphere will achieve a higher terminal velocity and will touch the ground before the wooden sphere.
While skydiving, even though a group of divers dive from the aeroplane one after another, in mid-air the group can hold hands and make beautiful patterns. This is possible because the skydivers can alter their terminal velocities by changing their position in air. A diver falling vertically can reduce his/her terminal velocity by acquiring a horizontal position with arms and legs spread out.
