Tag: Integrated Science,Physical Sciences

Thematteris made up of very small particles calledmolecules.Molecules are made up of a number of atoms of either the same or different elements which are bonded together.

The Greek philosopher calledLeucippusand his pupilsDemocritus (460 – 310 BC)are believed to be the founders of the atomic theory of matter. They speculated that all matter is made up of identical, indivisible particles calledatomsseparated by empty space.

The existence of molecules suggested by Leucippus and Democritus has been shown by experimental results produced by the English chemistJohnDaltonin 1808 whose theory proposed that matter was composed of tiny indivisible particles that he then called“ultimate”particles, now known asatoms.His theory further states that every pure substance is made up of a single type of atoms or a combination of different atoms called molecules.

• In 1600’sBoyletried to explain the observed properties of gases in terms of molecules, their smallest particles (this will be detailed in the gas laws).
• Daniel Bernoulli (1700 – 1782)was the first to explain the law which Boyle discovered. He also related temperature and molecular motion directly, calculated the pressure of a gas from collisions of the molecules with the walls of the containing vessel and inferred that when the temperature increases, the velocity of the molecules increases also.
• The Kinetic Molecular Theorystates that “the particles that the matter is made up with are always moving and they often bump into each other”.This random motion is present in varying amount in solids, liquids and gases. The assumptions of kinetic theory of matter can be summarised in the following points:
• All matter is made up of particles calledmolecules. In normal circumstances it exists in three states.
• All the molecules of a given substance are alike in all respects.
• The molecules are separated from one another by the intermolecular space which is more than the diameter of the molecule itself.
• Themoleculesattract each other with a force called inter-molecular force, which is strongest in solids and weakest in gases.
• Molecules are in constant motion.
• Thetemperatureof a substance is proportional to theaverage kinetic energyof all the molecules of the substance.

States of Matter

Matter exists in the three states of solids, liquids and gas. The physical Properties of Matter in these three states lies in the arrangement and behaviour of their molecules.

These differences can be explained in terms of theKinetic Theory Modelwhich states that:

• Matter is made up of very small particles called molecules
• These molecules are in constant motion
• The degree of movement which is the measure of kinetic energy depends upon the temperature.

The higher the temperature, the faster the molecules move and therefore the higher the kinetic energy.

Noticethat theheat energyof a substance is the total kinetic energy of all the molecules or atoms of that substance.

SOLIDS

In the solid state the molecules are closely packed together and are not free to move about randomly; they vibrate about their fixed position (known as themean position)

The ability of a solid to maintain its definite shape and fixed volume is the result of the strong intermolecular forces of attraction between the molecules. These are known ascohesive forces.

Solids are classified into two groups:crystalline and non-crystalline (amorphous)

The difference between the two groups is the result of the way in which the molecules are arranged. This molecular arrangement is called thelattice structureof the molecules

Crystalline solids

These are solids whose lattice structure has a definite regular pattern.E.g. sodium chloride (rock salt)

Amorphous solids

The lattice structure of an amorphous solid is an irregular, disordered pattern.E.g.:charcoal and sulphur.

Molecules are arranged differently in different solids and have varying magnitudes of intermolecular forces of attraction. These factors are responsible for some of the mechanical properties of solids (especially metals)

Elasticity:It is the ability of a material to return to its original state or its original shape after it has been stretched or compressed by an external force.

Plasticity:It is the property of a material by which some permanent deformation remains even after the force producing it has been removed.

Ductility:it is the property of a material which governs whether it can be stretched and drawn into wires of small cross-section where there will be considerable deformation without fracture.

Brittleness:This is the property of a material which is fragile and breaks suddenly when a force is applied to it.

Malleability:It is the property which allows a material to be rolled into thin sheets without breaking. E.g. Iron, copper and aluminium can be rolled into thin sheets

Change of solid state:

When a solid is heated, the vibrations of molecules and their kinetic energy increase. If the temperature goes to a certain level, the intermolecular forces are broken apart and molecules are released from each other. This results in the material changing its state from a solid to a liquid, the process known asmeltingorfusion. The opposite phenomenon is calledsolidificationorfreezing– change of liquid into solid.

LIQUIDS

Molecules in a liquid are more widely spaced than in a solid, are also vibrating to and fro about their mean position. Their intermolecular force is weak and the molecules movement is free. This enables them to take the shape of the containing vessel.

Under the same temperature conditions, the intermolecular forces of attraction between molecules are different in liquids and this is responsible for the variance in physical properties.

a.Surface tension:

It is the property whereby the molecules on the surface of the liquid do not experience the same intermolecular forces as those within the liquid.

The effect of surface tension

Surface tension tends to reduce the surface area of a liquid. The degree of reduction in the surface area is determined by the surface tension which in turn, depends on the intermolecular forces present. An example of two liquids with different surface tensions is water and mercury:

A droplet of water will tend to spread out, whereas a drop of mercury will be a spherical ball. This is because there is greater surface tension in a mercury droplet than there is in a drop of water.

Another example of the effect of surface tension is the perfectly spherical shape of soap bubbles. Soap solution, detergents and alcohol lower the surface tension of most liquids.

b.Viscosity

This is a property of certain fluids which causes them to offer resistance to the flow of other substances (either solids or liquids) through them.

COHESION AND ADHESION

The particles making substances are held together by intermolecular forces of attraction which are grouped in two major categories according to their action:

1. Cohesive forces (cohesion)

It is the intermolecular forces of attraction acting between molecules of the same substance. If these forces are strong, then the surface tension is strong and the surface area of the liquid is reduces.

• When the liquid with very strong cohesive forces are put in a container they form aconvexmeniscus.
• When the liquids with weak cohesive forces are put into a container, they form aconcave meniscus

2.Adhesive forces (adhesion)

This is the intermolecular forces of attraction acting between molecules of different substances. E.g.: The cohesive forces between the mercury molecules are stronger than the adhesive forces between mercury and glass molecules and subsequently the mercurymeniscusin glass container isconvex.This is opposite for water and the meniscus areconcave. (Itis even the reason why water wets glass whilst mercury does not.)

Another example of the effect of cohesion and adhesion is the rise of liquids in very narrow capillary tubes.

Change of liquid state:

When a liquid is heated at high temperature, the particles move faster, intermolecular forces of attraction decrease and intermolecular spaces reduce. This results in the liquid changing the state and turns into gas. This process in known as ‘evaporation’. Evaporation on the surface of a liquid occurs at any temperature but it is very slow. However, when a liquid is boiling, this change takes place everywhere in the liquid and is very fast. The process opposite to evaporation is calledcondensation– change of gas into liquid.

GASES

In the gaseous state, molecules are even further apart than in water. This implies that they have more freedom to move among themselves. In gaseous state the molecules move haphazardly and at high speed and they are constantly colliding with each other and the walls of the container. These collisions are perfectlyelastic,which means that the total kinetic energy of the particles before collision is equal to their total kinetic energy after collision.

Remark:The spaces between the molecules of a gas are relatively large, so that much of the volume occupied by a gas is actually empty space. As a result, the intermolecular forces are almost negligible and the molecules are independent to each other. The increased speeds and the greater spacing between the molecules allow the gas to both fill anyvolumeand also to be compressed (as opposed to expansion).

Change of gaseous state:

When a gas is cooled down, the motion of its particles slows down and the intermolecular spaces reduce. This results in the gas changing into liquid, the process known ascondensationorliquefaction. Some substances such as carbon change directly from gas to solid and vice versa. This process, in both cases is calledsublimation(e.g.: case of carbon dioxide or dry ice and naphthalene).

The following table summarises properties of gases, liquids, and solids and identifies the microscopic behaviour responsible for each property.

Table 1.1: Comparison of solid, liquid and gaseous state in terms of their particles’ behaviour

The change of state can be summarised in the following diagram:

If a solid substance is added to a liquid, the solid may dissolve forming a solution.

Liquids and gases may also dissolve in liquids. In that situation, the liquid is a solvent while the dissolving substance is called a solute. Sometimes, solids and liquids may not dissolve in a given liquid. The substances are said to be insoluble. The combination is called amixture. The constituents of the mixtures can be separated by physical means. Particular methods are used in the separation depending on the properties of the constituents. Methods of separating mixtures into their components are important because pure substances are needed for industrial processes.

Methods of Separating Mixtures

Chemists have developed many different methods of separating mixtures from simple or complex compounds. Methods used depend on what is in the mixture and properties of the substances present. Also depends on whether the substances to be separated are solid,liquidorgas.

Some methods of separating mixtures are:

Filtrationseparates a solid from a liquid using a filter paper supported in a funnel. Decanting also separates a solid from a liquid. However, in decanting, the mixture is left to settle. The liquid is then carefully poured off the solid.

Acentrifugeseparates a solid and a liquid by spinning a tube containing the mixture in a circle at several thousands of revolutions per minute. The solid is forced to the bottom of the tube allowing the liquid to be decanted easily.

Simple distillationis similar to fractional. The process involves the evaporation of a liquid from a mixture of a liquid and a solid followed by condensation of the vapour. The condensed liquid is called the distillate. For example sea water on distillation will produce pure water as the distillate and sea salt as a residue.

1. Fractional distillationseparates mixtures of liquids (fraction) into their components. Boiling the mixtures causes the components to vaporise in succession – the liquid with the lowest boiling point vaporising first. The vapours are condensed in a water-cooled condenser and collected as distillates. For example, wine on distillation will produce two distillates, ethanol (alcohol) and water, in that order.
2. Crystallisationsoccur when a hot, saturated solution is allowed to cool. The crystals that form will be the pure solid component if solvent is the only other component. Decanting the remaining solution allows the crystals to be dried on a tissue and so obtained pure.

vii.Paper chromatographyis the process of separating a mixture of solids, in solution, on a sheet of paper.

The table below summarises the types of separation techniques used in industry and in the laboratories.

When the substances are separated, the final products approach the original purity of the original substance.

Substances are made up of some basic substances called“elements”.Each element is composed of its own kind of particles called “atoms”. For example, aluminium is an element which is made up of only aluminium atoms. An Element cannot be broken any further. Elements can be classified as “metalsandnonmetals”.

The atoms of all elements are extremely small to be seen. The smallest atom known is hydrogen with diameter 7×10^-8mm. atoms of different elements have different diameters as well as different masses.

Chemist use shorthand symbols to label the elements and their atoms. The symbol consists of one, two or three letters, the first of which must be a capital. For example, C is used for Carbon, Ca for Calcium and Cl for Chlorine. Some symbols seem to have no relationship to the name of the element, for example Na for Sodium and Pb for lead. These symbols came from their Latin names Natrium for Sodium and Plumbum for Lead.

To make sense of the material world around us, we need methods for physically separating the many and varied mixtures that we come across. Being able to purify and identify the many substances present in these mixtures not only satisfies our curiosity but is crucial to our well­-being and health. A range of physical techniques are available to make the necessary separations. They all depend in some way on a difference in the physical properties of the substances in the mixture.

The most useful separation method for a particular mixture depends on:

• the type of mixture, and
• which substance in the mixture we are most interested in.

Separating heterogeneous mixtures

In some ways these are the easier mixtures to separate. Quite often, just leaving them to stand helps with the separation. This is often the first stage in separating mixtures of immiscible liquids. It is also often used to separate solid-in-liquid suspensions if the particles of solid are large enough. Once the solid has settled to the bottom (sedimented), the liquid can be carefully poured off (this is calleddecantation).

A more generally useful method than decantation for separating solids from liquids isfiltration. Here the insoluble material is collected as aresidueonfilter paper. Filtration is useful because both phases can be obtained in one process. The liquid phase is collected as thefiltrateThe process can be speeded up by using a vacuum pump to ‘suck’ the liquid through the filter paper in aBuchner funneland flask.

Various large-scale filtration methods are used in industry. Perhaps the most useful of these are the filter-beds to purify water for household use.

Table 2.5 Methods of separating substances from mixtures

Another means of separating an insoluble solid from a liquid iscentrifugationwhere the mixture is spun at high speed in a centrifuge. Here, it is no longer the force of gravity on the solid particles that causes settling. Instead, there is a hugecentrifugal forceacting on the particles due to the high-speed spinning of the samples. This causes the solid to be sedimented at the bottom of the centrifuge tube. The liquid can be decanted off carefully.

Mixtures of two immiscible liquids can be separated if the mixture is placed in aseparating funneland allowed to stand. The liquids separate into different layers. The lower, denser layer is then ‘tapped’ off at the bottom. This type of separation is useful in industry. For example, at the base of the blast furnace the molten slag forms a separate layer on top of the liquid iron. The two can then be ‘tapped’ off separately. The method is also very useful in organic chemistry as part of a process called ‘solvent extraction’.

The separation of a solid from a mixture of solids depends largely on the particular substance being purified. Some suitable difference in physical properties needs to be found. Usually it helps if the mixture is ground to a powder before any separation is attempted.

1.Separations based on differences in density

‘Panning’ for gold is still carried out in the search for new deposits. In Amazonia, river-beds are mechanically sifted (‘vacuum-cleaned’) to collect gold dust. These methods depend on the gold dust being denser than the other substances in the river sediment. This type of method is also used in purifying the ores of zinc and copper, although in these cases the metals are less dense than the ores and so float on the surface.

1. The mixture of immiscible liquids settles into two layers, as the liquids do not mix
2. The tap is opened to let only the bottom layer run info the beaker
3. The tap is closed and the beaker is changed. The tap is opened to let the top layer run out

2.Separations based on magnetic properties

Magnetic iron ore can be separated from other material in the crushed ore by using an electromagnet. In the Amazonian gold diggings, magnets are used to clean away iron-containing, red-brown dust from the powdered gold. In the environmentally and economically important processes of recycling metals, iron objects can be picked out from other scrap metal using electromagnets.

3.Separations based on differences in solubility

One very useful way of separating a soluble substance from a solid mixture is as follows. The mixture is first ground to a powder. A suitable liquid solvent is added. The solvent must dissolve one of the solid substances present, but not the others. The solvent is often water, but other liquids can be useful. The mixture in the solvent is then warmed and stirred. Care must be taken at the warming stage when using solvents other than water.

The warm mixture is then filtered. This gives the insoluble substances as a residue on the filter paper, which can be dried. The soluble substance is in the liquid filtrate. Dry crystals can be obtained by evaporation and crystallisation.

The gold prospectors in Brazil and Zimbabwe still use an immensely dangerous version of this method to extract the gold from other substances. The solvent they use is mercury, which dissolves the gold. The gold is then recovered from solution by evaporating off the mercury with a blowtorch.

The unwanted residues, contaminated with mercury, are thrown into the rivers. Damage to the environment from this activity is very likely because mercury is poisonous to living things.

4.Separations based on sublimation

A solid that sublimes can be separated from others using this property.

Separating homogeneous mixtures

The separation of homogeneous mixtures is often slightly more complicated because there is no physical separation of the phases in the original mixture. The methods of separation usually depend on solubility properties or on differences in boiling point.

For example, Ammonium chloride can be separated from a mixture because it sublimes. The crystals condense on the cooled surface.

Separating a solid from solution in a liquid can be carried out byevaporationorcrystallisation. Evaporation gives only a powder, but crystallisation can result in proper crystals. Both processes begin by evaporating away the liquid but, when crystals are needed, evaporation is stopped when the solution has been concentrated enough. Figure 2.10 shows how this can be judged and done safely. The concentrated solution is allowed to cool slowly. The crystals formed can then be filtered off and dried.

Separating a liquid from a solution is usually carried out by distillation. The boiling point of the liquid is usually very much lower than that of the dissolved solid. The liquid can easily be evaporated off in a distillation flask. It is condensed by passing it down a water-cooled condenser, and then collected as the distillate.

While the solvent is evaporating, dip a glass rod into the solution from time to time. When small crystals form on the rod, take the solution off the water bath and leave it to cool.

This method should not be used if the solvent is flammable. Instead, use an electrical heating element and an oil or water bath.

Separating the liquids from a mixture of two (or more) miscible liquids is again based on the fact that the liquids will have different boiling points. However, the boiling points are closer together than for a solid-in-liquid solution and fractional distillation must be used.

For example, ethanol boils at 78 °C whereas water boils at 100°C. When the mixture is heated, ethanol and water vapours enter the fractionating column. Glass beads in the column provide a large surface area for condensation. Evaporation and condensation take place many times as the vapours rise up the column. Ethanol passes through the condenser first as the temperature of the column is raised above its boiling point. Water condenses in the column and flows back into the flask because the temperature of the column is below its b.p. of 100°C.

The temperature on the thermometer stays at 78 °C until all the ethanol has distilled over. Only then does the temperature on the thermometer rise to 100°C and the water distils over. By watching the temperature carefully, the two liquids (fractions) can be collected separately.

Fractional distillationis used to separate any solution containing liquids with different boiling points. It can be adapted as a continuous process and is used industrially to separate:

• the various fractions from crude oil,
• the different gases from liquid air .

Solubility

The solubility of solids in liquids

Probably the most important and common examples of mixtures are solutions of solids in liquids.

Such a solution is made up of two parts:

• the solid that dissolves is known as the solute,
• the liquid in which it dissolves is called the solvent.

Kinetic theory has got more applications in daily life, in most of the tools and apparatus we use in our activities. This articles will focus on the major application.

RATES OF DIFFUSION AND KINETIC THEORY

Diffusion is the process by which the molecules of one substance mix with those of another as a result of their motion. The substances move freely from a region of high concentration to a region of low concentration at their own pace. The rate of diffusion depends on the temperature and thedensityof the substances involved as proved in experiment 1.1 and 1.2 below.

Experiment 1.1:Diffusion in gases

Aim:to demonstrate the diffusion in hydrogen

Requirements: hydrogen gas, porous jar, glass tube, coloured water, clamp and stand,

Hydrogen gaspasses quite easily through earthenware which is porous. Arrange the apparatus as shown in the figure below. Introduce hydrogen into a vessel surrounding porous jar. What do you observe?

Observation:

The bubbles of the gas start to escape through the water: The hydrogen will diffuse through the porous jar and increases the gas pressure in the jar and this will cause some of the gas to bubble out. Air molecules will diffuse out of the vessel at the same time as the hydrogen molecules diffuse in but because the air molecules are large and heavier they diffuse out at a much slower rate.

Conclusion:The rate of diffusion in gases depends on its density.

Air is a uniform mixture of various gases because of diffusion. However, diffusion does not only take place in gases, it even occurs in liquids as well. If convection currents and stirring are avoided, it can be demonstrated that diffusion in liquid is quite slow using the following setup.

Experiment 1.2: Diffusion in liquids

Aim:To show diffusion in water

Materials:beaker, water, dropper, copper sulphate (CuSO₄)

Procedure:

Fill a gas jar with water. Place a thistle funnel into it so that it extends to the bottom of the jar and carefully pour a solution of copper sulphate until the bottom of the jar is filled to a height about one centimetre. Let the jar stand undisturbed for a few days. What do you observe?

Observation:You will observe an upward diffusion of the blue copper sulphate against gravity.

Conclusion:The rate of diffusion in liquid depends on the temperature.

The diffusion in fluids can be explained using the kinetic theory model.

• When diffusion takes place, molecules move from one substance to another to fill up the empty spaces.
• The spaces between the molecules are such that molecules are able to pass through them.
• The continuous movement of the molecules ensures a thorough mixing of the substances as they collide.
• When temperature is increased, the kinetic energy increases through the increased speeds of molecules and consequently, the rate of diffusion increases.

BROWNIAN MOTION

In 1827 Robert Brown was studying pollen grain suspended in water using a microscope, he was very surprised to observe that they were in a state of continuous random motion. He could not account for this movement, but later scientists discovered that the movement of the pollen grain was due to the collision of the water molecules against the pollen grains.

If you observe smoke particles in air, you will notice that they experience similar movement. As a result of such observations, scientists concluded that the molecules of liquids and gases are in a state of constant, random motion, an idea known as the“kinetic theory of matter”

Experiment 1.3: Brownian motion

Aim:To demonstrate Brownian motion in air/ a smoke cell

Materials:glass (smoke) cell, optical microscope, converging lens, a lamp/ torch lamp

Procedure:

Set up apparatus as shown in the figure below; a small glass cell in which smoke has been trapped is view through a microscope. A converging lens or glass rod is used to focus the light from a lamp into the glass cell.

Observation:

When the light strikes the smoke particles it is scattered and the smoke particles are observed as bright specs of light. They are also seen to be moving about in a zig-zag manner.

Conclusion:

This zigzag movement is due to the collisions of the smoke particles with invisible air molecules that are also moving about randomly in the smoke cell. The zigzag pattern of movement is illustrated in figure below.

EVAPORATION

Evaporation is the change of a liquid into a gas at the surface. It occurs at any temperature but occurs more rapidly at higher temperatures because heat imparts more kinetic energy to the molecules and they escape from the surface faster.

Increased gas pressure on the surface of the liquid reduces the rate of evaporation because more collisions occur between the evaporating liquid molecules and the gas molecules, resulting in some of the evaporated liquid molecules bouncing back into liquid.

Some factors increase the rate of evaporation namely:

–Increase in temperature:If we observe the wet clothes, we will notice that on a warm day, they dry faster than on a cold day. This is because more particles of the water have enough energy to escape.

–Increase in surface area:Water in a puddle on the road will dry more quickly than water in a cup. This is because more water molecules in the puddle are close to the surface and escape in bigger number than in the cup.

–Air blowing across the surface:When a day is windy, the clothes dry faster than when it is quiet or you can dry your wet hand by getting them closer to the fan. This is because the moving air carries escaping water molecules away from the liquid, preventing them from bouncing back into the liquid as result of air pressure over the liquid.

–Reduction of humidity:when air contains more water vapour, it is said to be very humid. Wet clothes will take longer to dry in very humid air because molecules in the water vapour return to the liquid almost at the same rate as water molecules escape from it.

During the evaporation process, the molecules that escape from the liquid are the most energetic ones. The average kinetic energy of those that remain is therefore reduced, and the temperature reduces. This is cooling by evaporation.

COOLING EFFECT OF EVAPORATION

Evaporation results in a general cooling of the liquid. This cooling is brought about by the lowering of the overall kinetic energy of the liquid as the more energetic molecules escape from the surface.

If energy is supplied to the liquid molecules (for example by heating), the rate of evaporation increases. If this energy comes from the surroundings, then the surroundings lose energy and experience a drop in temperature. The cooling effect of evaporation can be demonstrated by the following experiment:

Experiment 1.4: Cooling effect of evaporation.

Aim:To demonstrate the cooling effect of evaporation.

Requirements:a small beaker, ether, wooden block, water, glass tube

Procedure:

Place a small beaker, about one third full of ether, on a flat piece of wood which has a layer of water on it. Place a glass tube obliquely into the beaker to allow air in the beaker.

Observation:

Air is bubbled through the ether and effectively increases the surface area of the liquid and allows a lot more evaporation to take place. As the evaporation continues, the water between the beaker and wooden block will gradually start to freeze and the beaker will stick to the wooden block. Water droplets will also form on the outside of the beaker.

Explanation:

The energy required for this increased rate of evaporation is drawn from the beaker, the water and the wooden block

Water droplets form on the outside of the beaker due to condensation of water vapour from the surrounding air coming into contact with the beaker.

The cooling effect of evaporation is utilised in refrigerators. A refrigerator basically transfers heat from the inside cabinet to the outside environment. The principle involves a liquid that is constantly evaporating and thereby drawing energy (or heat) from the inside of the cabinet. Thetransfer of the heatfrom the inside to the outside is quite rapid and the inside becomes very cool. Once outside the cabinet the heat is transferred to the surrounding environment through the thin tube containing the vapour and the vapour is condensed back into liquid. A pumping mechanism ensures the movement of the liquid vapour round a closed system of conducting pipes thereby keeping the same liquid flowing continually.

The figure above represents a typical refrigerator system. The electric compressor motor forces a gas at high pressure through a heat exchanger (condenser) on the rear outside wall of the refrigerator, where Q_His given off, and the gas cools to become liquid. The liquid passes from a high-pressure region, via a valve, to low-pressure tubes on the inside walls of the refrigerator; the liquid evaporates at this lower pressure and thus absorbs heat (Q_L) from the inside of the refrigerator. The fluid returns to the compressor, where the cycle begins again

Other Cooling effects of Evaporation

Another every day application of cooling by evaporation is well observed insweating: When animals sweat, fine drops of moisture are released either through the skin or through pores on the tongue. The passage of air over these drops results in evaporation which causes a cooling of the animal’s body in a similar way as described in the experiment above. Dogs, for example, remove excess heat from their bodies by panting.