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 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.

Boiling is a change from liquid to vapour state at a particular temperature. Evaporation is the change from liquid to vapour 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 (vapour). 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 and vice versa. 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. The melting points and boiling points of some common chemical substances are shown in Table 5.2.

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

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 (vapour) to liquid. Solidification is a change from liquid to solid state of a substance. Solidification is the same as freezing.

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.

Different mixtures can be separated through such processes as distillation, sublimation, evaporation and condensation. Let us have a look at an example of distillation. This process involves boiling, evaporation and condensation. Distillation as a process can be applied in separation of a mixture (solution) of two or more substances. A mixture of two or more substances with different boiling points e.g. water and alcohol can be separated by this means. In such a case, a container with the mixed-up liquids is heated. The liquid with a low boiling point evaporates and condenses first, leaving the one with a high boiling point in the container. The distillate (liquid with low boiling point) is collected, cooled down and transferred into another container.

Industrially, the process of distillation is applied in the production of pure substances such as beer and other alcoholic drinks such as wine, vodka, konyagi, etc. The manufacturing process involves boiling, evaporating and condensation.

Crude oil contains organic liquid components, each with a different boiling point. In the refinery, the components with lower boiling points evaporate first and get separated out, leaving those with higher boiling points behind. In this way, we get various types of oil components (fractions) such as petrol, diesel, kerosene, lubricating oil, etc.

When you suspend your clothing on a cloth line to dry, the moisture in it is lost through evaporation. Likewise, farmers in the village often spread crops on the ground to dry. They do this in order to reduce moisture content and hence prevent decaying. The moisture contained in crops leave by evaporation. Therefore, you can notice how evaporation, as a change in state, is important in everyday lives.

During hot weather, our bodies perspire a lot. When water evaporates from the body, it takes up the heat. This brings about the cooling effect, as heat is lost from the body surface. The cooling effect is more evident when the wind or air is blowing over the body. This is because wind increases the rate of evaporation. In this way, the body gets cooled down

Drinking water and beverages are usually cooled down in refrigerators. Alternatively, you can freeze water into ice and then use the resulting ice for cooling the drinking water, or beverage. Also, perishable products such as fish, meat, and milk are often packed in ice blocks to prevent them from spoiling. Selling ice blocks to beverage vendors is a source of income to some people

In metallurgical industries, a need may arise to mix two or more metals (alloys) together. This is only possible where two or more metals are first melted at high temperatures into liquids. Then the resulting liquid metals are mixed in appropriate proportions. This is followed by cooling down the mixture to a solid alloy. Normally alloys have better qualities than individual metals. Examples of alloys include; amalgam (consists of mercury, silver, and copper), brass (consists of copper and zinc), and bronze (consists of copper and tin)

The presence of impurity may lower or raise the boiling point of the substance. A pure substance melts and boils at definite temperatures (Table 5.4). The values for the melting point and boiling point are precise and predictable. This means that we can use them to test the purity of a sample. They can also be used to check the identity of an unknown substance. For example, seawater is impure. It freezes at a temperature well below the freezing point of pure water (0 °C) and boils at a temperature above the boiling point of pure water (100 °C). Other substances behave in a similar manner. So, boiling as a change in state can be used to test for the purity of substances (Table 5.4). In addition, the impurity also reduces the exactness of the melting point or boiling point. An impure substance melts or boils over a range of temperatures, not at a particular point

Perhaps the most important of all, as far as change in state is concerned, is the formation of rain. Rained is mainly formed through the process of evaporation and condensation. Water vapour, evaporating mostly from water bodies (oceans, seas, lakes, rivers, ponds, etc), land and plants rises up to the sky. As it rises, it cools down and condenses into tiny droplets

On further cooling as they rise up, these droplets form bigger water drops. Owing to gravitational force, these drops fall down as rainfall. Every one of you knows how important rain is to our life. Therefore, you have noticed how evaporation and condensation, as changes in state, contribute to rain formation.

We already know that matter is composed of atoms, ions or molecules. We have not yet considered the reason why the same substance, say water, can exist in more than one form, for example as solid ice, liquid water, and gaseous steam. But does matter behave like that?

The kinetic theory of matter has been used to explain the way in which the arrangement of the particles of a substance can determine the properties of that substance, and particularly the state in which it is likely to be found under a given set of conditions. The idea is that all matter is made up of tiny moving particles. The main points of the theory are as follows:

In the solid state, the particles are so closely packed as indicated in Figure 5.1(a). The particles are held together by strong forces of attraction that act as a chemical glue. Free movements of particles cannot take place. They cannot move around freely in this arrangement. Instead, they vibrate about a fixed position. They are arranged in a fixed pattern which forms a cluster of vibrating masses. This makes a solid to have a fixed shape, which cannot be changed except by applying strong external forces

The particles of a liquid are also closely packed but the forces of attraction between them are weaker than of a solid. These forces of attraction tend to bind them together. The particles have more kinetic energy and they can move around each other. It is thought that the particles of a liquid are fairly randomly arranged but consist of "clusters" closely packed together as shown in Figure 5.1(b). This property makes a liquid to have a definite volume. However, since the particles are fairly free to move, a liquid does not have any characteristic shape. Thus, a liquid will always take the shape of its container

The gaseous state is one in which the particles are moving independently of each other in all directions and at great speeds. The particles of a gas are relatively far apart as indicated in Figure 5.1(c). They exert no force of attraction on each other. They have more energy than the particles of solids and liquids. They move rapidly and randomly, colliding with each other and with the walls of the container. A typical speed for a molecule of hydrogen in air at ordinary temperature and pressure has been found to be approximately 500 ms^–1. It is estimated that a nitrogen molecule makes 10⁹ collisions each second. Thus, a gas will rapidly spread out to fill any container in which it is placed. A gas cannot have any shape of its own

The different states of matter are due to the variations in the characteristics of their constituent particles. Solids, liquids and gases differ in the ways their atoms are arranged, and the intermolecular forces of attractions between their particles. The differences in physical properties between the three states of matter are summarized in Table 5.1.

Physical change is any change that occurs without altering the chemical composition of a substance. Substances may undergo changes in their physical properties such as changes in colour, shapes (or forms), states, density, structures, and texture. If you take a stone and break it down into small particles, you will have only changed its form, but it will remain a stone. Likewise, melting ice to water or freezing water to ice do not change it, but it is still water in different forms. The same case happens when you dissolve salt in the water to get a solution of salt in water. You can still get back the original salt by evaporation, except that the crystals of the salt obtained will not look exactly the same as those of the original salt

These changes of state are examples of physical changes. Physical changes such as melting and boiling do not result in new substances being formed

For example, ice and water still contain the same particles whether in solid (ice) liquid (water) or gaseous (vapour) state.

In the explanation above, we find that in a physical change it is only the physical form, and not the actual nature, of a substance that changes. The changes are brought about by a mere addition or removal of heat, as in the case with water or ice. Such a change is called a physical change. It can be distinguished by the following characteristics:

A chemical change is a process in which one or more substances are altered into one or more new and different substances. Chemical changes usually involve changes in the chemical properties of substances. For example, when you burn a piece of wood in the fire, you get ash. The properties of wood and ash are very different. There is no way you can change ash back to wood. It is practically impossible. A permanent change in the chemical properties of a substance is called a chemical change. In a chemical change, a substance loses its physical and chemical properties. For example, when some roll sulphur is strongly heated on a deflagrating spoon, it burns to produce a new gaseous substance (sulphur dioxide). The magnesium ribbon burns on a Bunsen burner flame to produce a new substance, magnesium oxide. Likewise, iron nails kept in wet cotton will form rust after few days

Includes

A chemical symbol is the way of representing an element using initial letter(s). There are many different elements as you have seen above. Every element has a name and a symbol to represent it. Some symbols are just a single capital letter, such as H. Others have two letters, the first of which is always a capital, such as Mg.

Includes

All symbols are recognized and are used by all scientists all over the world. Some examples of elements and their symbols are given in the table below

Most symbols of elements are taken from their English names. For example, the symbol for aluminium is Al and for carbon is C. But for some elements, their symbols are taken from Latin names. For example, potassium has the symbol, K from its Latin name Kalium. Sodium has the symbol, Na from its Latin name Natrium. Table 5.8 shows examples of symbols of elements derived from Latin language

The reason for assigning some elements with Latin names was to avoid confusion among scientists when representing different elements. For example, the symbol for silicon is Si. It could be impossible to represent silicon by the symbol S and at the same time represent the element sulphur by the very symbol, S. Similarly, potassium could not be represented by the symbol P that was assigned to phosphorus. So in order to avoid such confusion, scientists decided to use Latin names to represent some elements. In so doing, the anticipated and unnecessary contradiction among scientists from different parts of the world was avoided.

Symbols are particularly useful when more than one atom is present in a substance. For example, hydrogen gas consists of pairs of hydrogen atoms joined together. So hydrogen gas is shown as H₂. When more than one atom is joined together like this, we call the substance formed a molecule. Atoms making up gases such as hydrogen, oxygen, nitrogen, etc, always exist as molecules. Sulphur exists as a hexagonal ring of eight atoms. Phosphorus exists as a tetrahedron of four atoms. Table bellow shows some elements that exist as molecules.

Elements can be classified as either metals or non-metals. Metals and non-metals have different physical and chemical properties. This is the criterion used for classification of these elements into metals on one hand and non-metals on the other hand. Table bellow summarizes the physical and chemical properties of some common elements.

Many elements differ in their chemical and physical properties, but some elements are similar in their behaviours. For example, some elements are shiny, malleable, and good conductors of heat and electricity. These are called metals. On the other hand, elements that appear dull, poor conductors of heat and electricity are known as non-metals. Some elements conduct heat and electricity moderately well, and possess some properties of metals and some properties of non metals; these are called metalloids. The differences between metals and non metals are shown in Table 5.11.

Mixtures can be classified as solutions, suspensions or emulsions. This classification is based on whether the mixed substances dissolve completely or not. It also depends on the nature of the mixtures that result upon mixing. Let us look at each category in detail.

A solution is a uniform mixture of two or more substances. Such mixtures may be a solid in a liquid, a liquid in a liquid, a liquid in a gas and, very rarely, a gas in a gas. (See table bellow). We most often think of a solution as being made of a solid dissolved in a liquid. For example, solutions of sugar or salt in water are quite common. A solid that dissolves in a liquid is called a solute while the liquid in which that solid dissolves is called a solvent. For example, sugar and salt are solutes and water is a solvent.

However, other substances that are not normally solids can be found dissolved in a liquid. For example, the gases, carbon dioxide and oxygen, dissolved in water are important for life to continue in oceans, seas, lakes, rivers, etc.

Less obvious perhaps, but quite common, are solutions of one liquid in another. Alcohol mixes (dissolves) completely with water. Beer, wine and whisky do not separate into layers of alcohol and water (even when the alcohol content is quite high). Alcohol and water are completely miscible, that, is they make a solution.

Solutions of gases in gases are very uncommon. Technically, air could be described as a solution of several gases in nitrogen, though this could be unusual everyday use of the term. However, it is interesting to note that different gases always mix completely with each other.

A suspension is a cloudy mixture of solid particles suspended in a liquid. A solid is said to be suspended in a liquid when small particles of it are contained in a liquid but are not dissolved in it. If the mixture is left undisturbed, the solid particles will slowly settle to the bottom of the containing vessel, leaving the pure liquid in a top layer. Muddy water is a typical suspension. The mud would settle after a time if left undisturbed leaving the residue on the bottom of the containing vessel and clear water in a top layer. The particles of mud would be retained by filtering whilst the water (and any solids in solution) would pass through. If you mix flour or chalk dust in the water, it forms a suspension. Their particles are simply dispersed (spread) throughout the water and would eventually settle down to the bottom of the vessel if left undisturbed for some time. Table 5.15 shows the differences between solutions and suspensions.

An emulsion is a cloudy mixture of tiny droplets of one liquid suspended in another liquid. Sometimes two immiscible liquids will not separate out into two layers when mixed together. One of the liquid may form droplets and spread throughout the other to form an emulsion. Cooking oil and water do not mix but they will form an emulsion when they are mixed and shaken. Droplets of oil will spread throughout the water. Unlike pure liquids, emulsions are cloudy (opaque). So you cannot see through them. The emulsion will not settle like a suspension. Which other liquids you know can form suspensions?

Mixtures can be formed from different substances in two major ways.

The first type constitutes homogenous mixtures, where the substances are totally mixed together uniformly. Examples include solutions of salts and sugars in water.

The second type constitutes heterogeneous mixtures, where the substances remain separate and one substance is spread throughout the other as small particles, droplets, or bubbles. All emulsions and suspensions fall under this category. Examples include suspensions of insoluble solids or oil droplets in water.

This method is best applicable in separation of components of mixtures called suspensions.

A mixture of chalk dust or flour with water can be separated by filtering the suspension. The suspended particles get trapped in the filter paper. The trapped particles are called the residue. The water is called the filtrate

This is another method that can be used to separate mixtures called suspensions. However, in this case, separation will be successful if the suspended particles are large enough. Otherwise, the decantation exercise should be accompanied by filtration if you want to get a clear liquid.

Once the solid has settled to the bottom of the container (sedimented), the liquid can be carefully poured off. This is called decantation. Decantation can be applied to separate such components as mixtures of mud, sand or gravel in water and so on.

This method is used to separate substances that form a solution. In such a mixture, the solute is completely dissolved in a solvent to make a uniform solution. To separate these substances, the solution is heated so that the solvent evaporates, leaving the solid residue behind.

A mixture of salt or sugar in water can be separated by applying this method.

Separating a liquid from a solution can be carried out by distillation. The boiling point of a 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

This method can be used to obtain pure water from impure water or from water with dissolved impurities. The process may be used to separate a liquid from a solution or to separate two liquids whose boiling points differ by an appreciable temperature interval. This is a way of getting a pure solvent out of a solution.

Separating the liquids from a mixture of two (or more) miscible liquids is again based on the fact that liquids will have different boiling points. However, the boiling points are closer together than for solid-in-liquid solutions. It is difficult to separate mixtures of liquids whose boiling points differ by only a few degrees. In this case, fractional distillation is used.

For example, ethanol boils at 78°C whereas water boils at 100°C. When a solution of ethanol and water is heated, ethanol and water vapours enters the fractionating column. Evaporation and condensation take place as the vapours rise up the column. Ethanol passes through the condenser first as the temperature of the column is raised above the boiling point. Water condenses in the column and flows back into the flask because the temperature of the column is below its boiling point of 100°C.

The temperature on the thermometer stays at 78°C until the ethanol has distilled over. Eventually, the thermometer reading rises above 78C°. This is a sign that all the ethanol has been separated, so heating can be stopped. By watching the temperature carefully, the two liquids (fractions) can be collected separately.

Various forms of fractionating column can be used. Their general purpose is to provide surfaces, e.g. flat discs, on which ascending vapour can condense. Glass beads in the column provide a large surface area for condensation.

This is a technique used to separate a mixture of solids where one of the solids sublimes. Examples of solids which sublime are ammonium chloride, iodine, solid carbon dioxide and naphthalene. A mixture of any of these solids with another solid can be separated by sublimation.

Let us consider a mixture of iodine and sodium chloride. The mixture is placed in a beaker and covered with a filter funnel as shown in the diagram below. Then, as the mixture is heated, the ammonium chloride sublimes. The ammonium chloride vapour rises and condenses on the cooler walls of the filter funnel. The sodium chloride is left in the beaker.

Chromatography is a method used to separate and identify the components of a mixture. The method works by allowing the molecules present in the mixture to distribute themselves between two phases; a stationary phase and a mobile phase. Thus, the separation is based on the differences in the rates at which components of the mixture move through a stationary phase (porous medium) under the influence moving phase (solvent, such as liquid or gas). During separation by chromatography, a mixture is placed on stationary phase; then, the mobile phase passes through the stationary phase and solubilizes the components. The mobile phase carries the individual components a certain distance through the stationary phase, depending on their attractions to both of the phases. Molecules of a component of the mixture that spend most of their time in the mobile phase are carried along faster. For instance, during separation of the components of black ink by paper chromatography, different colours of the ink are observed as shown in Figure 5.12. This pattern of colours is called a chromatogram. The blue ink has the fastest speed. This means it is the most soluble in the solvent and least absorbed by the paper. The green ink has travelled the least. This means, it is the least soluble in the solvent and most absorbed by the paper

As the solvent (water) moves up the paper, the dyes are carried with it and begin to separate. They separate because they have different solubilities in water and are absorbed to different degrees by the filter (chromatography) paper. As they rise, they are gradually separated.

The different colours of the ink make a pattern of colours formed during the process of chromatography. This pattern of colours is called a chromatogram.

Figure aboveshows a chromatogram of black ink. The blue ink has the fastest speed. This means it is the most soluble in water and least absorbed by the paper. The green ink has travelled least. This means it is the least soluble in water and most absorbed by the paper.

Chromatography is used in many different ways. The following are some of the application of chromatography:

Mixtures of two immiscible liquids can be separated with a separating funnel. The mixture is placed in a separating funnel and allowed to stand. The liquids separate into two different layers. The lower denser layer is then "tapped" off at the bottom.

For example, when a mixture of kerosene and water is poured into the funnel, the kerosene floats to the top as shown in figure above. When the tap is opened, the water runs out. The tap is closed again when all water has gone, leaving the kerosene in the funnel.

Solvent extraction, also known as liquid-liquid extraction, refers to the separation of materials of different chemical types and solubilities by selective solvent extraction. That is, some materials are more soluble in one solvent than in another. The method is used to refine petroleum products, chemicals, vegetable oils, and vitamins.

This method is used is to separate a solid from a solution in which there is more than one solid dissolved. An example of this is a water solution of iodine and sodium chloride.

The solvent extraction works on two principles:

A centrifuge is used to separate small amounts of suspension. Centrifugation is used with insoluble solids where the particles are very small and spread throughout the liquid. In centrifugation, test tubes containing suspensions are spun round very fast. The solid gets thrown to the bottom. Here, it is no longer the force of gravity on the solid that causes settling.

Instead, there is a huge centrifugal force acting on the particles due to the high speed spinning of the samples. This causes the solid to be deposited at the bottom of the centrifuge tube.

After centrifugation, the liquid can be decanted (poured out) from the test tube, or removed with a small pipette.

This makes the solid to be left behind.

If the solid mixture contains iron, the iron can be removed using a magnet. This method is used to separate scrap iron from other metals. Magnetic iron ore can be separated from other material in the crushed ore by using an electromagnet. In the process of recycling metals, iron objects can be picked out from other scrap metals using electromagnets.

This process involves evaporation but the speed of evaporation is much slower. In principle the salt solution can be left in the evaporating basin for a long period until all the water has evaporated but in practice this takes longer time. The process begins by evaporating away the liquid. However, because the crystals are needed, evaporation is stopped after the solution has been concentrated enough. The concentrated solution is allowed to cool slowly and crystallize. The crystals so formed can be filtered off and dried. A similar process is used to extract salt from the sea. Salty sea water is placed in wide basins and put in the sun. Water evaporates off, leaving the salt crystals in basins.

This is a method used to separate grains from husks or bran. The process makes use of the differences in density of the constituents in the mixture. When the winnower is shaken around, grains, being denser than husks or bran, sink to the bottom of the winnower

The less dense husks or bran moves to the top. They are then blown off the winnower by wind or breath, or sometimes picked by hand and separated from the grains.

Matter is anything that has mass and occupies space.

Matter exists in three states, namely gaseous, liquid and solid states. Changes in states from one state to another are caused by alterations in temperature and pressure

A physical change is a change that does not result in new substances being formed. A chemical change is a permanent change in the chemical properties of a substance.

An element can be defined as a substance that cannot, by any known chemical process, be split into two or more simpler substances

A chemical symbol is a way of representing an element using the letter(s)

Elements can be classified as either metals or non metals. Metals and non metals have different physical and chemical properties

A compound is a substance that contains two or more elements chemically combined together. A mixture is something that contains two or more substances not combined chemically. Mixtures can be classified as solutions, suspensions or emulsions. The different methods of separating mixtures include filtration, decantation, evaporation, simple distillation, fractional distillation, sublimation, chromatography, layer separation, solvent extraction, centrifugation, magnetic separation, crystallization and winnowing