Physical and Chemical changes: There are some changes during which no new substances are formed. On the other hand, there are some other changes during which new substances are formed. So, on the basis of whether new substances are formed or not, we can classify all the changes into two groups: Physical changes and chemical changes.
Fig: Physical and Chemical changes
Physical Properties: The properties that can be observed and specified like colour, hardness, rigidity, fluidity, density, melting point, boiling point etc. are the physical properties.
Physical changes: Those changes in which no new substances are formed are called physical changes. In a physical change, the substances involved do not change their identity. They can be easily returned to their original form by some physical process. This means that physical changes can be easily reversed to form the original substance. The changes in physical state, size and shape of substances are called physical changes.
Example: When ice is heated it melts to form water. Though ice and water look different, they are both made of water molecules. Thus no new chemical substance is formed during the melting of ice. So, the melting of ice to form water is a physical change. When water is cooled, then water solidifies to form ice. This is called freezing of water. The freezing of water to form ice is also a physical change. Some other examples of physical changes are Boiling of water, condensation of steam, the ringing of an electric bell and breaking of a glass.
Fig. Physical Change i.e. Melting of Ice
Chemical changes: Those changes in which new substances are formed are called chemical changes. A chemical change is also called a chemical reaction. In a chemical change, the substances involved, change their identity.
They get converted into entirely new substances. The new substances usually cannot be returned to their original form. This means that chemical changes are usually irreversible.
Example: When a magnesium wire is heated it burns in air to form a white powder called 'magnesium oxide'. This magnesium oxide is an entirely new substance. Thus a new chemical substance is formed during the burning of a magnesium wire is a chemical change. Some other examples of chemical changes are: Burning of a candle, Burning of character, and burning of hydrogen in oxygen to form water.
Fig: Burning of MagnesiumDifferences between Physical and Chemical Changes:
New substances are not formed
New substances are formed
A physical change is a temporary change
A chemical change is a permanent change
A physical change is easily reversible
A chemical change is usually irreversible.
The mass of substances does not alter in a change
The mass of substance does alter in a chemical physical change
Separating the components of a mixture : Many of the material around us are mixtures, these mixtures have two or more than two constituents mixed in them. It may not be possible to use a mixture as such in homes and in industries. We may require only one or two separate constituents of a mixture for our use. So, we have to separate the various mixtures into their individual constituents to make them useful in our daily life.
The various constituents of a mixture have different physical properties such as density, solubility, size of particles volatility, boiling points, etc.
"Heterogeneous mixtures can be separated into their respective components by simple physical methods such as hand picking, sieving, filtration etc. in everyday life."
However, for separating homogeneous mixtures special techniques are employed depending upon the difference in one or more.
Separation of coloured components (Dye) from Blue or Black ink : The blue ink (or black ink) used in fountain pens is a liquid mixture. It is a mixture of a 'dye' in water. We can separate the coloured component (dye) by the process of evaporation. In process of separation, we do not heat the china dish containing ink directly over the flame.This is because the 'dye' obtained from ink can get decomposed by the high temperature produced by the direct heating with a burner. We use a 'water bath' for evaporating ink.
Fig. Ink separation using evaporation method
Experiment to obtain coloured component (Dye) from ink : We take a beaker and fill it half with water (as a water bath). About 5 ml of ink (Blue ink or black ink) is put in a china dish. The china dish containing ink is then placed over the mouth of the beaker containing water, which is kept on a tripod stand.
We now start heating the beaker with a burner, soon the water in beaker starts boiling to form steam, this steam heats the ink in the china dish. Due to this heating, the water present in ink starts evaporating gradually. When all the water has evaporated from the ink. we stop heating. We will find that a small amount of a solid coloured material is left in the china dish. Thus we can separate the volatile component (Solvent) from its non-volatile solute by the method of evaporation.
Separation of Cream from milk : Sometimes the solid particles in a liquid are very small and pass through a filter paper. for such particles, the filtration technique cannot be used for separation. Such mixtures are separated by centrifugation.
Centrifugation : The method of separating finely suspended particles in a liquid, by whirling the liquid at a very high speed is called centrifugation.
Fig: CentrifugationPrincipal of centrifugation: It is based on that when a very fine suspension or a colloidal solution is whirled rapidly, the heavier particles are forced towards the bottom of liquid and the lighter stay at the top.
Method : Milk is a suspension of tiny droplets of oil cream in a water of liquids. The milk is put in a closed container in big centrifugation machine. When the centrifugation machine is switched on, the milk is rotated at a very high speed in its container. Due to this, the milk separates into 'cream' and 'skimmed milk'. The cream, being lighter, floats over the skimmed milk can then be removed. Thus, the cream is separated from milk by centrifugation.
Applications of centrifugation :
Separation of a mixture of two immiscible liquids : The separation of two immiscible liquids is based on the difference in their densities. The apparatus used for separation is separating funnel.
Methods : The mixture of two immiscible liquids is put in a separating funnel and allowed to stand for some time. The mixture separates into two layers according to the densities of the liquids in it.
Water and kerosene oil are two immiscible liquids. Pour the immiscible liquids mixture in the separating funnel. Allow the mixture to stand for half an hour or more. It forms two layers. Water being heavier forms the lower layers in the separating funnel. Whereas kerosene, being lighter, forms the upper layer:
Fig: Separation of liquids on opening the stop-cock of separating funnel, the lower layer of water comes out first and collected in a beaker. When the water layer has completely run off, then the stopcock is closed. The kerosene is left behind in the separating funnel. It can be removed in a separate breaker by opening the stop-cock again.
Separation of a mixture of common salt and Ammonium Chloride : This method is used in the separation of such solid-solid mixtures where one of the components sublimes on heating. However, it is useful only if the components of the mixture do not react chemically on heating. So, we can separate ammonium chloride from a mixture of common salt and ammonium chloride by this process.
Fig. Separation of Common salt and Ammonium Chloride
Methods : The mixtures of common salt and ammonium chloride are taken in a china dish and placed on a tripod stand. The china dish is covered with an inverted glass funnel. A loose cotton plug is put in the upper, open end of the funnel to prevent the ammonium chloride vapours from escaping into the atmosphere.
The china dish is heated by using a low Bunsen flame, on heating the mixture ammonium chloride changes into white vapour. These vapours rise up and get converted into solid ammonium chloride on coming in contact with the cold, inner walls of the funnel.
When the mixture gives off no more white fumes, lift the funnel, scrap the fine white powder from its sides on a piece of paper. This is pure ammonium chloride. The residue left behind in the funnel is sodium chloride. Some examples of solids which sublime are camphor, naphthalene and anthracene.
Chromatography : The process of separation of different dissolved constituents of a mixture by adsorbing them over an appropriate adsorbent material is called Chromatography. Kroma in Greek means colour. The adsorbent medium is generally magnesium oxide, alumina or filter paper.
There are many types of chromatography but the simplest form is the paper chromatography. This separation is based on the fact that the different constituents of a mixture get adsorbed differently on the same adsorbent material because they have different rates of movement. The rate of movement of each adsorbed material depends upon.
The relative solubility of the constituent of a mixture in a given solvent. The relative affinity of the constituents of the mixture for the adsorbent medium. Paper chromatography is very useful in separating various constituents of coloured solutes present in a mixture lime, ink, dyes etc.
Separation of coloured constituents present in a mixture of ink and water.
Method : The different coloured dyes present in black ink can be separated by performing a paper chromatography.
Fig: Layer Chromatography and Column Chromatography
Separation of the 'dyes' in black ink by paper chromatography
Separation of a mixture of two miscible liquids :
Those liquids which mix together in all proportions and form a single layer are called miscible liquids. Alcohol and water are miscible liquids because they mix together in all proportions and form a single layer on mixing.
To separate a mixture of two or more miscible liquids for which the difference in boiling points is less than 25 K, fractional distillation process is used.
A simple fractionating column is a long vertical glass tube filled with glass beads. The glass beads provide a large surface area for hot vapours to cool and condense respectively.
Separate a mixture of Ethyl alcohol and water:
Method : Ethyl alcohol and water are miscible liquids. The boiling point of ethyl alcohol is 78°C and the boiling point of water is 100°C, a mixture of ethyl alcohol and water can be separated by fractional distillation. The mixture of ethyl alcohol and water is heated in distillation flask fitted with a fractionating column. When the mixture is heated, both ethyl alcohol and water form vapours as their boiling points approach. The ethyl alcohol vapour and water vapour rise up in the fractionating column.
Fig: Fractional Distillation
The upper part of the fractionating column is cooler, so as the hot vapours rise up in the column, they get cooled, condense and trickle back into the distillation flask. As the experiment goes on, the fractionating column warms up by the heat released by the condensed vapours. After some time, the temperature at the top of the column is much less than at its bottom. When the temperature at the top of the fractionating column reaches 78°C, then ethyl alcohol vapour passes into the condenser, gets cooled and collects in a beaker kept at the end of the condenser. The ethyl alcohol water mixture is kept boiling at such a rate that the thermometer shows the boiling points of ethyl alcohol (78°C). In this way, all the ethyl alcohol distills over and gets separated.
Separation of the Gases of the air : Air is a mixture of gases like nitrogen, oxygen, argon, carbon dioxide, helium, neon etc. The major component of air is nitrogen (78.03%). The second major component of air is oxygen (20.99%) and the third major component of air is argon (0.93%).
All these gases are obtained from the air on a large scale. This is because air is the cheapest source of these gases.
Air is a homogeneous mixture and can be separated into its components by fractional distillation of liquid air.
Fig. Separation of components of air
Separation of components of air:
Fig. Separation of Components of air
Separation of pure copper sulphate from an impure sample :
The process involved in obtaining pure copper sulphate from its impure sample is crystallisation.
Crystallisation : The process of cooling a hot, concentrated solution of a substance to obtain crystals is called crystallisation. Crystallisation is a process that separates a pure solid in the form of its crystals from a solution.
1. Purification of salt that we get from sea water
2. Separation of crystals of alum (phitkari) from impure samples.
Supply of drinking water in a city : In cities, drinking water is supplied from water works. In water works, the methods like sedimentation, decantation, loading, filtration and chlorination etc are used to remove undesirable materials from water. The source of water supply in a city is either a nearby river or a lake called reservoir. The river water and lake water usually contain suspended solid substances and some germs, so, before this water can be supplied to homes, it must be purified to remove suspended impurities as well as germs.
Fig: Water purification
Water purification system in water works:
The purification of river water or lake water is done in the following steps.
(1) Sedimentation: The water is allowed to stand in big tanks, where heavier suspended impurities settle down. To increase the rate of sedimentation, alum is added to it. The impurities settle at the bottom.
(2) Filtration: The semi-clear water is allowed to pass through beds of sand, charcoal and gravel to remove suspended impurities.
(3) Removal of harmful - organism or sterilization: The harmful bacteria in filtered water can cause very serious diseases such as typhoid, cholera etc. Thus, to the filtered water bleaching powder or chlorine gas is added. This kills the microorganisms and hence the water becomes fit for drinking. This water is directly pumped into overhead tanks for supply to a city.