Elements, Compounds and Mixtures PDF

## Chapter 3: Elements, Compounds and Mixtures **Inside the Chapter** - Elements, compounds and mixtures: a review - Principles of separation of mixtures - Separation techniques **Warm-up** A centrifuge is used to separate suspended particles from a mixture. A model centrifuge is shown in the figu...

## Chapter 3: Elements, Compounds and Mixtures **Inside the Chapter** - Elements, compounds and mixtures: a review - Principles of separation of mixtures - Separation techniques **Warm-up** A centrifuge is used to separate suspended particles from a mixture. A model centrifuge is shown in the figure. In this lesson, you will learn how a real centrifuge works and where it is used. **Elements, Compounds and Mixtures: A Review** You have already learned about elements, compounds and mixtures. Matter is anything that occupies space and has mass. Matter can exist in two forms, that is, pure substances and mixtures. | | | |---|---| | Matter | * Pure substances <br> * Elements <br> * Compounds | | | * Mixtures <br> * Homogeneous <br> * Heterogeneous | **Pure Substances** A pure substance can be an element or a compound. It has a definite composition and fixed chemical properties. For example, copper and aluminium are elements and salt and sugar are compounds. Elements and compounds are pure substances. - A pure substance has a definite composition. For example, a molecule of salt consists of one atom of sodium and one atom of chlorine. You cannot have a molecule of salt that has a different number of sodium and chlorine atoms. - A pure substance has a fixed melting point, boiling point and density. Pure water at sea level boils at 100°C. - A pure substance has a characteristic colour, odour and taste. **Mixtures** A mixture is made up of two or more pure substances (elements, compounds or both) that retain their individual properties. The components that make up a mixture are not chemically combined. Lemon juice is a mixture of lemon extract, sugar and water. It has the sour taste of lemon and the sweetness of sugar. We can differentiate between the ingredients by their taste. Air contains a number of gases. Each gas shows its property. Air supports combustion because oxygen is present in it. Air turns lime water milky because it contains carbon dioxide. Milk is a mixture of cream and water. We can separate cream from milk by churning. Sugar can be separated from a sugar solution by evaporating the water. - The components are present in any proportion. - The components retain their properties. - A mixture does not have a specific boiling and melting point or a characteristic colour, smell or taste. - Components can be separated by easy physical means. **Types of Mixtures** Different substances in different states may form a mixture. These are given below: * **Solid-solid mixture:** For example, rice and sand; dal and rice; soil, salt, and flour. * **Solid-liquid mixture:** For example, sugar syrup, salt solution, starch solution. * **Liquid-liquid mixture:** For example, lemon juice, alcohol and water; oil and water; milk and water. * **Gas-solid mixture:** For example, smoke and dust particles in air. * **Gas-liquid mixture:** For example, aerated drinks, oxygen dissolved in water. * **Gas-gas mixture:** For example, air, acid vapours in air, water vapour in air. **Homogeneous Mixtures** In a homogeneous mixture, the components cannot be differentiated by colour, shape or size. They are evenly distributed. The mixture appears as one single entity. For example, in a salt solution, we cannot see salt particles. The solution appears just as a liquid. Homogeneous mixtures are generally found in the form of solutions and alloys. * **Solution:** A solution is a homogeneous mixture of two or more substances. Components of a solution can be easily separated. For example, solids can be separated from their aqueous solutions by evaporation or crystallisation. * **Alloys:** An alloy is a homogeneous mixture of two or more metals or a metal and a non-metal. Alloys are obtained by melting the metals together. Brass is an alloy of copper and zinc. Bronze is an alloy of copper and tin. Steel is an alloy of iron and carbon. Alloys are also called solid solutions. **Heterogeneous Mixtures** A heterogeneous mixture is a mixture having a non-uniform composition. The composition varies from one region to another, with at least two phases that remain separate from each other, with clearly identifiable properties. If you examine a sample of a heterogeneous mixture, you can see the separate components. Ice cubes in cola form a heterogeneous mixture. Some other examples of heterogeneous mixture are chalk powder in water, iodine and sand, rice and dal, etc. Heterogeneous mixtures are generally found in the form of suspensions and emulsions. * **Suspension:** A suspension is a heterogeneous mixture containing solid particles that are sufficiently large for sedimentation. The components can be separated by filtration. It is a mixture in which the solute particles are not dissolved, but get suspended throughout the bulk of the medium. Particles of the suspension are visible to the naked eye. A suspension is formed when particles are left floating around freely in a solvent. Some examples of suspensions are sand in water, chalk in water, dust in air, etc. * **Emulsion:** Two immiscible liquids form an emulsion. In most cases, one of the components is water. A mixture of oil and water is an emulsion. When one of the components is thicker and present in excess, the emulsion is a semi-solid or paste-like mass. Face creams, shaving creams and butter are examples of emulsion. **Pause and Do** 1. Complete the flowchart: ``` Matter ``` 2. Give two examples of each. (a) Pure substances (b) Mixtures 3. Write one property of pure substances. **Separation Techniques** We need many substances to make our life convenient and comfortable but most of these substances are available in the form of mixtures. These mixtures contain unwanted substances which may be harmful and may degrade the properties of mixtures. For example, seawater is rich in common salt, which is an important ingredient of our food to add taste and nutrients. But seawater cannot be directly used to get the salt. It is necessary to separate salt from seawater. The purpose of separating the components of a mixture is to: - Remove unwanted and harmful substances. - Obtain pure and useful substances. **Principles of Separation Techniques** There are different techniques of separating mixture known to us today. But every method cannot be applied to separate every mixture. The separation technique that should be used depends upon the properties of its components such as: - Physical state - Size, shape and colour - Boiling point, melting point, density, volatility, solubility, magnetic properties, ability to sublime, etc. A component can be separated from its mixture based on the property it possesses, which other components do not have. This is the differentiator while using a separation method. You have already learnt about several separation techniques in the previous class. Some of them, the principles behind them and examples are given below. | Separation Technique | Property of the components (principle involved) | Examples | |---|---|---| | Sieving | Size of the components | Sand separated from gravel at construction sites | | Handpicking | Size, shape and quantity | Stones separated from rice | | Sedimentation and decantation | Weight of the components | Muddy water separated into mud and clear water | | Filtration | Size of the components | Straining lemonade to separate lemon seeds | | Evaporation | Volatility | Separation of common salt from seawater | | Magnetic separation | Magnetisation | Separation of iron scrap from waste | **Handpicking** This method involves separating a mixture by picking out components using one's hands. If the components can be differentiated by colour, size and shape and one of the components is less in quantity, the mixture can be separated by handpicking. For example, rice mixed with stones or pulses containing clay particles can be separated by handpicking. **Sieving** This method of separating a mixture involves using a sieve. If particles differ in size, they can be separated by sieving. This method is usually preferred when the components are of different sizes and one of the components is small enough to pass through the holes of a sieve. **The distillation flask is fitted with a fractionating column, which is connected to the condenser. There is a thermometer inserted into the fractionating column. The temperature across the column varies; it is higher at the bottom and lower at the top. The vapours of the component with a higher boiling point condense at the bottom of the column. The liquid with the lower boiling point vapourises first and rises up. The vapours of the other liquid coming along in small quantities will be condensed and sent back into the flask by the fractionating column. The vapours of the liquid with the low boiling point rise and are condensed in the condenser. This liquid is then collected in a container. After all the first component is removed, the temperature starts rising till the boiling point of the second liquid is reached. Its vapours are also condensed and collected in a different container. Crude petroleum oil is refined using this principle. Refining is done in a fractionating tower. The fractions obtained are petroleum gas, petrol, kerosene, diesel, lubricating oil and paraffin wax. What is left out is asphalt, used for making roads.** **Experiment** **Aim:** To separate the components of a mixture of CaCO3 and NaCl, or BaSO4 and NaCl. **Materials required:** Beaker, conical flask, funnel, filter paper, water, and a mixture of CaCO3 and NaCl, or BaSO4 and NaCl. **Procedure:** - Pour the mixture in the beaker. - Add some water in the beaker and stir it thoroughly until all the salt (NaCl) is dissolved. If required, add some more water. - Make a cone of a filter paper, fix it in the conical funnel and set the apparatus, as shown. - Pour the content of the beaker gently into the funnel. - Heat the conical flask to dry up the water. **Observation:** CaCO3 or BaSO4 filters out and remains partly in the beaker and partly on the filter paper. The conical flask contains the flakes of NaCl. **Conclusion:** The combination of filtration and evaporation methods can be used to separate a mixture of CaCO3 and NaCl, or BaSO4 and NaCl, because CaCO3 or BaSO4 is insoluble while NaCl is soluble in water. **Combination of Methods for Separation** A mixture which has more than two components can be separated into individual components by using a combination of separation methods. The separation of mixtures by using a combination of methods will become clear from the following examples. **Separation of iron filings, ammonium chloride and sand from their mixture:** Whenever you want to separate the components of a mixture, you should first look for the properties which are different for different components. Here, you have three components, i.e., iron filings, ammonium chloride and sand. Iron is attracted by a magnet, so it can be removed by the magnetic separation. Now, we can heat ammonium chloride and sand. The fumes of ammonium chloride are condensed. Ammonium chloride undergoes sublimation. Now, you are left with sand. **Separation of salt, sand and grain from their mixture.** Grain is separated from the mixture by sieving. Now, you are left with the mixture of salt and sand. Add some water and make a solution. The solution is filtered to obtain sand (residue) and a salt solution. Sand is insoluble in water and can be easily removed by filtration. Salt can be obtained from the salt solution by evaporation. **Summary** - Elements and compounds are pure substances. - The properties of compounds are different from that of its constituent elements. - Mixtures are either homogeneous or heterogeneous. - An alloy is a mixture of a metal with another metal or non-metal. - In a suspension, one component does not dissolve completely into the solvent. - An emulsion is a mixture of two or more immiscible liquids. **Glossary** - **Compound:** a substance formed when two or more elements combine chemically in a fixed proportion - **Homogeneous mixture:** a mixture in which the components are distributed uniformly throughout - **Heterogeneous mixture:** a mixture in which the components are distributed non-uniformly - **Froth flotation:** a method of separation of a mixture based on the principle of differential wetting - **Distillation:** a method of separation of a liquid from a soluble component by first converting the liquid into its vapour and then condensing the vapour **Fractional Distillation** This is used to separate mixtures comprising miscible liquids like alcohol and water. It is based on the difference in the boiling points of the liquids to be separated. Fig. 3.12 shows the set-up for fractional distillation. **Experiment** **Aim:** To separate a mixture of immiscible liquids **Materials required:** Mixture of kerosene oil and water, or CCI4 (carbon tetrachloride) and water, separating funnel **Procedure:** - Pour the mixture of kerosene oil and water in a separating funnel. - Keep it undisturbed till two separate layers of oil and water are visible. - Open the stopcock of the separating funnel and pour the lower layer into a beaker carefully. - Close the stopcock immediately as the other layer reaches near the stopcock. **Observation:** One of the liquids is left in the separating funnel and the other is obtained in the beaker. **Procedure:** - Take the mixture of wheat and chaff (or small bits of paper) on a plate. Go to an open ground. - Now, stand on a raised structure and drop the mixture slowly on the ground, constantly shaking the plate. **Observation:** The chaff or bits of paper form a separate heap, some distance away from where the grains drop, and are thus separated. **Froth Floatation** The principle behind this method is the differential wetting of components of a mixture by water. It is used to separate metal ore from mud and other impurities in the case of ores lighter than the gangue or impurities. For example, sulphide ores such as galena (ore of lead) and zinc blende (ore of zinc). The impure ore is mixed in water in a tank, to which pine oil and ethyl xanthate are added. When air is bubbled through the mixture from the bottom of the tank, a thick froth is formed. As sulphide ores are insoluble in water and are lighter, they are carried away by the froth to the surface and are skimmed off. Impurities, as they get wet by the water, become heavy and settle at the bottom. **Sublimation** Substances which sublime are separated by this method. This method is also used to purify compounds. For example, a mixture of ammonium chloride and salt can be separated by sublimation as ammonium chloride sublimes. On heating, ammonium chloride vaporises and gets deposited near the tail end of the funnel. Salt is left in the china dish. **Distillation** If the liquid component is required in a pure form, the solution is subjected to distillation. The principle behind this separation method is the difference between the boiling points of the components of the mixture. **Centrifugation** Finely suspended solid components in a liquid are separated by spinning the mixture in a closed container at a very high speed. This is called centrifugation. This method is used to separate the components of a mixture based on the difference in density. **When a suspension of a solid in liquid is subjected to a spin, the heavier substance will settle at the bottom due to the centrifugal force. Heavier particles experience a larger centrifugation force than the lighter particles. Centrifugation is used in cases where filtration doesn't work due to the smaller size of the solid particles. Best examples are colloids like blood, separating cream and butter from milk, etc. This method is used in hospitals, pathology labs, dairy farms and industries.** **A simple laboratory centrifuge has a vertical shaft with two, four or more than four test tube holders. When using a centifuge, the mixture is taken in one test tube and water in the test tube kept just opposite it. This is called the balancing test tube. When the motor works, the shaft rotates. The solid particles settle at the bottom.** **Separating Funnel** A separating funnel is shown in Fig. 3.11. There is a stopcock at the tail end of the funnel. It is used to control the flow of the liquid. A liquid mixture, when poured into the funnel and allowed to stand, separates into different layers. Each layer can be drawn out into different containers.