JAMB Chemistry Note-Separation of Mixtures and Purification of Chemical Substances

Table of Contents

• 1. Pure and Impure Substances:
• 2. Boiling and Melting Points:
• 3. Elements, Compounds, and Mixtures:
• 4. Chemical and Physical Changes:
• 5. Properties of the Components of a Mixture:
• 6. Separation Processes:
• 7. Application of Separation Processes in Everyday Life:
• Summary:

1. Pure and Impure Substances:

• Pure Substances: A pure substance consists of only one type of atom or molecule. It has a fixed and uniform composition throughout. Examples include water (H₂O), gold (Au), and oxygen (O₂). Pure substances have fixed physical and chemical properties.
• Impure Substances: Impurities are foreign substances mixed with a pure substance. Impure substances do not have uniform composition, and their physical properties vary. Examples include air (a mixture of gases), soil, and seawater.

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2. Boiling and Melting Points:

• Boiling Point: The boiling point is the temperature at which a liquid changes into vapor. The boiling point is a characteristic property of a pure substance. For example, water boils at 100°C under standard conditions.
• Effect of Impurities on Boiling Point: When impurities are present in a substance, they typically increase the boiling point. This is known as boiling point elevation. The impurities disrupt the orderly arrangement of molecules in the liquid, making it harder for the liquid molecules to escape into the gas phase. As a result, the temperature at which the liquid boils is higher than that of the pure substance.
• Melting Point: The melting point is the temperature at which a solid turns into a liquid. For instance, pure ice melts at 0°C.
• Effect of Impurities on Melting Point: Impurities usually lower the melting point of a substance, a phenomenon called melting point depression. Impurities disrupt the regular arrangement of particles in the solid, making it easier for them to move and transition into the liquid phase at a lower temperature.

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3. Elements, Compounds, and Mixtures:

• Elements: These are pure substances made up of only one type of atom. Elements cannot be broken down into simpler substances by chemical means. Examples: oxygen (O₂), hydrogen (H₂), and carbon (C).
• Compounds: A compound is a substance formed when two or more elements chemically combine in a fixed proportion. Compounds can only be separated into their constituent elements by chemical reactions. Examples: water (H₂O), sodium chloride (NaCl), and carbon dioxide (CO₂).
• Mixtures: Mixtures consist of two or more substances (elements or compounds) physically combined. Each component retains its own properties. Mixtures can be separated by physical methods such as filtration, distillation, and chromatography. Examples: air, saltwater, and sand mixed with iron fillings.

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4. Chemical and Physical Changes:

• Chemical Changes: These are irreversible changes in which the composition of a substance is altered, and new substances are formed. Chemical reactions involve breaking and forming bonds. Examples include burning, rusting of iron, and digestion.
• Physical Changes: These are reversible changes where the composition of the substance remains the same, but its physical properties change. Physical changes involve changes in state or appearance without forming new substances. Examples include melting, freezing, and dissolving.

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5. Properties of the Components of a Mixture:

In a mixture, each component retains its own properties. For example:

• Salt in Water: Salt remains salt (sodium chloride), and water remains water, but when mixed, they form a saltwater solution. The properties of salt (such as solubility) and water (such as boiling point) are retained.
• Air: Air is a mixture of gases such as nitrogen (N₂), oxygen (O₂), and carbon dioxide (CO₂), each retaining its own properties.

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6. Separation Processes:

Different methods are used to separate mixtures based on the physical properties of their components. Below are some common separation techniques:

• Evaporation: This method involves heating a liquid to remove the solvent, leaving the dissolved solid behind. It’s commonly used to separate salt from seawater. In evaporation, the solvent (e.g., water) turns into vapor, while the solute (e.g., salt) remains.
• Simple Distillation: Simple distillation is used to separate a liquid from a mixture based on differences in boiling points. The mixture is heated, and the component with the lowest boiling point is vaporized and then condensed into a separate container. This is used for purifying liquids like water and alcohol.
• Fractional Distillation: This method is used to separate a mixture of liquids that have different boiling points. A fractionating column is used to separate the liquids into fractions. It is commonly used in oil refineries to separate crude oil into its components (e.g., gasoline, kerosene).
• Sublimation: This technique is used to separate a substance that changes directly from a solid to a gas without passing through the liquid phase. Substances like iodine, naphthalene, and dry ice (solid CO₂) undergo sublimation.
• Filtration: Filtration separates solid particles from a liquid or gas by passing the mixture through a porous material (filter paper, mesh, etc.). It’s used to separate sand from water or to purify liquids like drinking water.
• Crystallization: This process is used to purify solids. It involves dissolving a solid in a solvent and then evaporating the solvent slowly. As the solvent evaporates, the solute forms pure crystals. This method is used to purify substances like salt and sugar.
• Paper Chromatography: This technique is used to separate different components of a mixture based on differences in their solubility in a solvent. It is commonly used for separating pigments in inks, plant dyes, and food colorants.
• Column Chromatography: This method is similar to paper chromatography but involves using a column filled with a stationary phase (like silica gel). It’s used to separate compounds from complex mixtures, such as in drug purification.
• Magnetization: A magnet is used to separate magnetic materials from a mixture. For example, separating iron filings from sand or magnetic metals from non-magnetic substances.
• Decantation: This involves pouring off the liquid portion of a mixture while leaving the solid material behind. It’s often used to separate oil from water or liquid from sediment.

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7. Application of Separation Processes in Everyday Life:

• Evaporation: Salt is obtained from seawater by evaporating the water, leaving the salt behind. Similarly, sugar is purified by evaporating water from sugar solutions.
• Distillation: Distillation is used in the production of potable water, where contaminants are removed by distillation. It’s also used in the alcohol industry to distill alcoholic beverages like whiskey and vodka.
• Filtration: Filtration is used in water purification plants to remove dirt and debris from water before it’s made drinkable. It’s also used in coffee making and air filtration.
• Chromatography: Chromatography is employed in forensic science, medicine, and food testing to separate substances, identify chemicals, and ensure purity.
• Crystallization: Sugar and salt industries use crystallization to purify and collect sugar and salt from their respective solutions.

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Summary:

In this topic, we explored the principles and methods used in separating mixtures and purifying substances. The separation methods discussed above allow chemists to isolate components of mixtures based on their physical properties, such as boiling point, solubility, and magnetism. These techniques have practical applications in fields such as water treatment, food production, and pharmaceutical industries.

Each separation method works on a unique principle, and selecting the appropriate technique depends on the nature of the mixture and the physical properties of its components. Understanding the effects of impurities on the boiling and melting points of substances is crucial when evaluating the purity of a substance and selecting suitable purification methods.