Introduction
In this chapter, the concept of pure substances and mixtures is explored in detail, examining their properties, types, and methods of separation. The discussion explores the difference between elements, compounds, and mixtures, highlighting the importance of understanding the nature of matter in scientific study and everyday life.
What is Pure Substances?
A pure substances consist of a single type of matter. Pure substances can be elements or compounds.
Elements: Elements are substances that cannot be broken down into simpler substances by chemical means. Examples include hydrogen, oxygen, and iron.
Compounds: Compounds are substances composed of two or more elements chemically combined in fixed proportions. Examples include water (H₂O) and carbon dioxide (CO₂).
What is Mixtures?
Mixtures are combinations of two or more substances where each retains its individual properties. Mixtures can be homogeneous or heterogeneous.
1. Homogeneous Mixtures:
Definition: Uniform composition throughout. Examples: Air, Salt dissolved in water, sugar dissolved in water, stainless steel.
Properties of homogeneous mixture:
●Particle size <1 nm,
●Cannot see individual components with the naked eye,
●Even distribution of components, uniform color and texture,
●Do not scatter light, making the path of light invisible,
●Particles do not settle down,
●Indicating stability.
2. Heterogeneous Mixtures:
Non-uniform composition. Examples: mixtures of oil and water, soil and sand, salt and sulphur.
Properties heterogeneous mixture:
●Components are visible, non-uniform textureand
●can often be separated by physical means.
What is a Solution?
A solution is a homogeneous mixture of two or more substances. It can be a solid, liquid, or gas dissolved in a liquid.
Components of a Solution:
(i) Solvent: The component present in a larger amount that dissolves the solute.
(ii) Solute: The component dissolved in the solvent.
Example: In a sugar solution, sugar is the solute, and water is the solvent.
Properties of Solutions:
●Homogeneous at the particle level.
●Particle size less than 1 nm
●The solute particles cannot be seen by the naked eye.
●Solutions do not scatter light, making the path of light invisible.
●The components of a solution cannot be separated by filtration but can be separated by evaporation or distillation.
Concentration of Solutions
The concentration of a solution is the amount of solute present in a given amount of solution. It can be dilute, concentrated, or saturated depending on the amount of solute.
Saturated Solution: Contains the maximum amount of solute that can be dissolved at a given temperature.
Unsaturated Solution: Contains less amount of solute than the saturation level.
Methods of Expressing Concentration:
(i) Mass by Mass Percentage:
Mass by Mass percentage = (Mass of solute)×100 / (Mass of solution)
(ii) Mass by Volume Percentage:
Mass by volume percentage = (Mass of solute)×100 / (Volume of solution)
(iii) Volume by Volume Percentage:
Volume by Volume percentage = (Volume of solute)×100 / (Volume of solution)
What is a Suspension?
A suspension is a heterogeneous mixture in which the solute particles do not dissolve but remain suspended throughout the bulk of the medium.
Properties of Suspensions
●Particles are visible to the naked eye.
●They scatter light, making the path of light visible.
●Solute particles settle down when left undisturbed, indicating instability.
●They can be separated by filtration.
What is a Colloidal Solution?
A colloidal solution, or colloid, is a mixture where particles are uniformly spread but not dissolved, appearing homogeneous.
Properties of Colloids:
●Appears homogeneous but is actually heterogeneous.
●Particles cannot be seen with the naked eye but scatter light (shows Tyndall effect).
●Particles do not settle down on standing.
●Cannot be separated by filtration but by centrifugation.
Components of Colloids
Dispersed Phase: The solute-like component.
Dispersing Medium: The solvent-like component.
Common Examples of Colloids
Aerosols: Dispersions of liquid or solid particles in a gas (e.g., fog, cloud, smoke).
Foams: Dispersions of gas in a liquid or solid (e.g., whipped cream, sponge).
Emulsions: Dispersions of liquid in liquid (e.g., milk, mayonnaise).
Gels: Dispersions of liquid in a solid (e.g., jelly, butter).
Separating the Techniques of a Mixture:
Different techniques can be used to separate the components of a mixture based on their physical properties. Some common methods include:
Evaporation: Used to separate volatile solvents from non-volatile solutes (e.g., salt from water, separating dye from ink).
Centrifugation: Used to separate components with different densities (e.g., separating cream from milk).
Separating Funnel: Used for separating immiscible liquids (e.g., kerosene oil from water).
Sublimation: Separates substances that sublimate from those that do not (e.g., ammonium chloride from salt).
Chromatography: Separates different components of a mixture based on their movement on a stationary phase.
Distillation: Separates miscible liquids based on boiling points (e.g., separating acetone from water).
Fractional Distillation: Used for separating a mixture of different boiling points (e.g., separating gases from air).
Filtration: Used to separate solids from liquids or gases using a porous barrier that allows the fluid to pass through but not the solid particles.
Crystalisation: Crystallization is utilized to purify solids. For instance, salt extracted from seawater can contain impurities. Through crystallization, these impurities are removed, leaving behind pure salt crystals separated from the solution.
Physical and Chemical Changes:
Physical Changes: Changes that affect the form of a chemical substance but not its chemical composition. Examples include changes in state (melting, boiling) and physical deformation (cutting, bending).
Chemical Changes: Changes that result in the formation of one or more new substances with new chemical and physical properties. Examples include rusting of iron, combustion, and digestion of food.
Differences between Mixtures and Compounds:
Mixtures: Consist of two or more substances mixed together but not chemically combined. Components can be separated by physical means.
Compounds: Consist of two or more elements chemically combined in fixed proportions. Components can only be separated by chemical means.