01 · Explore
How Can We Classify Mixtures?
Matter around us is rarely pure; it usually exists as mixtures of different substances. We classify these mixtures based on how uniformly their components are distributed.
A homogeneous mixture, also known as a solution, has a uniform composition throughout. For example, in a well-stirred sugar-water solution, every sip is equally sweet because the sugar particles are spread evenly. Other examples include vinegar (acetic acid in water) and aerated drinks like soda (carbon dioxide in water).
In contrast, a heterogeneous mixture does not have a uniform composition. The components remain physically distinct and are often visible to the naked eye. A mixture of sand and water is a classic example; the sand particles do not dissolve and eventually settle at the bottom. Other examples include oil mixed with water or chalk powder in water.
Colloids, such as milk, represent a unique category. While they appear homogeneous to the naked eye, they are technically heterogeneous because their particles are larger than those in a true solution but smaller than those in a suspension. Unlike suspensions, colloidal particles do not settle when left undisturbed.
| Type of Mixture | Composition | Visibility of Particles | Example |
|---|---|---|---|
| Homogeneous | Uniform throughout | Not visible | Salt solution, Vinegar |
| Heterogeneous | Non-uniform | Visible to naked eye | Sand and water, Chalk |
| Colloid | Appears uniform | Not visible to naked eye | Milk, Blood, Fog |
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Why is a mixture of salt and water considered homogeneous?
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02 · Explore
Concentration of a Solution
Concentration describes the exact amount of solute present in a specific amount of solvent or solution. This precision is vital in medicine, agriculture, and cooking.
A solution consists of a solute (the substance being dissolved) and a solvent (the substance that does the dissolving). For instance, in Oral Rehydration Solution (ORS), salt and sugar are solutes, while water is the solvent. The effectiveness of ORS depends on the correct ratio of these components.
Concentration is expressed quantitatively to ensure safety and efficacy. In agriculture, using too little pesticide may fail to protect crops, while too much can damage the environment. Scientists use three main percentage methods to communicate these ratios clearly: mass by mass, mass by volume, and volume by volume.
Mass by Mass Percentage (% m/m)
Mass % = (Mass of solute / Mass of solution) × 100
If 10 g of salt is dissolved in 90 g of water, the total mass of the solution is 10 g + 90 g = 100 g. The concentration is (10 / 100) × 100 = 10% m/m.
Mass by Volume Percentage (% m/v)
Mass by Volume % = (Mass of solute / Volume of solution) × 100
If 5 g of glucose is dissolved to make 100 mL of solution, the concentration is (5 / 100) × 100 = 5% m/v. This is common for medical drips.
Volume by Volume Percentage (% v/v)
Volume % = (Volume of solute / Volume of solution) × 100
If 1 mL of liquid pesticide is mixed with water to form 100 mL of spray, the concentration is (1 / 100) × 100 = 1% v/v.
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A talcum powder contains 4% m/m zinc oxide. How much zinc oxide is in a 300 g pack?
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03 · Explore
Solubility and Crystallization
Solubility is the maximum amount of solute that can dissolve in a fixed amount of solvent at a specific temperature. When this limit is reached, the solution is 'saturated'.
Temperature significantly affects solubility. For most solid solutes in liquid solvents, solubility increases as the temperature rises. Conversely, the solubility of gases in liquids generally decreases with higher temperatures. This property is exploited in the process of crystallization.
Crystallization is a technique used to obtain a pure solid from its saturated solution. By preparing a hot saturated solution and cooling it slowly, the excess solute that can no longer remain dissolved separates out as pure crystals. This method is superior to simple evaporation because it prevents the decomposition of solids and removes soluble impurities that remain in the liquid (filtrate).
Naturally occurring crystals include rock salt, sugar (mishri), and snowflakes. In the laboratory, adding a drop of dilute sulfuric acid during the preparation of copper sulfate crystals helps ensure purity by preventing unwanted chemical reactions.
- 1
Take 1 g of impure copper sulfate in a beaker and add 25 mL of water.
- 2
Add a drop of dilute sulfuric acid and heat the mixture while stirring.
- 3
Continue adding copper sulfate until no more dissolves (saturated).
- 4
Filter the hot solution to remove insoluble impurities.
- 5
Cover the filtrate with a watch glass and allow it to cool slowly and undisturbed.
- 6
Filter the resulting shiny blue crystals, rinse with cold water, and dry.
Pure, well-shaped crystals of copper sulfate are obtained, leaving impurities behind in the solution.
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What happens to the solubility of a solid in a liquid when the temperature increases?
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04 · Explore
Separating Miscible Liquids: Distillation
Distillation is used to separate a mixture of two miscible liquids (liquids that mix completely) or to recover a solvent from a solution.
The principle of distillation relies on the difference in boiling points of the components. The mixture is heated in a distillation flask; the liquid with the lower boiling point vaporizes first. These vapors are then passed through a water condenser, where they cool and condense back into a liquid called the distillate, which is collected in a separate vessel.
Simple distillation is effective when the boiling points of the two liquids differ by at least about 25 °C. For example, acetone (boiling point 56 °C) and water (boiling point 100 °C) can be easily separated this way. If the difference in boiling points is less than 25 °C, a more complex process called fractional distillation is used.
Fractional distillation is essential in petroleum refineries to separate crude oil into various fractions like petrol, diesel, and kerosene. In Kannauj, Uttar Pradesh, traditional distillation (the Deg-Bhapka method) is used to capture the earthy fragrance of the ground after rain, known as Mitti ka Ittar.
The Distillation Process
- 1
Heating
The mixture is heated in a flask until the component with the lower boiling point turns into vapor.
- 2
Vaporization
Vapors rise and enter the condenser, leaving higher-boiling components behind.
- 3
Condensation
Cold water circulating in the condenser cools the vapors back into liquid form.
- 4
Collection
The purified liquid (distillate) is collected in a separate conical flask.
Sequence of steps in separating miscible liquids using their boiling point differences.
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When is fractional distillation preferred over simple distillation?
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