Grade 8 Science Lesson: Separation of Mixtures and Solutions
Welcome, Grade 8 students, to our lesson on Separation of Mixtures and Solutions! In our daily lives, we are surrounded by countless mixtures—from the syrup in bubble tea and saline solution for cleaning wounds, to harvesting sea salt in salt fields or extracting essential oils from herbs. The concepts in this unit will help you understand the nature of solutions and teach you how to separate components from mixtures for practical use. If it seems tricky at first, don't worry! Take your time, read through, and let's master these concepts step-by-step together!
Part 1: Solutions and Solubility
A solution is classified as a homogeneous mixture made up of extremely tiny particles evenly distributed throughout. A solution always consists of two main components:
• Solvent: The substance present in a greater amount, or the one that has the same state of matter as the final solution.
• Solute: The substance that is dissolved and dispersed, present in a smaller amount, or the one whose state changes to match the solvent.
Criteria for Identifying the Solvent and Solute
1. When the mixed substances are in different states of matter: The substance that shares the same state as the final solution is the solvent. For example, when granulated sugar (solid) dissolves in water (liquid) to form sugar syrup (liquid), water is the solvent and sugar is the solute.
2. When the mixed substances are in the same state of matter: The substance present in the greater quantity is the solvent. For example, 70% rubbing alcohol consists of 70 cubic centimetres of ethanol and 30 cubic centimetres of water. Here, ethanol is the solvent and water is the solute.
Solubility and Factors Affecting It
Solubility is the maximum amount of a solute that can dissolve in a given amount of solvent at a specific temperature and pressure. When no more solute can dissolve, we call it a saturated solution. The key factors that affect solubility include:
• Nature of the solute and solvent: Different substances have different dissolving abilities. Some substances dissolve very well in water, while others dissolve better in alcohol or oil.
• Temperature: For most solids and liquids, solubility increases as temperature rises. However, for gases, solubility decreases as temperature increases.
• Pressure: This significantly affects gases. When pressure increases, the solubility of a gas in a liquid increases.
Did you know?
When you open a bottle of fizzy soft drink and hear a "fizz" while bubbles rapidly rise, that's because the pressure inside the bottle drops suddenly! This decreases the solubility of the dissolved carbon dioxide gas, causing it to escape from the liquid immediately!
Part 2: Concentration of Solutions in Percentage Units
The concentration of a solution expresses the amount of solute present in a given amount of solution. In Grade 8, we learn to calculate concentration in percentage units, which are divided into three types based on the states of matter:
1. Percentage by Mass (% by mass or % w/w)
Commonly used when both the solute and the solvent are solids, or when both are measured by mass, such as in metal alloys, rose gold (tombac), or chemical powders.
Formula:
\( \text{ร้อยละโดยมวล} = \frac{\text{มวลของตัวละลาย}}{\text{มวลของสารละลาย}} \times 100 \)
Example: A 200-gram sugar solution contains 20 grams of dissolved sugar. What is the concentration of this solution in percent by mass?
Solution: \( \text{ร้อยละโดยมวล} = \frac{20}{200} \times 100 = 10\% \) by mass
2. Percentage by Volume (% by volume or % v/v)
Commonly used when both the solute and the solvent are liquids or gases, such as rubbing alcohol, cooking gas (LPG), or air.
Formula:
\( \text{ร้อยละโดยปริมาตร} = \frac{\text{ปริมาตรของตัวละลาย}}{\text{ปริมาตรของสารละลาย}} \times 100 \)
Example: You want to prepare \( 500 \text{ cm}^3 \) of ethanol solution with a concentration of \( 20\% \) by volume. How many cubic centimetres of ethanol do you need?
Solution: \( 20 = \frac{\text{ปริมาตรเอทานอล}}{500} \times 100 \implies \text{ปริมาตรเอทานอล} = \frac{20 \times 500}{100} = 100 \text{ cm}^3 \)
3. Percentage by Mass per Volume (% by mass/volume or % w/v)
Commonly used when the solute is a solid (measured in mass, grams) dissolved in a liquid solvent to form a liquid solution (measured in volume, cubic centimetres), such as saline solution, syrup, or potassium permanganate solution.
Formula:
\( \text{ร้อยละโดยมวลต่อปริมาตร} = \frac{\text{มวลของตัวละลาย (กรัม)}}{\text{ปริมาตรของสารละลาย (ลูกบาศก์เซนติเมตร)}} \times 100 \)
Example: Dissolving 15 grams of table salt in water produces \( 300 \text{ cm}^3 \) of salt solution. What is the concentration of this solution?
Solution: \( \text{ร้อยละโดยมวลต่อปริมาตร} = \frac{15}{300} \times 100 = 5\% \) by mass per volume
Common Mistakes to Avoid:
Be very careful not to confuse the mass of the solution with the mass of the solvent! The denominator in the formula must always be the total mass or volume of the entire solution.
Relationship formula: \( \text{มวลของสารละลาย} = \text{มวลของตัวละลาย} + \text{มวลของตัวทำละลาย} \)
For instance, if a problem states: "10 grams of salt is dissolved in 90 grams of water," the mass of the solution is \( 10 + 90 = 100 \) grams, not 90 grams!
Key Review Points:
• % mass/mass = used for solid in solid (mass/mass)
• % volume/volume = used for liquid in liquid, or gas in gas (volume/volume)
• % mass/volume = used for solid in liquid (solute mass/total solution volume)
Part 3: Methods of Separating Mixtures
Mixtures in nature are typically composed of multiple substances combined together. To obtain pure substances for practical use, we must choose an appropriate separation technique based on the physical properties of each component. In Grade 8, there are five primary methods to master:
1. Evaporation to Dryness
• Principle: Used to separate a solution consisting of a solid solute dissolved in a liquid solvent, where the solvent has a much lower boiling point than the solute. Upon heating, the liquid completely evaporates away as vapour, leaving the solid residue behind in the container.
• Applications: Solar salt production (evaporating seawater to collect table salt), rock salt processing.
2. Crystallisation
• Principle: Used to separate a solid dissolved in a liquid to obtain a highly pure solid substance. This is done by heating the solution until it becomes a saturated solution at a high temperature, then allowing it to cool down slowly. As the temperature drops, solubility decreases, causing the excess solute to precipitate out as solids with characteristic geometric shapes called crystals.
• Applications: Refining sugar, crystallising alum, manufacturing pharmaceuticals and fine chemicals.
3. Simple Distillation
• Principle: Used to separate a solution consisting of a liquid mixed with a solid or liquids mixed together that have significantly different boiling points (at least \( 20 - 30^\circ\text{C} \) apart). The component with the lower boiling point vaporises first. As the vapour passes through a chilled condenser, it cools and condenses back into a pure liquid called the distillate, while the substance with the higher boiling point remains in the distillation flask.
• Applications: Producing distilled water for laboratory use, desalinating seawater to produce fresh water.
4. Paper Chromatography
• Principle: Used to separate mixtures that are present in tiny amounts and contain multiple components. It relies on two combined properties:
1. Solubility in the solvent (substances that dissolve better travel farther).
2. Adsorption by the adsorbent (paper) (substances that are adsorbed weakly move farther and faster, while substances that are strongly adsorbed move slower and cover less distance).
• Calculating the Retention Factor (\( R_f \)):
\( R_f = \frac{\text{ระยะทางที่สารเคลื่อนที่}}{\text{ระยะทางที่ตัวทำละลายเคลื่อนที่}} \)
(The \( R_f \) value is always less than or equal to 1. For a given substance, solvent, and adsorbent at a constant temperature, the \( R_f \) value remains constant.)
• Applications: Separating plant pigments from leaves or flowers, testing fabric dyes or food colourings, screening for drugs and toxins in blood samples.
5. Solvent Extraction
• Principle: Used to extract a desired substance from a mixture by choosing a suitable solvent that dissolves the target substance very well, but does not dissolve unwanted components. The solvent must not react chemically with the target substance and should preferably have a low boiling point so that it can easily be evaporated away afterward.
• Applications: Extracting essential oils from herbs (such as citronella or kaffir lime), brewing tea and coffee (using hot water as a solvent to extract caffeine and aromatic compounds), extracting natural food colourings from pandan leaves or butterfly pea flowers.
Common Mistakes to Avoid:
• Confusing evaporation to dryness with distillation: An easy way to remember is: if you only want to collect the solid and discard the liquid, use evaporation to dryness. But if you want to collect the liquid, or keep both components, use distillation.
• Solvent depth in chromatography: When immersing the paper into the solvent, the baseline spot of the mixture must always stay above the solvent level! Otherwise, the mixture will dissolve directly into the beaker instead of travelling up the paper.
Quick Review: Choosing the Right Separation Method
• Solid dissolved in liquid, want pure, well-formed solids \( \implies \) Crystallisation
• Solid dissolved in liquid, want the solid quickly \( \implies \) Evaporation to Dryness
• Solution where you need to separate liquids with very different boiling points \( \implies \) Simple Distillation
• Very small sample quantity, coloured or complex mixture \( \implies \) Paper Chromatography
• Want to selectively draw out a specific substance from plants or a mixture \( \implies \) Solvent Extraction
Fantastic job, everyone! Understanding mixtures and solutions provides an essential foundation for your future studies in science and chemistry. Practice your calculations, review these separation techniques often, and you'll realise that science isn't as intimidating as it seems. Keep up the great work!