Separating Mixtures (Junior Cert Science): Revision Notes
Separating Mixtures
What are mixtures?
A mixture is made up of two or more different substances that are mixed together but not chemically combined. For example, when you mix iron filings with sulfur, you create a mixture. The iron and sulfur remain as separate substances that can be separated again.
In this unit, you will learn about different methods scientists use to separate the components of mixtures in the laboratory. Each method works best for specific types of mixtures and separation needs.
Filtration
What is filtration?
Filtration is a way to separate an insoluble solid from a liquid. An insoluble solid is one that does not dissolve in the liquid. The method uses a special material, usually filter paper, that allows the liquid to pass through but stops the solid particles.
Some key points about filter paper:
- It contains tiny gaps (pores) that liquids can pass through
- The gaps are too small for solid particles to pass through
- It acts like a very fine sieve
How filtration works
When you filter a mixture:
- You pour the mixture into filter paper shaped like a cone in a funnel
- The liquid passes through the tiny gaps in the filter paper
- The solid particles are too large to pass through, so they remain on the filter paper
Understanding the Particle Theory of Filtration
The particle theory helps us understand why filtration works. The small water particles can pass through the gaps in the filter paper, but the larger solid particles cannot fit through these tiny spaces. Think of it like trying to pour sand through a kitchen sieve - the water flows through easily, but the sand stays behind.
Important terms
Key Filtration Terms:
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Residue: The solid material that remains behind on the filter paper after filtering. For example, when filtering soil and water, the soil is the residue.
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Filtrate: The clear liquid that passes through the filter paper. For example, when filtering soil and water, the water is the filtrate.
Remember: Residue Remains, Filtrate Flows!
Examples of filtration in everyday life
Filtration is used in many situations.
Real-World Applications of Filtration
Personal protective equipment (PPE): During the COVID-19 pandemic, face masks worked by filtering out tiny virus particles from the air. The mask material acts like filter paper, trapping harmful particles while allowing air to pass through for breathing.
Food preparation: A colander is used when separating vegetables or pasta from water. The water passes through the holes while the solid food remains behind.
Surgical masks: Surgeons use masks in operating theatres to filter out germs from the air they breathe, preventing infection during operations.
Evaporation
What is evaporation?
Evaporation is the process where a liquid changes into a gas. We can use evaporation as a method to separate a dissolved solid from a liquid.
How to separate mixtures using evaporation
When a solid is dissolved in water (such as salt in seawater), we can use evaporation to get the solid back:
Worked Example: Separating Salt from Water
- Pour the solution into an evaporating basin
- Heat the basin gently (using a Bunsen burner with a hotplate)
- The water gradually evaporates and turns into water vapour
- When most of the water has evaporated, the dissolved solid remains behind in the basin
- The remaining water can be left to evaporate naturally
Result: At the end of this process, you are left with pure solid salt.
The key to evaporation is that only the liquid changes state and escapes as vapour. The solid cannot evaporate at these temperatures, so it stays behind in the basin.
Distillation
What is distillation?
Distillation is a method used to separate a liquid from a mixture by heating it to form vapour and then cooling the vapour to turn it back into liquid. This technique is particularly useful when you want to keep the liquid rather than the solid.
The process involves two key changes of state:
- Vaporisation: The liquid turns into gas when heated
- Condensation: The gas turns back into liquid when cooled
How distillation works
Understanding the Distillation Apparatus
The distillation apparatus includes several important parts:
The distillation flask: This is where you put the impure liquid (such as seawater). It is heated using a Bunsen burner. Anti-bumping chips are added to help the liquid boil smoothly.
The Liebig condenser: This is a special cooling device invented by German chemist Justus von Liebig (1803-1873). It consists of:
- An inner tube where steam travels
- An outer jacket where cold water flows
- Cold water enters at the bottom and exits at the top
- The cold water cools the steam, turning it back into liquid
The thermometer: This measures the temperature of the steam entering the condenser.
The collection flask: This collects the purified liquid.
The Distillation Process - Step by Step:
- Heat the impure liquid in the distillation flask
- The water turns into steam (changes from liquid to gas)
- The steam enters the condenser
- Cold water flowing around the condenser cools the steam
- The steam condenses back into pure water (changes from gas to liquid)
- The pure water drips into the collection flask
Remember: Heat it up, cool it down, collect the pure liquid!
Important term
Distillate: This is the purified liquid produced by the distillation process. It is the product you collect at the end.
Uses of distillation
Practical Applications of Distillation
Purifying water: In countries where fresh water is scarce, seawater can be distilled to obtain pure drinking water. The salt and other impurities remain in the distillation flask.
Manufacturing alcoholic drinks: Distillation is used to produce spirits such as whiskey, gin, and brandy. Alcohol is produced through a chemical reaction called fermentation. The alcohol found in alcoholic drinks is called ethanol, which has a lower boiling point () than water (). This difference allows the alcohol to be separated and concentrated through distillation.
Paper chromatography
What is paper chromatography?
The word chromatography comes from 'colour writing'. Paper chromatography is a method of separating mixtures of coloured substances in solution. In this technique, substances dissolved in a solvent are passed along a length of special paper, and the different substances separate out as they travel at different rates.
How chromatography works
The Chromatography Process:
- Draw a pencil line near the bottom of chromatography paper (pencil is used because it doesn't dissolve)
- Place small spots of the mixture (such as ink) on this pencil line
- Place the bottom of the paper in a small amount of solvent (such as water) in a gas jar
- Make sure the solvent level is below the spots of mixture
- Allow time for the solvent to soak up the paper
- As the solvent moves up the paper, it carries the dissolved substances with it
- Different substances travel at different speeds, so they separate into different positions on the paper
The paper showing the separated colours is called a chromatogram.
Why substances separate
Each substance in the mixture has different properties:
- Some substances are more soluble in the solvent and travel further up the paper
- Other substances are less soluble and don't travel as far
- This causes the substances to separate into distinct bands or spots at different heights on the paper
Uses of chromatography
Real-World Applications of Chromatography
Forensic science: Scientists use chromatography to identify paint samples. In a hit-and-run accident, paint from a suspect's car can be analyzed using chromatography to see if it matches the paint found at the crime scene.
Analyzing inks and dyes: Chromatography can be used to separate the different coloured substances that make up inks, paints, and dyes. Many inks that appear to be one colour are actually made from a mixture of different coloured substances.
Exam Tip: When drawing a diagram of chromatography apparatus, make sure to draw the spot of ink above the level of the solvent. The solvent must travel up the paper and carry the ink with it.
Remember!
Key Points to Remember:
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Filtration separates insoluble solids from liquids using filter paper. The solid (residue) stays behind while the liquid (filtrate) passes through.
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Evaporation separates dissolved solids from liquids by heating the liquid until it turns to vapour, leaving the solid behind.
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Distillation separates and purifies liquids by heating them to form vapour, then cooling the vapour to collect pure liquid (distillate). It uses a condenser to cool the vapour back to liquid.
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Paper chromatography separates coloured substances in mixtures by allowing a solvent to carry them up paper at different rates, creating a chromatogram.
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Each separation method is chosen based on the type of mixture and what you want to separate or collect.