Lesson is taken from SLS for my own learning.
Separation Techniques [LSS]
This lesson is designed for Secondary One or Secondary Two Express and Normal Academic students, taking Lower Secondary Science.
- Knowledge of mixtures and solutions
- Basic understanding of processes such as evaporation, boiling and condensation
- This lesson consists of
- a short introduction to the need for separation of mixtures
- a video to explain magnetic attraction
- a video to explain paper chromatography
- a video to explain filtration
- a video to explain evaporation and distillation
- questions to check students' understanding of concepts covered
1 The need to separate
This segment is a short introduction on the need for separation of mixtures to be done so as to obtain constituents which are useful to us in our daily lives. It will be useful to highlight to students that more than one separation technique will be used in most instances to obtain the constituents.
To separate or not to separate? That may not be the only question we are interested in when we are working with mixtures.
Why do we need to separate mixtures? And, how do we then carry out the separation? Also, what are we actually separating? These are the more common questions that we will need to ask when we look at mixtures.
Many substances that we see around us are mixtures. These mixtures contain constituents that are useful to us in our daily lives. For example, crude oil must be separated into its different constituents before each constituent can be used efficiently.
In order to carry out the separation of these mixtures successfully, we will need to appreciate how the differences in physical properties of the constituents will enable us to separate them successfully. At the same time, many mixtures may contain more than two constituents and the successful separation of these constituents will involve two or more separation techniques used in the correct sequence.
2 Common separation techniques
In this section, the common separation techniques will be discussed using short videos.
For magnetic separation, it is vital to highlight to students that this method is not used to separate metals from non-metals as some metals, like gold , are not attracted to magnets. Hence, this method is commonly used to separate some metals (that are magnetic) from non-metals (that are all non-magnetic) in the context of LSS.
For paper chromatography, it should be highlighted to the students that this method will only work if the constituents are soluble in the solvent provided. Hence, solubility tests are often conducted to select an appropriate solvent before chromatography can be carried out. It will be good to remind students that there are solvents other than water even though water is the more common solvent at LSS.
For filtration, it can be supplemented with decanting (pouring away the top layer of liquid) first to remove part of the liquid so as to speed up the process of filtration. However, it is advisable that decanting should be done only if the residue does not float on the surface of the liquid. In the context of the video, decanting can be used to pour some of the salt solution into a separate beaker and not into the sink before the filtration is done to obtain the green beans. Students must understand that the solution contains salt and thus cannot be discarded using decanting in order to ensure the maximum possible amount of salt can be obtained at the end of the experiment.
For heating to dryness, the solid should be thermally stable to withstand the strong heating towards the end of the process. In the context of LSS, the term "high melting point" is used instead of "high thermal decomposition temperature" to facilitate understanding at LSS level. For reference, the melting point of salt is 808 °C and the boiling point of pure water is 100 °C. Hence, it is possible to heat to dryness without having to worry that salt may thermally decompose. It will be good to get students to think why this method cannot be used in the case of sugar dissolved in water.
For simple distillation, the key idea is that the two constituents in the mixture to be separated should have significantly different boiling points in order for this method to be successful. If the boiling points of the two constituents are too close to each other, distillation may not be effective in separating the constituents. Fractional distillation, which will be used for mixtures with more than two liquid constituents with significantly different boiling points, will be covered at higher levels.
There are different separation techniques that we can use to separate the different mixtures that we see around us. Successful separation of mixtures is dependent on a clear understanding of the physical properties of the constituents in each mixture. The differences in the physical properties of the constituents will be used to select the most appropriate separation technique.
With the aid of videos, we will examine some of the more common separation techniques in greater detail.
This separation technique is used mainly to separate magnetic materials (e.g. metals like iron) from non-magnetic materials.
Let us watch the following video to understand this separation technique better.
poly course https://www.rp.edu.sg/SAS/full-time-diplomas/Details/diploma-in-materials-science Diploma in Materials Science (R17)
This separation technique is used to separate small amounts of substances from mixtures, based on the different solubilities of the substances in a particular solvent.
Let's watch the following video to help us understand this technique better.
poly course https://www.rp.edu.sg/schools-courses/courses/full-time-diplomas/full-time-courses/modules/index/A364 A364 Analytical Instrumentation
Filtration is a separation technique used to separate a solid substance from the liquid in an insoluble solid-liquid mixture.
In the following video, we will examine how filtration can be used in our daily lives.
Evaporation and Distillation
Evaporation is a separation technique used to obtain a dissolved solid substance in a solid-liquid mixture. The melting point of the solid in the mixture must be relatively higher than the boiling point of the liquid in the mixture in order for this technique to be used successfully. Usually, the mixture will be heated to dryness to evaporate the liquid completely in order to leave the solid behind.
Distillation is a separation technique used to obtain a pure liquid from a dissolved solid-liquid or liquid-liquid mixture. The basis of this technique is centred on the significantly different boiling points of the substances present in the mixture. The liquid constituent with the lower boiling point will evaporate to form a gaseous substance. This gaseous substance will eventually be condensed to obtain the pure substance in the liquid form.
Let's watch the following video to understand more about the use of these two techniques in our daily lives.
Applied Chemistry , Biomedical Science , Chemical & Biomolecular Engineering , Chemical & Pharmaceutical Technology, Chemical Engineering, Environmental & Water Technology, Environmental Science, Food Science & Nutrition, Food, Nutrition & Culinary Science, Materials Science
Q1. MCQ Evaporation is a separation technique that can be used to obtain __________ solids from a solution.
That is right! This separation technique allows us to obtain the dissolved solids from a solution by removing the solvent.
This is incorrect. Since the insoluble solids are in a different state from the liquid solvent, a more suitable and convenient separation technique is by the use of filtration.
Q2. MCQ In which of the following cases can filtration be used to separate a mixture of two solid substances with the help of a solvent?
Both substances are soluble in the same solvent.
Good try but this is incorrect. If both substances are soluble in the same solvent, a solution containing both substances is formed. Filtration cannot separate one substance from the other.
Both are insoluble in the same solvent.
Good try but this is incorrect. If both substances are insoluble in the solvent, filtration will only separate the solvent from the mixture and not one solid substance from the other.
One is soluble while the other is insoluble in the same solvent.
Well done! When only one of the two substances in a mixture is soluble in the solvent, it dissolves and leaves the other insoluble substance behind. This insoluble substance can then be filtered off.
Q3. MRQ Which of the following mixtures cannot be separated by performing the following separation techniques in sequence: dissolving in water followed by filtration? (You may choose more than one answer.)
Iron filings and sulfur powder
Well done! Both iron filings and sulfur powder are insoluble in water and hence cannot be separated with this proposed sequence as filtration will not be able to separate these two substances from each other.
Sand and salt.
That is incorrect. The proposed sequence is workable. Salt is soluble in water but sand is not. The salt dissolves in water and the sand can then be filtered off.
Sugar and salt.
Well done! Both sugar and salt are soluble in water to form a solution. Subsequent filtration cannot separate them from each other.
Iron filings, sulfur powder and sand are insoluble in water. Salt and sugar are soluble in water.
Q4. MRQ Which of the following statement(s) is/are true about distillation? (You may choose more than one answer.)
Distillation is a separation technique used to separate a solid from a solid-liquid mixture.
That is incorrect. Distillation is used to separate a pure liquid from a solid-liquid mixture, or a liquid-liquid mixture.
Distillation is a separation technique used to separate a pure liquid from a liquid-liquid mixture.
That's right! Distillation is used to separate a pure liquid from a solid-liquid mixture, or a liquid-liquid mixture.
The two main processes involved in distillation are boiling and condensation.
That's right! The main processes involved are boiling/evaporation and condensation.
Q5. MRQ Which of the following statement(s) is/are true about chromatography? (You may choose more than one answer.)
Chromatography is a separation technique used to separate a solid from a liquid-solid mixture only.
That is incorrect!Chromatography is used to separate small quantities of a liquid from a liquid-liquid mixture.
Chromatography can be used to separate small quantities of a liquid from a liquid-liquid mixture.
That is correct!Chromatography is used to separate small quantities of a liquid from a liquid-liquid mixture.
The more soluble a solute is in a solvent, the further it will travel on a chromatography paper.
That is correct!The more soluble a solute is in a solvent, the longer the distance it will travel on a chromatography paper.
Q1. FILL-IN-THE-BLANKS State the most appropriate separation technique for the following mixtures:
1. Obtain water from a mixture of sand and water
Distillation Water can be separated from sand by distillation.
2. Obtain salt from a beaker of salt solution
Evaporation can be done to remove the water, leaving the salt behind.
3. Obtain sand from a mixture of sand and water
Filtration can be used to separate sand from water as sand does not dissolve in water.
4. Obtain iron filings from a mixture of iron filings and sand
Iron is magnetic but sand is non-magnetic and thus, magnetic attraction can separate iron filings from sand.
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