Separation Techniques [LSS]
DescriptionThis lesson is designed for Secondary One or Secondary Two Express and Normal Academic students, taking Lower Secondary Science.
Pre-requisites
- 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.
Magnetic attraction
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.
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.
Filtration
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.
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