II.2.1. Methods for separating homogeneous mixtures.
II.2.1.1. Crystallization.
Crystallization is the method of separating a solid substance in the form of crystals from a homogeneous liquid mixture (solution), by evaporating the liquid substance.
👀 Experiment: Crystallization
🔥 Warning! This experiment is for teachers only!
Required Materials:
Berzelius Glass, Water, Bruise Stone Crystals, Wand, Asbestos Sieve, Alcohol cooking lamp, Match, Capsule, Tripod.
Experiment description:
- In the glass, mix the water with the bruised stone (copper sulfate) with the help of the wand.
- Put the formed mixture in a capsule that you place on the asbestos sieve on the tripod and heat it in the flame of the alcohol cooking lamp.
- What do you notice?
The water evaporates and bruised stone (copper sulfate) crystals are obtained in the capsule.
Experiment conclusion: Through crystallization, we separated a solid substance in the form of crystals from a homogeneous liquid mixture (solution), by evaporating water.
👀 Experiment: How to Grow Crystals
🔥 Caution! This experiment is performed only in the presence of an adult!
🔥 Warning! When working with fire sources make sure to keep your hair tight and don't wear clothes with wide sleeves! Be careful not to burn yourself when working with hot water!
🔥 Warning! The bruise stone (copper sulfate) is toxic. Don't touch it or taste it!
👀 Experiment: Crystal Growth Result
🔥 Caution! This experiment is performed only in the presence of an adult!
🔥 Warning! When working with fire sources make sure to keep your hair tight and don't wear clothes with wide sleeves! Be careful not to burn yourself when working with hot water!
🔥 Warning! The bruise stone (copper sulfate) is toxic. Don't touch it or taste it!
Applications of crystallization
- Extraction of salt from sea water.
- Extraction of sugar from cane/sugar beet.
👀 Experiment: Crystallization of some salts with release of heat
🔥 Warning! This experiment is for teachers only!
🔥 Warning! Sodium acetate is a compound with very low toxicity. It is slightly irritating to the eyes. Respiratory tract may be irritated after inhalation. Frequent and constant contact with the skin may cause irritation. Sodium acetate and sodium thiosulfate can be reused repeatedly. Just follow the basic principles of handling reagents in the chemistry lab.
Required Materials:
4 pahare Berzelius foarte curate (preferabil ca acestea să fie noi - să nu fie zgâriate), acetat de sodiu, tiosulfat de sodiu, spirtieră, stativ, sită de azbest, sticlă de ceas, termometru, dispozitiv Heat In a Click / Hot Pack sau comprese calde, apă.
Experiment description:
- Take a Berzelius glass, preferably new (so as not to be scratched) and very clean.
- Add sodium acetate trihydrate (CH3-COONa x 3H2O).
- Heat the sodium acetate and add a little water until you notice that a clear solution has formed (approx. 80-90°C). If a ring of crystals forms above the solution, on the inner edge of the beaker, pour the entire contents of the beaker into another clean beaker.
- Cover the beaker with a watch glass and let the solution cool. Note that the solution remains colorless and clear even when cooled to room temperature.
- Add a small crystal of sodium acetate to that solution and observe its behavior.
Sodium acetate crystallizes in mass, and this crystallization raises the temperature to 38-40°C.
Another reagent with a behavior similar to sodium acetate is sodium thiosulfate.
- Take a Berzelius glass, preferably new (so as not to be scratched) and very clean.
- Add sodium thiosulfate pentahydrate (Na2S2O3 x 5H2O).
- Heat until you notice that a clear solution has formed (approx. 80 - 90 °C). The salt dissolves in its own water of crystallization generating a clear, saturated solution at 90°C.
- Cover the beaker with a watch glass and let the solution cool. Note that the solution remains colorless and clear even when cooled to room temperature. It measures the temperature of the solution.
- Add a small crystal of sodium thiosulphate to that solution and observe its behavior. Measure the temperature again.
Allowed to cool, the solution becomes supersaturated and crystallizes in mass if a single sodium thiosulphate crystal is thrown into it to "seed the solution". Mass crystallization of sodium thiosulfate raises the temperature to 50 °C.
Experiment conclusion:
The increase in temperature justifies the use of this solution in the medical field, the respective devices being called Heat In a Click / Hot Pack or warm compresses. Supersaturated solutions of certain salts can be used in medical practice to generate heat to keep the wounded warm until they are safely in the hospital. The principle is simple: a supersaturated solution when it comes into contact with its own crystals generates mass crystallization of the entire solution, the process being exothermic, the released heat being used in medical practice.
II.2.1.2. Distillation.
Distillation is the separation of liquid components from a homogeneous mixture based on their different boiling points.
During distillation, two physical phenomena occur: vaporization, followed by condensation.
If there is a mixture of water and alcohol in the flask, by heating, the temperature will rise, and when it reaches 78 °C, the alcohol begins to boil.
Throughout the boiling of the alcohol, the temperature remains constant at 78 °C. Alcohol vapors reaching the refrigerant are cooled and condense.
When the temperature starts to rise above 78 °C, the alcohol has separated from the water and the distillation stops. The water separated from the alcohol.
🔦 Remark
In general, the component parts of the mixture have boiling points that do not differ by more than 25 °C from each other at one atmosphere pressure. On the other hand, if the difference between the boiling points is greater than 25 °C, a simple distillation can be used to separate the components.
Applications of distillation
1. Obtaining distilled water (chemically pure), used in laboratories, industry, pharmacy.
2. Obtaining alcoholic beverages in industrial installations with alembic.
3. Through the fractional distillation of oil, in petrochemical refineries, propane, butane, gasoline, lamp oil, diesel, fuel oil are obtained. These products of oil distillation are used as fuels, to obtain plastics, medicines, artificial rubber, artificial fibers, detergents, insecticides, pesticides, etc.
4. Fractional air distillation with separation of nitrogen from oxygen. Consider a distillation flask containing a liquid mixture of oxygen (the heavy component) and nitrogen (the light component). If the liquid mixture is heated it will begin to boil, producing vapors that contain more nitrogen (the light component) than the original liquid. These nitrogen-rich vapors condense in a condenser.
