C12 Flashcards
What is a mixture ?
A mixture is made up of two or more substances (elements or compounds)
that are not chemically combined together.
What are the differences between compounds and mixtures ?
Mixtures :
-Mixtures have no fixed composition (the proportions vary depending on the amount of each substance mixed together).
-The different elements or compounds in a mixture can be separated again more easily (by physical means using the differences in properties of each substance in the mixture).
-There are no chemical bonds between atoms of the different substances in a mixture.
Compounds:
-Compounds have a fixed composition (the ratio of elements present is always the same in any particular compound).
-Chemical reactions must be used to separate the elements in a compound.
-There are chemical bonds between atoms of the different elements in the compound.
How can mixtures be separated ?
Mixtures can be separated by physical means, such as filtration, crystallisation, and simple distillation, fractional distillation and chromatography
What is and how can you do the filtration separation teqnique ?
The technique of filtration is used to separate substances that are insoluble in a particular solvent from those that are soluble in the solvent.
For example, you have probably tried to separate a mixture of sand, salt (sodium chloride, and water) before in science lessons
The sand that you collect on the filter paper can then be washed with distilled water to remove any salt solution left on it. The wet sand is finally dried in a warm oven to evaporate any water off and leave the pure, dry sand.
What is and how can you do crystilisation ?
To obtain a sample of pure salt (sodium chloride, NaCI) from the salt solution following filtration, you would need to separate the sodium chloride in the solution (called the filtrate) from the water. You can do this by evaporating the water from the sodium chloride solution.
The best way to do this is by heating it in an evaporating dish on a water bath. Using a water bath is a gentler way of heating than heating the evaporating dish directly on a tripod and gauze.
Heating should be stopped when the solution is at the point of crystallisation.
This is when small crystals first appear around the edge of the solution or when crystals appear in a drop of solution extracted from the dish with a glass rod.
The rest of the water is then left to evaporate off the saturated solution at room temperature to get a good sample of sodium chloride crystals. A flat-bottomed crystallisation dish or Petri dish can be used for this final step, to give a large surface area for the water to evaporate from.
What is and how do you do simple distillation ?
Crystallisation separates a soluble solid from a solvent but sometimes you need to collect the solvent itself instead of just letting it evaporate off into the air. For example, some countries with a lack of fresh water sources purify seawater to obtain usable water. Distillation allows us to do this.
In simple distillation, a solution is heated and boiled to evaporate the solvent. The vapour given off then enters a condenser. This is an outer glass tube with water flowing through it that acts as a cooling ‘jacket’ around the inner glass tube from the flask. Here the hot vapour is cooled and condensed back into a liquid for collection in a receiving vessel. Any dissolved solids will remain in the heated flask.
What do you do in crystillisation ?
You separate a soluble solid (solute) from a solvent (liquid)
What do you do in simple distillation ?
To separate (and recover) the solvent from the solute
What’s the principle of fractional distillation ?
To separate liquids of different bullet points
What does misclible mean?
Mixable so for e.g. water and oil are not mixable
How do you do fractional distillation ?
You put your mixture in a flask and stick a fractionating column on top. Then you heat it.
The different liquids will all have different boiling points - so they will evaporate at different temperatures.
The liquid with the lowest boiling point evaporates first. When the temperature on the thermometer matches the boiling point of this liquid, it will reach the top of the column.
Liquids with higher boiling points might also start to evaporate. But the column is cooler towards the top. So they will only get part of the way up before condensing and running back down towards the flask.
When the first liquid has been collected, you raise the temperature until the next one reaches the top.
How do you do the paper chromatography practical ?
1)Draw a line near the bottom of a sheet of filter paper.
(Use a pencil to do this - penoil marks are insoluble and won’t dissolve in the solvent.)
2) Add a spot of the ink to the line and place the sheet in a beaker of solvent, e.g water.
3) The solvent used depends on what’s being tested. Some compounds dissolve well in water, but sometimes other solvents, like ethanol, are needed.
4) Make sure the ink isn’t touching the solvent - you don’t want it to dissolve into it.
5) Place a lid on top of the container to stop the solvent evaporating.
6) The solvent seeps up the paper, carrying the ink with it.
7) Each different due in the ink will move up the paper at a different rate so the dyes will separate out. Each dye will form a spot in a different place - 1 spot per dye in the ink.
8) If any of the dyes in the ink are insoluble (won’t dissolve) in the solvent you’ve used, they’ll stay on the baseline.
9) When the solvent has nearly reached the top of the paper, take the paper out of the beaker and leave it to dry.
10) The end result is a pattern of spots called a chromatogram
What is a pure substance ?
The melting and boiling points of an element or a compound are called its fixed points.
What are fixed points ?
The melting and boiling points of an element or a compound are called its fixed points.
What’s the test for pure water ?
The test for pure water is that it’s melting point is exactly 0 degrees and it’s boiling point is exactly 100 degrees
