Element, compounds and mixtures Flashcards
what is an element?
a substance made from only one type of atom
what is a compound?
substance made from two or more elements that have reacted chemically with each other
what is a mixture?
- consists of 2 or more elements or compounds not chemically combined together
- chemical properties of each substance in the mixture are unchanged
what is a pure substance?
a single element or compound, not mixed with any other substance
what is a pure substance (in everyday language)?
substance that has had nothing added to it, its in its natural state
when do pure substances melt and boil?
in specific temperatures
what are five ways for separating mixtures?
- simple distillation
- fractional distillation
- filtration
- crystallisation
- paper chromatography
how do you do simple distillation
- it works when the liquid has different boiling points
- commonly used to separate ethanol from water
- sequence of distillation is as follows: heating -> evaporating -> cooling -> condensing
what is simple distillation used for?
separating a pure liquid from a mixture of liquids
how do you do fractional distillation
- Solution is heated to the temperature of the substance with the lowest boiling point and vapors of this substance will rise and evaporate.
- Vapors pass through a condenser where it cools and condenses, turning into a liquid that is collected in a beaker.
- After all of the substance is evaporated and collected, a mixture of substance will be left behind.
why do we use Filtration?
To separate an undissolved solid from a mixture of the solid and a liquid / solution ( E.g Sand from a mixture of Sand and Water)
how to do Filtration?
- Filter paper is placed in a filter tunnel, which is placed above another beaker
- A mixture of insoluble solids and liquids is poured into the filter tunnel, which only allows small liquids particles to pass through as filtrate
- Solid particles are too large to pass through as filtrate, so this particles will stay behind as residue
why do we use Crystallisation?
to separate a dissolved solid from a solution (solids are more soluble in hot solved than in cold ones) (e.g copper sulphate from a solution of copper sulphate in water)
how to do Crystallisation?
- The solution is heated, allowing solvent to evaporate and leave a saturated solution
- The saturated solution is left to cool down, and solids will start coming out of the solution to grow into crystals
- Crystal are collected and allowed to dry
why do we use Paper chromatography?
to separate substances that have different solubilities in a given solvent (e.g different coloured inks mixed to form black ink)
how to do Paper chromatography?
- Draw a line with a pencil on chromatography paper and the concentrated spot of ink/dye is placed on it.
- Lower the paper into a bucket of solvent, which allows the solvent to travel up the paper taking the particles of coloured substances with it.
- Different substances have different solubilities so the solvent will travel at different rates, causing the substance to be spread along the vertical length of paper.
- This will show the different components of the ink/ dye.
Practical: investigate paper chromatography by using inks/food colourings
(there are 6 points you need to make for this question)
- Draw a line 1 cm (baseline) from the bottom of the paper (use pencil not ink. Ink dissolves into the solvent and a pencil doesn’t).
- One drop of dye of each sample must be dropped at different points along the baseline.
- The paper must be then suspended in a beaker which contains a small amount of solvent. The bottom of the paper must be touching the solvent , but the baseline with the dyes should be above the level of the solvent (this is so that the dryer doesn’t simply dissolve into the solvent in the beaker).
- Put a lid to cover the beaker so that the atmosphere becomes saturated with the solvent (and so that the solvent does not evaporate from the surface of the beaker).
- When the solvent is near the top, the paper should be removed from the solvent and a pencil line drawn (and labelled) should reach up the paper. This is called the solvent font.
- The chromatography is then left to dry so that all the solvent evaporates.
how does chromatogram provide information about the composition of a mixture?
- Chromatogram shows the composition of a mixture as different coloured substances. It will spread apart as they have different solubilities so it will travel at different rates.
- Pure substances will only produce one spot on a chromatogram during paper chromatography.
how to calculate Rf values?
Rf = distance of dye from baseline/ distance of solvent front from baseline
