Elements, Compounds, Mixtures Flashcards
What is an element?
A substance made up of atoms that all contain the same number of protons (one type of atom) and cannot be split into anything simpler. There are limited types of element.
Example: Hydrogen, Oxygen, Carbon.
What is a compound?
A pure substance made up of two or more elements chemically combined together. There are unlimited types of compound. Cannot be separated by physical methods of separation.
Example: Copper (ii) Sulfate, Calcium Carbonate.
What is a mixture?
A combination of two or more substances (elements and/or compounds) that are not chemically joined together. Can be separated by physical methods of separation. Has a range of melting and boiling points as they consist of different substances that melt or boil at different temperatures
Example: Sand and water, oil and water.
What is a pure substance?
Contains only one substance, with no other substances mixed together
Has fixed melting and boiling point as they consist of only one substance so all molecules will have the same melting and boiling points
E.g. Water has a boiling point of 100°C and a melting point of 0°C.
How do you test for purity?
The physical property of boiling/melting point can therefore be used to test if a substance is pure or not.
For example, if a sample of water melts at exactly 0°C and boils at exactly 100°C then the water is pure.
If the melting and boiling points of the water aren’t these exact values then the water must be impure and contain other substances i.e. it must be a mixture.
What is simple distillation?
Use: To separate a liquid and soluble solid from a solution (e.g. water from a solution of salt water)
Explanation:
Solution is heated and water vapors will rise and evaporate
Water vapors will pass through the condenser, where it cools and condenses, turning into a liquid that will be collected in a beaker
After all the water is evaporated from the solution, the solute will be left behind.
What is fractional distillation?
Use: To separate two or more liquids that are miscible with one another (e.g. ethanol and water from a mixture of the two)
Explanation:
Solution is heated at temperature of substance with the lowest boiling point
This substance will rise and evaporate, and vapours will pass through a condenser, where it cools and condenses, turning into a liquid that will be collected in a beaker
All of this substance is evaporated and collected, leaving behind a mixture or a substance
For water and ethanol: Ethanol has a boiling point of 78°C and water of 100°C. The mixture is heated until it reaches 78°C, the ethanol boils and distils out of the mixture and condenses into the beaker. When temperature starts to increase to 100°C heating should be stopped. Water and ethanol are separated.
What is filtration?
Use: 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)
Explanation:
Filter paper is placed in a filter funnel above another beaker
Mixture of insoluble solid and liquid is poured into the filter funnel
Filter paper will only allow small liquid particles to pass through as the filtrate
Solid particles are too large to pass through the filter paper so will stay behind as a residue.
What is crystallization?
Use: To separate a dissolved solid from a solution, when the solid is much more soluble in hot solvent than in cold (e.g. copper sulphate from a solution of copper (ii) sulphate in water)
Explanation:
Solution is heated, allowing the solvent to evaporate to leave a saturated solution. Test the solution is saturated by dipping a clean, dry, cold glass rod into the solution. If the solution is saturated, crystals will form in the glass rod.
Saturated solution is allowed to cool and solids will come out of the solution, as the solubility increases, and crystals will grow
Crystals are collected by filtering the solution. Then the crystals are washed with cold, distilled water to remove impurity. Dry the crystals.
What is paper chromatography?
Use: To separate substances that have different solubilities in a given solvent (e.g. different coloured inks that have been mixed to make black ink)
Explanation:
Pencil line is drawn on chromatography paper and spots of ink / dye is placed on it.
Paper is lowered into a bucket of solvent, allowing the solvent to travel up the paper, taking some of the coloured substances with it.
Different substances will have different solubilities so will travel at different rates, causing the substances to spread apart. Those with higher solubility will spread more than the others.
This will show the different components of the ink / dye.
How do you use retardation factor to identify components in a mixture?
Retardation factor (Rf) values:
Used to identify the components of mixtures
The Rf value of a particular compound is always the same
Calculating the Rf value allows chemists to identify unknown substances because it can be compared with Rf values of known substances under the same conditions.
Equation:
Rf value = distance moved by compound ÷ distance moved by solvent
