elements, compounds and mixtures Flashcards
What is an element?
An element is a substance that cannot be broken down into any other substance.
What is a compound?
A compound is a substance that contains two or more elements that are chemically combined.
What is a mixture?
A mixture consists of two or more different substances, not chemically joined together.
Why do pure substances have a fixed boiling and melting point, but a mixture may melt or boil over a range of temperature?
Pure substances melt and boil at specific and sharp temperatures e.g. pure water has a boiling point of 100 °C and a melting point of 0 °C. Mixtures have a range of melting and boiling points as they consist of different substances that tend to lower the melting point and broaden the melting point range.
What is simple distillation?
This is used to separate a liquid and soluble solid from a solution (e.g., water from a solution of salt water) or a pure liquid from a mixture of liquids. The solution is heated, and pure water evaporates producing a vapour which rises through the neck of the round bottomed flask. The vapour passes through the condenser, where it cools and condenses, turning into the pure liquid that is collected in a beaker. After all the water is evaporated from the solution, only the solid solute will be left behind.
What is fractional distillation?
This is used to separate two or more liquids that are miscible with one another (e.g., ethanol and water from a mixture of the two). The solution is heated to the temperature of the substance with the lowest boiling point. This substance will rise and evaporate first, and vapours will pass through a condenser, where they cool and condense, turning into a liquid that will be collected in a beaker. All of the substance is evaporated and collected, leaving behind the other components(s) of the mixture. For water and ethanol
Ethanol has a boiling point of 78 ºC and water of 100 ºC. The mixture is heated until it reaches 78 ºC, at which point the ethanol boils and distills out of the mixture and condenses into the beaker. When the temperature starts to increase to 100 ºC heating should be stopped. Water and ethanol are now separated.
What is filtration?
Used 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). A piece of filter paper is placed in a filter funnel above a beaker. A mixture of insoluble solid and liquid is poured into the filter funnel. The filter paper will only allow small liquid particles to pass through as filtrate. Solid particles are too large to pass through the filter paper so will stay behind as a residue.
What is crystallisation?
Used 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). The solution is heated, allowing the solvent to evaporate, leaving a saturated solution behind. Test if the solution is saturated by dipping a clean, dry, cold glass rod into the solution
If the solution is saturated, crystals will form on the glass rod. The saturated solution is allowed to cool slowly.
What is paper chromatography?
This technique is used to separate substances that have different solubilities in a given solvent (e.g., different coloured inks that have been mixed to make black ink). A pencil line is drawn on chromatography paper and spots of the sample are placed on it. Pencil is used for this as ink would run into the chromatogram along with the samples. The paper is then lowered into the solvent container, making sure that the pencil line sits above the level of the solvent, so the samples don’t wash into the solvent container. The paper is called the stationary phase. The solvent travels up the paper by capillary action, taking some of the coloured substances with it; it is called the mobile phase. Different substances have different solubilities so will travel at different rates, causing the substances to spread apart. Those substances with higher solubility will travel further than the others. This will show the different components of the ink / dye. If two or more substances are the same, they will produce identical chromatograms
If the substance is a mixture, it will separate on the paper to show all the different components as separate spots. An impure substance will show up with more than one spot, a pure substance should only show up with one spot.
How does a chromatogram provide information about the composition of a mixture?
Pure substances will produce only one spot on the chromatogram. If two or more substances are the same, they will produce identical chromatograms. If the substance is a mixture, it will separate on the paper to show all the different components as separate spots. An impure substance therefore will produce a chromatogram with more than one spot.
How do you use the calculation of Rf values to identify the components of a mixture?
These values are used to identify the components of mixtures. The Rf value of a particular compound is always the same but it is dependent, however, on the solvent used. If the solvent is changed then the value changes. 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. These values are known as reference values
Calculation
The Retention factor is found using the following calculation:
Rf = distance travelled by substance ÷ distance travelled by solvent.
The Rf value will always lie between 0 and 1; the closer it is to 1, the more soluble is that component in the solvent.
