experimental techniques ch2 Flashcards
apparatus and unit of measurement for time
seconds (s) minutes (min)
stopwatch
apparatus and unit of measurement for temperature
degrees Celsius °C
thermometer
apparatus and unit of measurement for mass
grams (g)
kilograms (kg)
1000g=1kg
balance
apparatus and unit of measurement for volume of liquid
cm^3 or ml (=)
dm^3 or l (=)
1000cm^3 = 1dm^3 = 1 l
measuring cylinder -> burette-> pipette
(increasing order of accuracy)
mixture
two or more substances not chemically combined
solution
a mixture you make by dissolving a substance in a solvent
A homogeneous mixture where the substances are fully dissolved and evenly dispersed
solute
the substance you dissolve
solvent
the liquid in which a solute is dissolved, to make a solution
volatile
describe a volatile liquid
1) a substance that evaporates readily at room temperatures
forces between its particles are weak so volatile liquids have low boiling points
What is paper chromatography used for
To find out how many substances are present in a mixture
To check on the purity of a substance; if impurities are present, they separate out
To identify the substances in a mixture, by measuring how far they travel
How do you separation occur in paper chromatography
There is a stationary phase (chromatography paper) and a mobile phase (solvent)
Substances have different solubilities in the mobile phase so will travel at different rates causing separation. More soluble substances travel further.
They separate because a different solubility in the solvent, and attraction to the paper they travel over.
The most soluble a substance is in the solvent, the further it will travel. The more attracted it is to the paper, the less far will travel.
What is an Rf value
It is the ratio between this distance traveled by the dissolve substance and the distance traveled by the solvent
Rf=Distance travelled by substance/distance travelled by solvent
A paper chromatography, what affects the Rf values of a substance?
The solvent. Repeating the experiment with a different solvent will change the Rf value
How many spots will be observed and a chromatogram of a pure substance
1
Locating agent
A substance used to analyse and identify colourless substances
What is a pure substance
A single element or compound not mixed with any other substance
How can a pure substance be identified using melting or boiling points
Pure: Melts and boils at temperatures are unique to it, has sharp melting and boiling points
Impure: Melt/boils over a range of temperatures, not sharply.
Melts at a lower temperature than a pure substance
Boils at a higher temperature than a pure substance
Why is the purity of substances important in every day life
Impurities in drugs and vaccines may cause dangerous side-effects
Impurities in food and drink may cause health problems of ingested
Crystallisation
The process in which crystals form, as a saturated solution cools
Distillation
A separation method that involves heating a substance for evaporation and then obtaining it through condensation. The works because different liquids have different boiling points.
Separate an insoluble solid from a liquid
Filtration
The insoluble substance will not pass through the filter paper and will remain there as the residue. You can rinse it with distilled water.
The soluble substance, the filtrate, passes through the paper.
Separating a solvent from a solution
Separating a liquid from a solution of a solid in a liquid
Simple distillation e.g. producing pure water, distilled water, from seawater
The solvent boils off as a gas at it’s boiling point. The gas condenses back into a liquid in the cool condenser, and drips into the beaker. The solute remains in the flask.
Separating a liquid from a mixture of miscible liquids
Fractional distillation e.g. removing ethanol from a mixture of ethanol and water
Supposing A has a lower boiling point:
- Gas A passes into the condenser first, at A’ boiling point. It condenses there.
- Collect liquid A. Stop collecting when the temperature on the thermometer rises
- Keel heating. Gas B passes into the condenser, at B’s boiling point. Collect liquid B in a separate beaker.
This method works because different liquids have different boiling points. When the mixture is heated, ethanol and some water vapour rises. They rise up the fractionating column, which is cooler the higher you go. When the glass beads reach 78°C, the water vapour condenses and drips back into the flask. The ethanol vapour continues to go up and through the condenser (as it’s boiling point is lower than water: 100°C.) The ethanol condenses due to the lower temperature from the cold water, and drips into the beaker. You now have distilled ethanol.
Separating a mixture of immiscible liquids
Separating funnel. E.g. separating a mixture of water and hexane
Immiscible liquids settle into two layers: the denser one at the bottom (ie hexane)
When the taps opened, the water runs to the beaker. With the bottom layer is drained, the remaining layer can be separately drained.
Separating a mixture of similar compounds in a solution
Substances from a mixture
Chromatography e.g. separating the dyes present in sample of ink
The mixture is dissolved in a suitable solvent. The solutions allowed to travel across the paper.
The substances in a solution travel at different speeds and separate.
They separate because of the different solubility in the solvent and attraction to paper they travel over.
The more soluble, the higher travels
The more attracted to the paper it is the lower it travels
Saturated solution
No more of the solute will dissolve in it, at that temperature
Separating a solute which sublimes from a solid which does not sublime
Solute from a solution
Crystallisation
Crystallisation works because a solvent can dissolve less and less solute as it’s temperature falls
Heat the solution until it is saturated then let it cool. Filter off the salts that form. Wash them with a little distilled water but be careful since they are soluble.
