12: Experimental Techniques

stopwatches

seconds, 0.01 s accuracy. accurate to two decimal places, subject to human error

thermometers

degrees celcius, 1-0.1C accuracy

temperature probe

more precise than liquid-glass thermometers, easy to make multiple repeat readings. more expensive than liquid-glass thermometers, corroded by some reagents.

balances

grams, 0.01 g accuracy. accurate to two decimal places, easy to make multiple repeat readings. Sensitive to environmental fluctuations (e.g. wind currents)

burettes

dispenses variable volumes in titration experiments, cm^3, 0.1 cm^3 accuracy. accurately measures variable volumes, meniscus may be difficult to read.

volumetric pipettes

measures a fixed volume of liquid, cm^3, 0.1 cm^3 accurately measures fixed volumes. harder to use than a normal pipette, only measures one fixed volume.

measuring cylinders

cm^3, 0.1 cm^3 accuracy. Can also be used for downward displacement. available in a range of volumes, cheap to acquire. gives less precise readings due to larger meniscus than volumetric pipettes and gas syringes

gas syringes

cm^3, 0.1 cm^3 accuracy. easy to set up, keeps gas dry, precise readings, expensive and fragile, prone to sticking, only collect limited volumes of gas.

Evaluating experimental methods

Is the apparatus appropriate for the quantity being measured? Have repeat experiments been carried out? Have appropriate control measures been put in place?

Solvent

substance that dissolves a solute

solute

substance that is dissolved in a solvent

solution

a mixture of one or more solutes dissolved in a solvent

saturated solution

a solution containing the maximum concentration of a solute dissolved in the solvent at a specified temperature

filtrate

a liquid or solution that has passed through a filter

residue

a substance that remains after evaporation, distillation, filtration or any similar process

Common indicators include

Phenolphthalein: Colourless in acids, turns pink in alkalis,
Methyl orange: Red in acids, yellow in alkalis.’

Acid-Base Titration

method used to find out the concentration of an acid or base by neutralising it with a solution of known concentration.
Steps: 1. Use a volumetric pipette to transfer a fixed volume of the solution with an unknown concentration (e.g., acid) into a conical flask.

   2. Add a few drops of the suitable indicator to the solution in the flask.

   3. Fill the burette with the standard solution (e.g., alkali) and note the initial reading.

   4. Slowly add the solution from the burette to the flask while swirling the flask.

   5. Stop adding when the indicator changes color, showing the reaction has reached its endpoint.

   6. Record the final burette reading and calculate the volume of solution added.

   7. Repeat the process to get consistent results within 0.1 cm'

End-point

the moment when the acid and base have completely acted, and the solution is neutralized. Add a few drops of indicator to the solution in the conical flask.
Slowly add the titrant from the burette while swirling the flask.
Stop when the indicator shows a permanent colour change (e.g., colourless to pale pink for phenolphthalein).
Record the exact volume of titrant used at the end-point.

Chromatography

1. Draw a pencil line at the bottom of the paper 


2. Place a spot of the test substance on the line


3. Lower paper into an appropriate solvent, ensuring the level of the solvent is below the pencil line


4. Solvent travels up the paper by capillary action, taking some of the coloured substances with it


5. Substances separate, as those with higher solubility move faster than those with lower solubility

Pure/impuresubstances

Pure substances (produce one spot), contain only one type of particle so has a sharp and fixed melting/boiling point. Impure substance (produces more than one spot), mixed particles alter melting and boiling point.

Locating agent

the locating agent reacts with the components of the mixture to produce coloured spots

Rf value

distance travelled by substance/distance travelled by solvent

Suitable solvent

substances based on solubility in a specific solvent. Steps: The dissolved salt can be separated by filtering out the sand, or by evaporating the water to get the salt back. Eg. Salt dissolves in water, but sand doesnt

Filtration

an insoluble solid from a liquid. Steps: Using funnel and filter paper, The solid (sand) stays on the filter as residue while the liquid (water) passes through as filtrate. Eg. Sand in water

Crystallisation

a dissolved solid from a solution. Steps: Evaporate to point of crystallisation. Let solution cool to form crystals or use glass rod to initiate crystallization. Dry with filter paper. Eg. Salt from saltwater

Simple Distillation

a liquid from a liquid mixture based on differences in boiling points. Steps: The mixture is heated with bunsen burner and the liquid with the lowest boiling point evaporates first. This vapour is then cooled and condensed back into a liquid in jacketed condensor, and goes into flask. Leaves the higher-boiling liquid behind. Eg. Water from saltwater

Fractional Distillation

liquids with similar boiling points using a fractionating column.
Steps: 1. Heat the mixture in a flask. At 78°C, ethanol (with the lower boiling point) boils first and rises into the fractionating column. However, some water may evaporate too.
2. The fractionating column creates a temperature gradient (hot at the bottom, cool at the top).
3. Inside the column, the ethanol vapour repeatedly condenses and re-evaporates, becoming purer with each cycle.
4. This ensures only ethanol vapour reaches the condenser first, where it cools and is collected as a liquid.
5. Once ethanol is separated, the temperature rises to 100°C, allowing water to vaporise, condense, and be collected in another flask.

Paper Chromatography

different substances from a solution.

1. Draw a pencil line at the bottom of the paper


2. Place a spot of the test substance on the line


3. Lower paper into an appropriate solvent, ensuring the level of the solvent is below the pencil line


4. Solvent travels up the paper by capillary action, taking some of the coloured substances with it


5. Substances separate, as those with higher solubility move faster than those with lower solubility