Chapter 12 - Experimental techniques and chemical analysis

Time can be measured using a stopwatch which shows time to one or two decimal places. The units used to measure time are usually seconds or minutes. 1 minute = 60 seconds.

Temperature is measured with a thermometer. Laboratory thermometers usually have a precision of half or one degree. The units of temperature are degrees Celsius (C)

Mass is measured using a digital balance which normally gives readings to two decimal places. Balances should be set to zero before use and should be allowed time to settle on a final measurement before the reading is recorded. 1000 grams = 1 kilogram

Volumes of liquid can be determined using burettes, volumetric pipettes or measuring cylinders. Burettes are the most accurate way of measuring a variable volume of liquid between 0cm3 and 50cm3. Read the burette scale form top to bottom as 0.00cm3 is at the top of the column. Volumetric pipettes are the most accurate way of measuring a fixed volume of liquid. They have a scratch mark on the next which is matched to the bottom of the meniscus to make the measurement. Measuring cylinders are used when approximate volumes are required. 1 millilitre = 1cm3.

Volumes of gas can be measured using a gas syringe or the downward displacement of water. A gas syringe is more precise and accurate than the downward displacement of water. Downward displacement of water is where a measuring cylinder is inverted in water to collect the gas produced. This method doesn’t work if the gas is soluble in water.

More precise readings Easy to make multiple repeat readings Can be automated to run over long periods of time

Can be corroded by some reagents More expensive to replace

Accurate measurement of a fixed volume

Harder to use than a normal pipette Only measures one fixed volume

Easy to set up Keeps the gas dry

The syringe can stick Collects limited volumes Expensive and delicate

Less wasteful Saves energy Safer

Hard to see what is happening Lose a lot of material when separating or purifying the product

A solvent is a substance that dissolves a solute

A solute is a substance that is dissolved in a solvent

A solution is a mixture of one or more solutes dissolved in a solvent

A saturated solution is a solution containing the maximum concentration of a solute dissolved in the solvent at a specific temperature

Soluble is a substance that will dissolve

Insoluble is a substance that will not dissolve

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

Filtrate is a liquid or solution that has passed through a filter

Titrations are a method of analysing the concentration of solutions. They can determine exactly how much alkali is needed to neutralise a quantity of acid and vice versa.

25cm3 volumetric pipette Pipette filler 50cm3 burette 250cm3 conical flask Small funnel 0.1mol/dm3 sodium hydroxide solution Sulfuric acid of unknown concentration A suitable indicator Clam stand, clamp and white tile

• Use the pipette and pipette filler and place exactly 25cm3 sodium hydroxide solution into the conical flask. • Fill the burette with hydrochloric acid, place an empty beaker underneath the tap. Run a small portion of acid through the burette to remove any air bubbles. • Record the starting point on the burette to the nearest 0.05cm3. • Place the conical flask on a white tile so the tip of the burette is inside the flask. • Add a few drops of a suitable indicator to the solution in the conical flask. • Perform a rough titration by taking the burette reading and running in the solution in 1 – 3cm3 portions, while swirling the flask vigorously. • Quickly close the tap when the endpoint is reached. The endpoint is when one drop causes a sharp colour change. • Record the volume of hydrochloric acid added, in a suitable results table as shown below Make sure your eye is level with the meniscus. • Repeat the titration with a fresh batch of sodium hydroxide. • As the rough end-point volume is approached, add the solution from the burette one drop at a time until the indicator just changes colour. • Record the volume to the nearest 0.05cm3. • Repeat until you achieve two concordant results (two results that are within 0.1 cm3 of each other) to increase accuracy.