(c) use of laboratory glassware apparatus for a variety of experimental…

The idea

This practical technique requires you to select and correctly use laboratory glassware — such as graduated pipettes, burettes, volumetric pipettes, measuring cylinders and volumetric flasks — across a range of experimental procedures, and specifically to carry out serial dilutions. A serial dilution is a stepwise series in which each dilution is made from the previous one, giving a geometric fall in concentration. For example, to make a 1:10 series you transfer a fixed volume of stock into a known final volume, mix thoroughly, then take the same fixed volume from that tube into the next, repeating along the series. Correct glassware choice matters: a volumetric pipette delivers one accurate fixed volume, a measuring cylinder suits approximate volumes, and a volumetric flask makes up to a single calibrated volume.

What earns the marks

• Selects glassware matched to the accuracy required: volumetric pipette or burette for precise fixed volumes, measuring cylinder for approximate volumes, volumetric flask for making up to a calibrated volume.
• Describes a serial dilution as a stepwise series in which each dilution is prepared from the previous one, producing a geometric decrease in concentration.
• States that a fixed volume is transferred at each step into a known final volume and mixed thoroughly before the next transfer.
• Explains that reading the meniscus at eye level and correct rinsing/mixing reduce systematic error in the delivered volumes.
• Recognises that the dilution factor multiplies along the series, so overall dilution after n steps is the product of the individual factors.

Common mistakes and how to put them right

• Thinking a serial dilution is several independent dilutions made directly from the stock.

  Put it right: Each step is made from the previous dilution, so errors and factors compound along the series.

• Choosing a measuring cylinder when a precise fixed volume is needed.

  Put it right: Match glassware to the accuracy required, using a volumetric pipette for accurate fixed volumes.

• Reading the meniscus from above or below eye level.

  Put it right: Align the eye with the meniscus to avoid parallax error in the volume reading.

Exam tips

• Link each named piece of glassware to the accuracy it provides rather than listing apparatus without purpose.
• When describing a serial dilution, state the fixed transfer volume, the final volume and the mixing step explicitly.
• Check whether a question asks for the overall dilution factor and multiply the individual step factors correctly.