22.12 - Other Question Types You'll Meet

Other question types you'll meet

• Cambridge practical papers are mostly built from a few big question types, each covered in its own subtopic:
  • The results table
  • The graph
  • The 6-mark plan
  • The specimen drawing
• Around these, the papers set a scatter of smaller questions, each turning on a single idea and usually worth 1 or 2 marks.
• On its own, none is big enough for a full subtopic, but together they account for several marks on most papers - and they're easy marks once you know the move.

The smaller skills covered here

• Explaining why a step is in a method
• Reading a measuring instrument to the right precision
• Making up a solution by dilution
• Measuring an area by counting squares
• Reading an unknown colour against a set of standards

There is no single mark scheme across these - they are different skills gathered for convenience, so each one teaches its own move and shows the answer that scores.

Explaining why a step is in the method

• Every paper prints a method and asks why one particular step is in it - e.g. why yeast was left two minutes before counting, why tubing was rinsed before weighing, why a tube was covered.
• There's no set answer to memorise, because the step changes each time.
• The move that works almost every time: ask what the experiment would get wrong if the step were left out. Whatever error or unfairness the step prevents is the reason for it.

The reasons fall into a small, repeating set

• To let something reach the right temperature (equilibrate) - The most common. A step that waits a minute or two before measuring is nearly always letting the contents reach the water-bath temperature. For the two-minute yeast wait, the mark-scheme answer is that it lets the temperature "become the same as the water", so the rate is measured at the temperature being tested.
• To remove something that would interfere - Rinsing or drying a surface so leftover liquid doesn't add to a mass or volume reading. For rinsing a dialysis tubing bag, the answer is "to remove any sucrose solution from the outside of the tubing", so it doesn't add to the mass.
• To act as a control - An identical set-up with only the tested variable left out, so any difference must be caused by that variable and nothing else.
• To prevent an unwanted change - Covering a tube to stop evaporation, or to keep out light, so the only thing that changes is the variable being tested.
• To spot anomalies - Repeating a reading so an odd result shows up and a mean can be taken.

How to phrase the answer

• Phrase it as what the step achieves, not as what the step was.
• A sentence frame that scores: "this step is there to [achieve something], so that [the result is fair, accurate, or measured at the right condition]".
  • Example: "the yeast is left two minutes to reach the water-bath temperature, so that the rate is measured at the temperature being tested".
• Where a step is worth 2 marks, give two distinct reasons, not the same idea twice.

Reading a measuring instrument

• Read a value off a scale to the precision the scale actually allows - and no finer.
• Read to the nearest marked division, or to the nearest half-division if you can confidently judge halfway between two marks.
• Never invent digits the instrument cannot show.

Two instruments to know the rules for

1. Thermometer (or any linear scale) - Read to the nearest division, or half a division at most. A thermometer marked every degree is read to the nearest degree, or nearest half-degree at most.
2. Liquid in a measuring cylinder - The surface curves up at the edges. Read the volume at the bottom of the curve (the meniscus), with your eye level with it. Reading from above or below at an angle gives a value that is too high or too low.

Making up a solution by dilution

• Some questions ask you to make a weaker solution from a stronger stock by mixing it with water, and to work out the two volumes.
• The key idea: the water makes up the rest of the total volume, so the stock and the water always add up to the total volume you want.

The two-step method

1. Find the volume of stock - Multiply the total volume you need by the fraction of the concentration you want.
2. Find the volume of water - It's whatever is left to reach the total.

Making a whole dilution series

• The same method gives every row of a dilution series.
• To make 10 cm³ from a 100% stock:

Each row adds up to the same 10 cm³ - only the concentration changes.

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Worked example - Making 10 cm³ of a 20% solution

Make 10 cm³ of a 20% solution from a 100% stock. Work out the volume of stock and the volume of water needed.

Measuring an area by counting squares

• To find the area of an irregular shape (such as a leaf), place it on a grid of known squares, draw round it, and count the squares it covers.
• The outline cuts through some squares, so use the rule for part-covered ones:
  • Count a square if the shape covers more than half of it.
  • Ignore a square if it covers less than half.
• The half-and-more squares roughly cancel the half-and-less ones, so the count comes out about right.

Turning the count into an area

1. Work out the area of one square from the grid - if each square is 1 cm by 1 cm, one square is 1 cm².
2. Multiply the area of one square by the number of squares you counted.
3. Example: a leaf covering 34 squares on a 1 cm grid has an area of about 34 cm².

The three marks credited here are: counting the covered squares, applying the "more than half" rule for edge squares, and multiplying the count by the area of one square.

Reading an unknown against colour standards

• Some results come out as a colour, and the question asks you to turn that colour into a value - a pH or a concentration.
• You do this by comparing the unknown against a set of known colour standards: a colour chart, or a row of tubes each made up to a known value.
• You find the standard the unknown matches best and read off its value.

How it works for pH and concentration

• pH - Add universal indicator to the sample, then match the colour it turns against the pH colour chart. Read off the pH of the closest match.
• Concentration - Set up a series of tubes at known concentrations, each a slightly different colour. Place the unknown alongside and read off the concentration of the tube it matches.

The estimate is only as good as the standards - the more closely spaced they are, the finer the value you can read.

Limitation - Colour matching is subjective

• It is an estimate by eye, so it is subjective - two people may judge the nearest standard differently.
• If a question asks for a limitation of the method, that subjectivity - judging the colour by eye - is the point to make.