It’s the weekend, it’s lunchtime, the sun is shining. You’ve just prepared yourself a nice, refreshing salad to munch in the garden. You go to add the salad dressing and notice the label: “Shake the bottle well before use”. Well before use? Is 10 minutes before use long enough? Should I have shaken before slicing my cucumber? Salad stress on a sunny Saturday – it’s not what you want! What the label really means, of course, is “shake well” because the contents are a mixture that separates out when the bottle is left standing. Most products are mixtures, though not many separate out. Either way, if the proportions of the ingredients are wrong, customers won’t be happy. So, quality control must include checking the composition.
Have you ever wondered how to analyse something that is not a single substance?
You might like to …
• find out how ‘representative sampling’ differs from just taking several samples; suggest when and why it is necessary
• check the composition of a homogeneous mixture (for example, determine the concentration of citric acid in lemonade or peroxide in hair bleach); compare your result with the value on the container, if stated; suggest reasons for any significant difference
• devise a sampling strategy and method to check the composition of a non-homogeneous mixture (for example, the proportions of ingredients in muesli; or oil, vinegar and solids (mainly herbs) in a French dressing); labels list the ingredients in order of quantity, highest first – does the product conform to this specification?