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Context: My son is studying in 6th grade, he had a question like this.

Vinegar, lemon juice, wheat flour, diesel and mustard oil can be distinguished on the basis of a property called

  1. Hardness
  2. Appearance
  3. Solubility
  4. Magnetism

Son answered 2. Appearance and as per their key, the answer is 3. Solubility

Which is correct and why? Or are all of them wrong?

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    $\begingroup$ That's a dreadful question. I don't see how solubility would allow vinegar and lemon juice to be distinguished, except for the fact that lemon juice contains suspended particles. We rarely make decisions based on one feature. I would also have answered appearance, but odor, density, viscosity, pH, solubility would be useful. Magnetism and hardness would not be very. $\endgroup$
    – Buck Thorn ♦
    Commented Jul 7 at 18:31

2 Answers 2

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You can distinguish them all by appearance.

You would have difficulties to differentiate by solubility

  • the water based solutions like vinegar and lemon juice
  • oily substances like diesel and plant oil.

The answer why the answer 3 and not 2 has been chosen can be provided only by the author of the question.

Numbers 1 and 4 are obvious and total nonsenses.

There is an argument that "appearance" is not a property. How valid is that?

It is half true, as appearance consists of a set of objective and subjective properties.

  • Flour being solid is both the subjective and objective property.
  • Low viscosity (vinegar, lemon juice) is both subjective and objective property.
  • Producing acidic vapor (vinegar) is both subjective and objective property.
  • Not being fully clear solution (lemon juice) is both subjective and objective property.
  • Having technical smell (diesel) is subjective property. Objective property would require instrumentation.

Solubility is objective property, but it would distingush just three groups:

  • Water solutions (vinegar, lemon juice)
  • Oily substances insoluble in water (diesel, mustard oil)
  • Insoluble solids (flour)
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    $\begingroup$ There is an argument that "appearance" is not a property. How valid is that? $\endgroup$
    – codeandcloud
    Commented Jul 7 at 11:50
  • $\begingroup$ Your answer gave me lots of insights. <3 $\endgroup$
    – codeandcloud
    Commented Jul 8 at 1:37
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Hardness is a parameter that is only valid for water containing a small amount of calcium or magnesium ions. It cannot be measured in a fruit juice, or in oils.

Appearance is a criteria difficult to define for a given substance. Is it the colour, the density, the melting point ? A combination of these parameters ?

Solubility has no meaning here, because, first, vinegar and lemon juice are already both solutions. Solutions don't have any solubility. Then, wheat is not a pure substance. Only pure substances have a measured solubility. Third, Diesel and mustard oil are all insoluble in water : their low solubility can be determined, but it is rather low, and often not known with precision.

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