All solutions are considered homogenous: a solution has all solutes dissolved in solvent with no excess
A. salt from salt solution – homogeneous; paper filtration
What is paper filtration?
A semi-permeable barrier is used to separate fine particles from a liquid or gas. Requires a heterogenous mix to separate.
We are told it is a solution so we know it should be homogenous. Paper filtration will not work for separating this substance because paper filtration only works for heterogenous substances. Therefore A cannot be correct.
B. water from salt solution – heterogeneous; simple distillation
As stated before, we know that salt in water is a homogenous mixture, therefore we can instantly eliminate B.
C. chlorophyll from a solution of different plant pigments – homogeneous; chromatography
Chlorophyll in a solution of different plant pigments is homogenous (keyword solution).
Chromatography is used to separate colours in a dye, separate pigments from natural colour, and separate drugs from the blood.
Pigments are coloured, so we use this method in order to separate these colours based on colour.
The different pigments present in a chloroplast can be separated by paper chromatography.
\fcolorbox{red}{grey!30}{Therefore the answer is C}
D. red blood cells from blood – homogeneous; centrifuge
First off, blood is a heterogeneous mixture. It includes a mixture of plasma, platelets, white blood cells, and red blood cells. It is heterogenous because there are two different phases, a solid one, and a liquid one. We can distinguish between the two phases during separation because the solid phase will aggregate (cells at the bottom).
Centrifugation - separates insoluble materials from a liquid when normal filtration is not possible. Usually involves spinning a test tube at high speeds, resulting in heavier things aggregating on the bottom and lighter things on the top.
E. gasoline/petrol from crude oil – heterogeneous; simple distillation
Difference between simple and fractional distillation
Simple: closed container, heat up liquid to the boiling point (BP) of liquid that has the lowest boiling point in the liquid mixture. The liquid with the lowest BP will then boul and the gas (vapour) will be collected in a tube (where it condenses back into liquid).
We use this technique when there is a large difference in the BP of two liquids.
Fractional distillation: similar process but more complex, several simple distillations are done in one.
Used when the difference in BP of the liquids is much closer together. Example: crude oil refining.
Separating gasoline from crude oil (where there are multiple liquids that have varying boiling points) can be a complex process that requires fractional distillation (multiple liquids that are similar in terms of composition so similar BP). The mixture is homogenous and this is one of the factors that explains the difficulty in separating the substances. Therefore E is incorrect.