ELEMENTS, COMPOUNDS AND MIXTURES [dchemlc]

which of the following is not an element?

• water

• oxygen gas

• diamond

• magnesium

Answer explanation

Elements are substances that can't be split into anything simpler by chemical means. An element contains only one type of atom.

which of the following is not a compound?

• air

• methane

• sodium chloride

• carbon dioxide

Answer explanation

Compounds are formed when two or more elements chemically combine. The elements always combine in fixed proportions.

an impure sample of aspirin would have a

• higher boiling point

• lower boiling point

• fixed boiling point

• no boiling point

Answer explanation

The presence of impurities lowers the melting point of a substance and raises the boiling point.

Filtration can be used to separate a solid from a liquid. What do you call the liquid that is filtered through?

Answer explanation

The substance left in the filter paper is called the residue and the liquid that comes through is called the filtrate.

which method of seperation would you use to separate a mixture of sand and water?

• filtration

• crystallisation

• distillation

• centrifuging

Answer explanation

sand can be separated from water by filtration.

which two methods can be used to obtain salt from rock salt?

• filtration

• crystallisation

• distillation

• centrifuging

• chromatography

Answer explanation

We can use filtration and crystallisation to obtain pure salt from rock salt.

If you crush the rock salt and mix it with hot water, the salt dissolves, but the impurities don't. The impurities can be filtered off, and remain on the filter paper. The filtrate is then a salt solution. The solid salt can be obtained from the solution by crystallisation.

copper sulphate is soluble in water. Which method would you use to obtain copper sulphate from water?

• crystallisation

• centrifuging

• chromatography

• filtration

Answer explanation

Crystallisation can be used to separate a solute from a solution.

which method can be used to obtain ethanol from a mixture of ethanol and water?

• fractional distillation

• simple distillation

• crystallisation

• chromatography

Answer explanation

Fractional distillation is used to separate a mixture of liquids such as ethanol and water. Ethanol gets collected in the beaker.

if you need to determine what kind of food colourings are there in a food sample, which method would you use?

• chromatography

• fractional distillation

• filtration

• crystallisation

Answer explanation

Paper chromatography can be used to separate a variety of mixtures such as coloured inks and food colourings.

which of the following is not true about mixtures and compounds?

• both mixtures and compounds can be seperated by physical means

• both mixtures and compounds can contain more than one element

• In a mixture of elements, each element keeps its own properties, but the properties of the compound are quite different

• the ratio of elements in a mixture can vary but not in compounds

Answer explanation

Only mixtures can be separated by physical means.