Which one of the following is not an allotrope of carbon?
- A.Coal
- B.Diamond
- C.Graphite
- D.Graphene
Correct answer
Explanation
The correct answer is A, Coal. Coal is an impure fossil fuel containing hydrogen, oxygen, nitrogen and sulphur besides carbon, so it is not a pure form of the element.
Allotropes are different physical forms of the same element, made of the same atoms arranged differently. Carbon's allotropes include diamond, in which each atom is bonded to four others in a hard three-dimensional network; graphite, made of stacked layers that slide over each other; fullerenes such as C-60; and graphene, a single sheet of carbon atoms. Coal, formed from buried plant matter, is a mixture rather than an allotrope.
B is wrong: diamond is a crystalline allotrope of carbon, the hardest natural substance.
C is wrong: graphite is a crystalline allotrope of carbon and a good conductor.
D is wrong: graphene, one atom-thick layer of graphite, is also an allotrope.
Exam tip: allotropes of carbon are diamond, graphite, fullerene and graphene; coal, coke and charcoal are impure forms.
Practice Questions
View allWhich of the following properties is true for a tooth paste?
- A.It is acidic
- B.It is neutral
- C.It is basic
- D.It is made up of Calcium phosphate, the material of tooth enamel
Show answer
Toothpaste is mildly basic so that it can neutralise the acid that causes tooth decay. Bacteria in the mouth break down sugar and food particles left on the teeth and produce acids.
Read the full explanation →A stainless steel chamber contains Ar gas at a temperature T and pressure P. The total number of Ar atoms in the chamber is n. Now Ar gas in the chamber is replaced by CO₂ gas and the total number of CO₂ molecules in the chamber is n/2 at the same temperature T. The pressure in the chamber now is P'. Which one of the following relations holds true? (Both the gases behave as ideal gases)
- A.P' = P
- B.P' = 2P
- C.P' = P/2
- D.P' = P/4
Show answer
For an ideal gas in a fixed volume at a fixed temperature, pressure is proportional to the number of molecules, so half the molecules give half the pressure.
Read the full explanation →In which of the following pairs are the ions isoelectronic?
- A.Mg²⁺, Ar
- B.Na⁺, O²⁻
- C.Al³⁺, Cl⁻
- D.K⁺, Ne
Show answer
Both have 10 electrons, the same number as a neon atom, so they are isoelectronic. Isoelectronic species have the same number of electrons.
Read the full explanation →A solid disc and a solid sphere have the same mass and same radius. Which one has the higher moment of inertia about its centre of mass?
- A.The disc
- B.The sphere
- C.Both have the same moment of inertia
- D.The information provided is not sufficient to answer the question
Show answer
About an axis through the centre, a solid disc has I = ½MR² while a solid sphere has I = ⅖MR², and ½ is larger than ⅖.
Read the full explanation →Numerically two thermometers, one in Fahrenheit scale and another in Celsius scale shall read same at
- A.−40°
- B.0°
- C.−273°
- D.100°
Show answer
At this one temperature the Celsius and Fahrenheit scales show the same number. The two scales are linked by F = (9/5)C + 32.
Read the full explanation →