Pearl is a hard object produced within the soft tissues of a mollusk. Which one of the following is the main constituent of pearl?
- A.Calcium carbonate
- B.Calcium oxide
- C.Calcium nitrate
- D.Calcium sulphate
Correct answer
Explanation
The correct answer is A, Calcium carbonate. A pearl is built up from layers of nacre, which is mostly calcium carbonate held together by a protein called conchiolin.
When an irritant such as a grain of sand or a parasite gets inside an oyster or mussel, the mantle covers it with layer after layer of nacre, the same mother-of-pearl that lines the shell. The calcium carbonate is present mainly as tiny crystals of aragonite, which give the pearl its lustre. In cultured pearls a bead or a piece of tissue is placed inside the mollusc on purpose to start this process.
B is wrong: calcium oxide is quicklime, which reacts with water and is not found in pearls.
C is wrong: calcium nitrate is a soluble salt used as a fertiliser.
D is wrong: calcium sulphate is gypsum, used in plaster of Paris.
Exam tip: pearl, shells, chalk, marble and limestone are all mainly calcium carbonate.
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 →