Which one of the following metals does not react with oxygen, water or acids?
- A.Na
- B.Cu
- C.Fe
- D.Au
Correct answer
Explanation
The correct answer is D, Au. Gold is a noble metal at the bottom of the reactivity series, so it does not react with oxygen, water or ordinary acids.
That is why gold keeps its shine for thousands of years and is used in jewellery, coins and electrical contacts. Gold dissolves only in aqua regia, a freshly made mixture of concentrated hydrochloric acid and concentrated nitric acid in the ratio 3:1, which forms a soluble gold chloride complex. Platinum is another noble metal that behaves in much the same way. The purity of gold is measured in carats, with 24-carat gold being pure; 22-carat gold is mixed with copper or silver to make it harder.
A is wrong: sodium (Na) reacts violently with water and is stored under kerosene.
B is wrong: copper (Cu) reacts slowly with moist air, forming a green coating, and with nitric acid.
C is wrong: iron (Fe) rusts in moist air and reacts with dilute acids to give hydrogen.
Exam tip: Gold and platinum dissolve only in aqua regia (3 HCl : 1 HNO₃).
Practice Questions
View allConsider the following animals :
1. Hedgehog
2. Marmot
3. Pangolin
To reduce the chance of being captured by predators, which of the above organisms rolls up/roll up and protects/protect its/their vulnerable parts?
- A.1 and 2
- B.2 only
- C.3 only
- D.1 and 3
Show answer
Hedgehogs and pangolins curl into a tight ball when threatened, covering the soft belly and face with spines or scales; marmots do not.
Read the full explanation →Which one of the following is used in preparing a natural mosquito repellent?
- A.Congress grass
- B.Elephant grass
- C.Lemongrass
- D.Nut grass
Show answer
Lemongrass and its close relative citronella grass yield aromatic oils that keep mosquitoes away, so they are the base of many natural repellents.
Read the full explanation →Which one of the following devices is non-ohmic ?
- A.Conducting copper coil
- B.Electric heating coil
- C.Semi conductor diode
- D.Rheostat
Show answer
A diode does not obey Ohm's law: the current through it is not proportional to the voltage across it. An ohmic conductor gives a straight-line graph of current against voltage, so its resistance stays constant at a fixed temperature.
Read the full explanation →What is the formula mass of anhydrous sodium carbonate? (Given that the atomic masses of sodium, carbon and oxygen are 23 u, 12 u and 16 u respectively)
- A.286 u
- B.106 u
- C.83 u
- D.53 u
Show answer
Anhydrous sodium carbonate is Na₂CO₃, and 2 × 23 + 12 + 3 × 16 = 106 u. Formula mass is the sum of the atomic masses of all the atoms in one formula unit of an ionic compound.
Read the full explanation →Which one of the following minerals is used as a fuel in nuclear power stations ?
- A.Bauxite
- B.Quartz
- C.Feldspar
- D.Pitchblende
Show answer
Pitchblende, or uraninite, is the chief ore of uranium, the fuel used in most nuclear power stations. Pitchblende is a heavy, black mineral made mostly of uranium oxide.
Read the full explanation →