Which one among the following substances is a compound?
- A.Sugar
- B.Air
- C.Milk
- D.Tea
Correct answer
Explanation
The correct answer is A, Sugar. Cane sugar, or sucrose, is made of carbon, hydrogen and oxygen joined chemically in a fixed ratio, so it is a compound.
A compound has a fixed composition, a definite formula and properties different from those of its elements; it can be broken into its elements only by chemical means. Sucrose has the formula C₁₂H₂₂O₁₁. A mixture, on the other hand, has no fixed composition, its parts keep their own properties, and they can be separated by physical methods such as filtration, evaporation or distillation.
B is wrong because air is a mixture of nitrogen, oxygen, argon, carbon dioxide, water vapour and other gases in varying amounts. C is wrong because milk is a mixture, a colloid of fat droplets and proteins in water with dissolved sugar and salts. D is wrong because tea is a solution of many substances from the leaves, along with milk and sugar, in water.
Exam tip: fixed formula means compound; variable composition means mixture.
Practice Questions
View allIn a molecule of water, hydrogen and oxygen are combined in a fixed ratio by mass of:
- A.1 : 8
- B.2 : 1
- C.1 : 16
- D.8 : 1
Show answer
A water molecule has two hydrogen atoms of atomic mass one and one oxygen atom of atomic mass sixteen, so the masses are two and sixteen, which is a ratio of one to eight; this fixed ratio in every sample of water, whatever its source, is the classic illustration of…
Read the full explanation →Which of the following is a water-borne disease?
- A.Malaria
- B.Dengue
- C.Cholera
- D.Tuberculosis
Show answer
Cholera is caused by the bacterium Vibrio cholerae and spreads through water and food contaminated with faecal matter, which is why it breaks out after floods and wherever sewage mixes with the water supply; typhoid, hepatitis A, dysentery and polio spread the same way.
Read the full explanation →Hard water does not lather readily with soap because the calcium and magnesium salts present in it:
- A.Make the water acidic
- B.Form insoluble salts with soap, seen as scum
- C.Raise the boiling point of the water
- D.Decompose the soap into glycerol
Show answer
Soap is the sodium or potassium salt of a long chain fatty acid.
Read the full explanation →In a nuclear reactor, heavy water is used mainly as:
- A.A fuel
- B.A moderator
- C.A control rod
- D.A shield against radiation
Show answer
A moderator slows down the fast neutrons released by fission so that they can cause further fission, and heavy water is the best of them because it slows neutrons well while absorbing very few; in Indian pressurised heavy water reactors it also serves as the coolant that carries heat away…
Read the full explanation →A reverse osmosis water purifier works by:
- A.Passing ultraviolet light through the water
- B.Forcing water through a semi-permeable membrane at a pressure above the osmotic pressure
- C.Boiling the water and condensing the steam
- D.Adding chlorine to the water
Show answer
In reverse osmosis the water is pushed through a semi-permeable membrane at a pressure greater than its osmotic pressure, so the flow runs opposite to natural osmosis; the membrane lets water molecules through and holds back dissolved salts, heavy metals and most microbes, which is why reverse osmosis is also…
Read the full explanation →