Permanent hardness of water cannot be removed by which one of the following methods?
- A.Treatment with washing soda
- B.Calgon's method
- C.Boiling
- D.Ion exchange method
Correct answer
Explanation
The correct answer is C, Boiling. Boiling removes only temporary hardness; permanent hardness comes from salts that do not break down on heating.
Temporary hardness is caused by the bicarbonates of calcium and magnesium. On boiling they decompose into insoluble carbonates that settle out, so the water softens. Permanent hardness is caused by the chlorides and sulphates of calcium and magnesium, which stay dissolved however long the water is boiled. To remove them the calcium and magnesium ions must be taken out chemically, either by precipitating them or by swapping them for sodium ions.
A, washing soda (sodium carbonate), precipitates calcium and magnesium as insoluble carbonates, removing permanent hardness.
B, Calgon (sodium hexametaphosphate), binds calcium and magnesium ions into soluble complexes so they cannot act on soap.
D, the ion exchange method, using zeolite (permutit) or resins, replaces calcium and magnesium ions with sodium or hydrogen ions.
Exam tip: temporary hardness, bicarbonates, removed by boiling or Clark's method; permanent hardness, chlorides and sulphates, removed by washing soda, Calgon or ion exchange.
Practice Questions
View allThe principle involved in paper chromatography is
- A.Absorption
- B.Partition
- C.Solubility
- D.Volatility
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Paper chromatography separates substances because each one divides, or partitions, itself differently between two liquid phases. In paper chromatography the stationary phase is the water held in the cellulose fibres of the paper, and the mobile phase is the solvent that rises up the paper by capillary action.
Read the full explanation →Which one of the following phenomena CANNOT be exhibited by sound waves ?
- A.Reflection
- B.Refraction
- C.Interference
- D.Polarisation
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Sound in air is a longitudinal wave, and only transverse waves can be polarised. Polarisation means restricting the vibrations of a wave to a single plane.
Read the full explanation →Which one of the following is NOT a luminous object ?
- A.Sun
- B.Electric lamp
- C.Candle
- D.Moon
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The Moon gives out no light of its own; it only reflects sunlight, so it is a non-luminous object. A luminous object produces and emits its own light, while a non-luminous object is seen only because it reflects light falling on it.
Read the full explanation →In experiment #1, a bar magnet is moved towards a conducting wire loop axially, with the magnet's north pole facing the loop. In experiment #2, the same process as in experiment #1 is repeated except that the south pole of the magnet faces the loop. Which one of the following statements is true in this context?
- A.The direction of current in the loop will be of opposite nature in both the experiments.
- B.The direction of current in the loop will be the same in both the experiments.
- C.No current will flow in either of the two experiments.
- D.More current will flow in the loop in experiment #1.
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Reversing the pole that approaches the loop reverses the change in magnetic flux, so the induced current reverses too. When a magnet moves towards a closed loop, the magnetic flux through the loop changes and an electric current is induced (Faraday's law of electromagnetic induction).
Read the full explanation →Which one of the following statements about classification of plants is correct?
- A.Thallophytes have well differentiated body design
- B.Funaria is a fungus
- C.All Pteridophytes are Phanerogams
- D.Vascular system is not found among Bryophytes
Show answer
Mosses and liverworts have no xylem or phloem, so water moves through them cell to cell. Bryophytes, such as Funaria (a moss) and Marchantia (a liverwort), are called the amphibians of the plant kingdom because they live on land but need water for fertilisation.
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