Temporary hardness in water is due to which one of the following of Calcium and Magnesium?
- A.Hydrogencarbonates
- B.Carbonates
- C.Chlorides
- D.Sulphates
Correct answer
Explanation
The correct answer is A, hydrogencarbonates. Temporary hardness comes from dissolved calcium and magnesium hydrogencarbonates (bicarbonates), and it can be removed simply by boiling.
On boiling, calcium hydrogencarbonate breaks down into insoluble calcium carbonate, water and carbon dioxide, and the carbonate settles out; this is the scale seen inside kettles. Magnesium hydrogencarbonate similarly gives a precipitate. Temporary hardness can also be removed by Clark's method, adding a calculated amount of lime. Hard water does not lather well with soap because calcium and magnesium ions form a scum.
B, carbonates, are almost insoluble in water, so they do not stay dissolved to cause hardness; they are what boiling produces.
C, chlorides, cause permanent hardness, which boiling cannot remove.
D, sulphates, also cause permanent hardness, removed by washing soda, ion exchange or the Calgon process.
Exam tip: temporary hardness, bicarbonates, removed by boiling or Clark's method; permanent hardness, chlorides and sulphates, removed by washing soda or ion exchange.
Practice Questions
View allConsider the following actions :
1. Detection of car crash/collision which results in the deployment of airbags almost instantaneously
2. Detection of accidental free fall of a laptop towards the ground which results in the immediate turning off of the hard drive
3. Detection of the tilt of the smartphone which results in the rotation of display between portrait and landscape mode
In how many of the above actions is the function of accelerometer required?
- A.Only one
- B.Only two
- C.All three
- D.None
Show answer
An accelerometer measures acceleration, including the pull of gravity, and each of the three actions depends on reading it. An accelerometer is a small sensor, today usually a MEMS (micro-electro-mechanical system) chip, that senses changes in speed and the direction of gravity.
Read the full explanation →Consider the following statements :
1. Carbon fibres are used in the manufacture of components used in automobiles and aircrafts.
2. Carbon fibres once used cannot be recycled.
Which of the statements given above is/are correct?
- A.1 only
- B.2 only
- C.Both 1 and 2
- D.Neither 1 nor 2
Show answer
Carbon fibre is widely used in cars and aircraft for its strength and low weight, and it can be recovered and recycled.
Read the full explanation →Which one of the following cell organelles contains DNA?
- A.Golgi apparatus
- B.Mitochondrion
- C.Lysosome
- D.Endoplasmic reticulum
Show answer
Mitochondria carry their own small, circular DNA and their own ribosomes. Mitochondria are the powerhouses of the cell, where aerobic respiration makes ATP.
Read the full explanation →Which set of vegetables is underground stem or modified stem of a plant?
- A.Potato, artichoke, round gourd
- B.Onion, garlic, carrot
- C.Potato, onion, garlic
- D.Potato, garlic, turnip
Show answer
All three are underground modified stems: the potato is a stem tuber, and onion and garlic are bulbs. A stem can be told from a root by its nodes and buds.
Read the full explanation →A sound wave having frequency of 300 Hz is travelling in an unknown medium. Its wavelength is not known. It travels a distance equal to 150 times its wavelength in time t. The value of t is :
- A.0.5 s
- B.1 s
- C.1.5 s
- D.2 s
Show answer
A wave moves forward by one wavelength in each cycle, so 150 wavelengths take 150 cycles, and 150 cycles at 300 per second take half a second.
Read the full explanation →