Consider the following statements : 1. The temporary hardness in water is due to the presence of bicarbonates of calcium. 2. The permanent hardness in water is due to the presence of soluble chlorides and sulphates of calcium and magnesium. 3. The permanent hardness in water is removed by adding Na₂CO₃ to water. Which of the statements given above are correct?
- A.1 and 2 only
- B.2 and 3 only
- C.1 and 3 only
- D.1, 2 and 3
Correct answer
Explanation
The correct answer is D, 1, 2 and 3. All three statements describe hard water correctly.
Statement 1 is correct: temporary hardness comes from dissolved bicarbonates, chiefly calcium bicarbonate (magnesium bicarbonate too). Boiling turns them into insoluble carbonates that settle, which is why it is called temporary. Statement 2 is correct: permanent hardness is due to soluble chlorides and sulphates of calcium and magnesium, which boiling cannot remove. Statement 3 is correct: washing soda, Na₂CO₃, reacts with these salts to precipitate calcium and magnesium carbonates, softening the water.
A is wrong because it leaves out statement 3, and washing soda does remove permanent hardness.
B is wrong because it leaves out statement 1, which correctly names bicarbonates as the cause of temporary hardness.
C is wrong because it leaves out statement 2, the correct cause of permanent hardness.
Exam tip: temporary hardness = bicarbonates, removed by boiling or Clark's method (lime); permanent = 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 →