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General ScienceEasyAsked in: CDS · CDS-I 2021, GK

Which one of the following sodium compounds is used for softening hard water ?

  1. A.Na₂CO₃
  2. B.NaHCO₃
  3. C.NaOH
  4. D.Na₂SO₄

Correct answer

A. Na₂CO₃

Explanation

The correct answer is A, Na₂CO₃. Sodium carbonate, called washing soda in its hydrated form Na₂CO₃·10H₂O, removes both temporary and permanent hardness of water.

Hard water contains dissolved calcium and magnesium salts, which stop soap from lathering. When washing soda is added, its carbonate ions combine with the calcium and magnesium ions to form insoluble calcium carbonate and magnesium carbonate, which settle out. With these ions removed, soap lathers freely. Washing soda is made by the Solvay process and is also used in the glass, soap and paper industries and as a household cleaner.

B, NaHCO₃, is baking soda, used in baking powder, antacids and soda-acid fire extinguishers, not as a softener. C, NaOH, is caustic soda, a strong alkali used to make soap and paper; it is too corrosive for softening household water. D, Na₂SO₄, is sodium sulphate (Glauber's salt) and does not remove calcium or magnesium ions.

Exam tip: washing soda Na₂CO₃·10H₂O softens water; baking soda NaHCO₃ is for baking and antacids.

Practice Questions

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Q1.General ScienceAsked in: NDA · NDA-II 2021, GATEasy

A negative work is done when an applied force F and the corresponding displacement S are

  1. A.perpendicular to each other.
  2. B.parallel to each other.
  3. C.anti-parallel to each other.
  4. D.equal in magnitude.
Show answer
Correct answer: C. anti-parallel to each other.

Work is negative when the force acts opposite to the displacement, because the angle between them is then 180°. Work done is W = F S cos θ, where θ is the angle between the force and the displacement.

Q2.General ScienceAsked in: NDA · NDA-II 2017, GATMedium

Which one of the following statements about a satellite orbiting around the Earth is correct ?

  1. A.Satellite is kept in orbit by remote control from ground station.
  2. B.Satellite is kept in orbit by retro-rocket and solar energy keeps it moving around the Earth.
  3. C.Satellite requires energy from solar panels and solid fuels for orbiting.
  4. D.Satellite does not require any energy for orbiting.
Show answer
Correct answer: D. Satellite does not require any energy for orbiting.

Once a satellite is placed in orbit with the right speed, the Earth's gravity keeps it going round without any engine.

Q4.General ScienceAsked in: NDA · NDA-II 2022, GATEasy

A microphone converts

  1. A.electrical signals to sound waves
  2. B.sound waves to electrical signals
  3. C.microwaves to sound waves
  4. D.sound waves to microwaves
Show answer
Correct answer: B. sound waves to electrical signals

A microphone is a transducer that changes the pressure variations of sound into a matching electrical signal. In a common dynamic microphone, sound waves make a thin diaphragm vibrate.