CDS (II) 2024 General Knowledge · Question 8 of 120

Correct — B, Sea water. Sound travels faster the stiffer a medium is — the harder it is to compress — and slower the denser it is. Three of the four options are simply water: distilled water and drinking water differ only in a trace of dissolved matter, and neither has any property that would separate it from the other, so neither can be the intended answer. Sea water is the one option whose composition is materially different, and the salt dissolved in it raises the speed. The standard figures show the size of the effect: in salt water free of air bubbles and suspended sediment sound travels at about 1,500 metres per second, against about 1,481 metres per second in fresh water at 20 degrees Celsius, with each part per thousand of salinity adding roughly one metre per second. Methanol, the only non-aqueous option, is an alcohol and far more compressible than water, so sound moves through it distinctly more slowly.

• (a)Distilled water — Distilled water is water with its dissolved matter removed, so it is the low end of the water group, not the high end. Adding solutes is what raises the speed here, and distillation takes them away.
• (c)Drinking water — Drinking water carries only small quantities of dissolved salts, nothing like the salinity of the sea. Its speed of sound is effectively that of fresh water, which is why it cannot be told apart from distilled water in an item of this kind.
• (d)Methanol — Methanol is an organic liquid and much easier to compress than water. Since the speed of sound rises with the stiffness of the medium, an alcohol carries sound more slowly than water does, and it is the slowest of the four options rather than the fastest.

Sound is a mechanical wave and needs a material medium to travel through. Its speed in that medium is set by two competing properties: the elasticity or stiffness, which restores each layer after it is disturbed and speeds the wave up, and the density or inertia, which resists the disturbance and slows it down. Stiffness varies far more between materials than density does, which is why sound is fastest in solids, slower in liquids and slowest in gases.

This item hides a shortcut. Whenever three options are variants of one substance and the fourth is a different substance, the setter is usually asking you to separate the three rather than to compare across the divide — and here distilled water and drinking water are the same thing for this purpose, so the answer must be the third water, the salty one. The physics that backs it is the salinity term in the standard sea-water formula, in which the speed rises with salinity, with temperature, and with depth through pressure. This is also why an accurate sonar or echo-sounding calculation needs the local salinity and temperature profile, not a single textbook number.

• Sound travels at about 1,500 m/s in salt water free of bubbles and sediment, against about 1,481 m/s in fresh water at 20 degrees Celsius.
• In sea water each additional part per thousand of salinity raises the speed of sound by roughly 1 m/s.
• The speed of sound rises with the stiffness of a medium and falls as its density rises.
• The order across states of matter is solids fastest, then liquids, then gases — for example about 5,900 m/s in stainless steel against about 343 m/s in air at 20 degrees Celsius.
• In air the speed rises with temperature, which is why the same Mach number means different ground speeds at different altitudes.

• Assuming a denser medium always carries sound faster; density alone slows it, and it is the stiffness that makes solids fast.
• Expecting distilled water and drinking water to differ measurably; for this purpose they do not.
• Thinking a higher-pitched sound travels faster; in a given medium all frequencies travel at the same speed.

Almost always as an in-which-medium-is-it-maximum item, sometimes across states of matter and sometimes, as here, within one state; the temperature dependence in air is the other recurring hook.

• practice — not a real PYQ

  The speed of sound in a medium depends on which of the following properties of that medium?

  • (a)Its elastic property only
  • (b)Its inertial property only
  • (c)Both its elastic and its inertial properties
  • (d)Neither of the two

  Answer(c) Both its elastic and its inertial properties — stiffness speeds the wave up and density slows it down, and the speed depends on the balance of the two.

• practice — not a real PYQ

  As the temperature of air rises, the speed of sound in it

  • (a)increases
  • (b)decreases
  • (c)remains unchanged
  • (d)first decreases and then increases

  Answer(a) increases — warmer air molecules move faster, and the speed of sound in air rises with temperature, which is roughly 343 m/s at 20 degrees Celsius.