Which one of the following, on adding to water, will not scatter a beam of light ?

Correct — A, (a) Copper sulphate. Scattering a beam of light — the Tyndall effect — needs dispersed particles large enough to get in the light's way. Colloidal particles, roughly 1 to 1000 nanometres across, do it; the still larger particles of a suspension do it too. Copper sulphate does neither, because it does not stay as particles at all: it is an ionic salt that dissolves to give a TRUE SOLUTION, splitting into hydrated Cu2+ and sulphate ions that are less than a nanometre across, far smaller than the wavelength of visible light. A true solution is homogeneous and transparent, and a light beam passed through it goes straight on without its path becoming visible from the side. Copper sulphate solution is a strong blue, which is what makes this option feel wrong at first glance, but colour and scattering are different things — the blue comes from the hydrated copper ion ABSORBING part of the spectrum, not from anything scattering light. The stem's emphasised 'not' is asking for the one substance that forms a true solution, and only this one does.

Why the others are wrong

• (b)Chalk powder — Chalk powder in water forms a SUSPENSION, so it does scatter a beam and cannot be the answer. Calcium carbonate is practically insoluble in water, so the powder simply disperses as solid particles well above a micrometre in size — big enough to make the mixture cloudy and opaque, big enough to be caught on ordinary filter paper, and big enough to settle to the bottom of the beaker if the mixture is left standing. A suspension is the coarsest of the three kinds of mixture and the easiest to identify: it is visibly heterogeneous, it is unstable, and it blocks and scatters light strongly.
• (c)Milk — Milk is the standard textbook COLLOID and one of the standard demonstrations of the Tyndall effect, so it scatters light and is not the answer. It is an emulsion — tiny globules of liquid fat dispersed through an aqueous medium — with particle sizes in the colloidal range. That is also the reason milk is white and opaque rather than transparent: the fat and protein particles scatter light of all visible wavelengths more or less equally. A few drops of milk stirred into a beaker of water and lit from the side make the beam's path glow visibly, which is the classroom experiment this option comes from.
• (d)Ink — Ink is a colloidal sol — finely divided solid pigment or dye particles dispersed through a liquid — so it scatters a beam of light and is not the exception being asked for. It behaves like a colloid in every diagnostic respect: it passes through ordinary filter paper, because the particles are far smaller than the pores, but it does not pass through a membrane such as parchment, and it does not settle out on standing, because Brownian motion and the charge on the particles keep it dispersed. The presence of colour is not what decides the question — copper sulphate solution is coloured too — the particle SIZE is.

Mixtures are classified by the size of the dispersed particles, and almost every property follows from that one number. A TRUE SOLUTION has particles below about 1 nanometre — individual ions or molecules. It is homogeneous and transparent, it passes through filter paper and through a parchment membrane, its particles never settle, and it does NOT scatter light. Salt water, sugar solution and copper sulphate solution are examples. A COLLOID has particles between about 1 and 1000 nanometres. It looks homogeneous to the eye but is heterogeneous in fact, it passes through filter paper but not through parchment, its particles do not settle because Brownian motion keeps them suspended, and it DOES scatter light — the Tyndall effect, named after John Tyndall. Milk, ink, fog, smoke, soap solution, blood and jelly are colloids, and the type is named for the phases involved: an emulsion is liquid in liquid, a sol is solid in liquid, an aerosol is solid or liquid in gas, a foam is gas in liquid and a gel is liquid in solid. A SUSPENSION has particles above about 1000 nanometres. It is visibly heterogeneous and often opaque, it is caught by filter paper, it settles on standing, and it scatters and blocks light. Muddy water and chalk in water are the standard examples. The Tyndall effect is therefore a practical test that separates a true solution from the other two, and it is why a beam of sunlight becomes visible when it enters a dark room through a small opening carrying dust, and why a vehicle's headlight beams stand out in fog.

Key facts

• The Tyndall effect is the scattering of a beam of light by particles in a colloid or a suspension, making the beam's path visible from the side.
• It is named after John Tyndall.
• True solutions do not show the Tyndall effect; their particles are smaller than 1 nanometre, far below the wavelength of visible light.
• Colloidal particles measure roughly 1 to 1000 nanometres; suspension particles are larger still.
• Copper sulphate dissolves in water to give a true solution of hydrated Cu2+ and sulphate ions; its blue colour comes from absorption, not scattering.
• Milk is an emulsion of fat globules in water and is the standard colloid used to demonstrate the Tyndall effect.
• Ink is a colloidal sol of pigment particles in a liquid.
• Chalk powder in water is a suspension: it makes the water cloudy, is retained by filter paper and settles on standing.
• A colloid passes through filter paper but not through a parchment membrane; a suspension is retained by filter paper.
• Brownian motion — the random movement of colloidal particles struck by the molecules of the medium — is what keeps a colloid from settling.

Common traps

• Missing the emphasised 'not' and choosing the substance that scatters light most strongly.
• Assuming a coloured liquid must be scattering light. Colour comes from absorption; scattering depends on particle size.
• Treating milk as a solution because it looks uniform. It is a colloid, and its whiteness is scattering in action.
• Calling ink a solution because it flows freely and does not settle. Not settling is a property of colloids as well.
• Forgetting that a suspension scatters light too. The Tyndall effect separates true solutions from the rest, not colloids from suspensions.
• Confusing the Tyndall effect with the scattering that colours the sky, which is scattering by air molecules rather than by colloidal particles.

• practice — not a real PYQ

  Which one of the following mixtures will NOT show the Tyndall effect ?

  • (a)Milk in water
  • (b)Common salt dissolved in water
  • (c)Muddy river water
  • (d)Smoke in air

  Answer(b) Common salt dissolved in water

• practice — not a real PYQ

  Milk is an example of which type of colloidal system ?

  • (a)Sol
  • (b)Gel
  • (c)Emulsion
  • (d)Aerosol

  Answer(c) Emulsion