Which one of the following ingredients does NOT constitute paint?

Correct — C, Thiourea (accelerator). An accelerator is a rubber-industry additive: it speeds up vulcanisation, the cross-linking of rubber with sulphur. Paint has no vulcanisation step and so has no use for an accelerator. The other three are genuine paint components — a thinner to adjust flow, an anti-skinning agent to stop a skin forming on stored paint, and a pigment to supply colour and opacity.

• (a)Turpentine oil (thinner) — Turpentine is the classic thinner or solvent of oil-based paint, cutting the viscosity so the paint spreads and then evaporating as it dries.
• (b)Poly-hydroxy phenols (anti-skinning agent) — Polyhydroxy phenols are used exactly as the bracket says. They interrupt the oxidation that would otherwise harden the surface of paint in a part-used tin, which is what a skin is.
• (d)Titanium dioxide (pigment) — Titanium dioxide is the most widely used white pigment in the world, valued for its very high refractive index and hiding power.

A paint is built from four functional parts. The binder or vehicle is the film-forming resin that sticks the coating to the surface; the pigment supplies colour and opacity; the solvent or thinner controls flow and evaporates as the paint dries; and a small set of additives — driers, anti-skinning agents, biocides, anti-settling agents — tunes the behaviour of the rest.

The examiner has helpfully printed each ingredient's role in brackets, so the item turns entirely on whether 'accelerator' is a paint term. It is not; it belongs to rubber technology, alongside vulcanisation and antioxidants. Thiourea does appear in industry, in photographic fixing and in metal-surface treatment, but a can of paint is not one of its homes. The bracket is not there to help, it is there to make an unfamiliar word look plausible.

• Paint is made of a binder, a pigment, a solvent or thinner, and additives.
• Titanium dioxide is the dominant white pigment because of its exceptionally high refractive index.
• Anti-skinning agents block the surface oxidation that forms a skin on stored oil paint; polyhydroxy phenols do this job.
• An accelerator is a rubber-industry additive that speeds up vulcanisation, the sulphur cross-linking of rubber.
• Turpentine, obtained from pine resin, is the traditional thinner of oil-based paints.

• Trusting the bracketed job title instead of checking whether the job exists in the product asked about.
• Assuming an unfamiliar chemical name must be the odd one out; here the unfamiliar names in options (b) and (d) are both genuine paint ingredients.
• Mixing up a thinner, which evaporates, with a binder, which stays behind as the film.

An odd-one-out on industrial composition, made harder by printing a plausible functional label beside each name.

• practice — not a real PYQ

  The white pigment most widely used in modern paints is

  • (a)zinc sulphide
  • (b)titanium dioxide
  • (c)calcium carbonate
  • (d)barium sulphate

  Answer(b) titanium dioxide — its very high refractive index gives the best opacity per unit of pigment.

• practice — not a real PYQ

  In the rubber industry, the process of heating raw rubber with sulphur to improve its strength and elasticity is called

  • (a)polymerisation
  • (b)vulcanisation
  • (c)galvanisation
  • (d)saponification

  Answer(b) vulcanisation — sulphur cross-links the polymer chains, and accelerators such as thiourea speed the process up.