Latex

Latex is a colloid in which tiny rubber particles are dispersed in water. It is the milky liquid tapped from the rubber tree, and it is the raw material from which natural rubber is obtained. Malay: lateks · Chinese: 乳胶.

In latex, each rubber particle is a droplet of poly(isoprene) surrounded by a thin protein layer that carries a negative charge. Because the particles all carry the same charge, they repel one another and stay evenly spread through the water instead of clumping together, which is what keeps the latex liquid and stable. This state does not last for ever: if the charge is removed, the particles join up and the rubber separates out as a solid. Understanding latex as a charged colloid is the key to explaining both why it flows and how it can be made to set.

Example. When latex is collected in a cup at the rubber tree, it is a smooth white liquid. Left in the open air, bacteria produce acid that slowly makes it coagulate, so a preservative such as ammonia is added to keep the latex liquid until it reaches the factory.

Confusion to avoid. Do not describe latex as a solution. In a solution the solute dissolves to give single molecules or ions, but in latex the rubber exists as much larger suspended particles, which is why it is classed as a colloid rather than a true solution. Nor is latex the same as solid rubber, latex is the liquid form before coagulation.

A common exam task is to explain, in terms of charge, why adding acid makes latex coagulate: the acid supplies hydrogen ions that neutralise the negative charge on the particles, removing the repulsion so the particles can join. Being able to give this charge-based reason, rather than just naming the acid, is what separates a full-mark answer from a partial one.

Latex is the starting point of the natural-rubber story in this chapter, leading directly into coagulation and then vulcanisation, all of which are tested together in SPM Chemistry.