Pure competition is also called perfect competition. It happens to be some kind of monopolistic competition where there are many firms within an industry. What usually happens is that the different firms produce closely related goods. As a result the prices of the goods are almost the same. In case one of the firms decides to increase their prices there is always the tendency that people will substitute the commodity for others produced by other firms. As a result the prices are almost similar and only the tastes and preferences and or the role of advertisement will make people make their choice of good.
Pure competition in most instances, do not appear in an economic perspective. This is due to the fact that we sometimes as economists overlook the role played by tastes and preferences. In reality we have seen instances where even as the price of a good that has close substitutes increases the demand of the good is never affected. Regardless of this fact there are instances where the demand of the good, goes down in cases of an increase in its price.