The following are common errors students make when discussing supply and demand. What is the mistake in each?

a. At equilibrium, demand equals supply.
b. The quantity of demand is greater than the quantity of supply.
c. They move along the line from both ends to an equilibrium in the middle.
d. The increase in demand causes an increase in supply.

a. Demand never equals supply because the entire demand curve never equals the entire supply curve. At the equilibrium price, quantity demanded equals quantity supplied.
b. Quantity of demand is a meaningless term in economics. The proper term is "quantity demanded."
c. This phrase is apparently an attempt to graphically describe supply and demand. However, it is difficult to understand and does not describe the process whereby a market reaches an equilibrium.
d. An increase in demand would lead to an increase in quantity supplied, but not an increase in supply. There would be a movement along the supply curve to a new quantity supplied at a higher price. Generally only one curve, either supply or demand, will shift at one time, causing equilibrium to move along the other curve.