What is the shape of an indifference curve when one of the goods is a free natural good?

When one of the goods is a free natural good, the indifference curve typically becomes a straight line parallel to the axis representing that good. This occurs because the consumer can obtain unlimited quantities of the free good without sacrificing any quantity of the other good. As a result, the consumer's utility remains constant regardless of the amount of the free good consumed, leading to a linear relationship in the indifference curve.