Q.If the price of a substitute(Y) of good X increases, what impact does it have on the equilibrium price and quantity of good X?

Let’s start with something you do every day. You walk into a shop to buy a cold drink. You see a bottle of Coke for ₹40 and a bottle of Pepsi for ₹40. You pick one — doesn’t matter which, because they both do the same job: quench your thirst with a fizzy cola. If the shopkeeper tells you Coke is now ₹50, you’ll probably put it down and pick up the Pepsi instead. That instinct — swapping one thing for another when the price changes — is the entire idea behind substitute goods.

The precise meaning

Two goods are substitute goods (or substitutes) when an increase in the price of one leads to an increase in the demand for the other. In other words, they can be used in place of each other. The consumer sees them as roughly similar in purpose, so when one becomes more expensive, they switch to the cheaper alternative.

The NCERT Class-11 Microeconomics textbook (Chapter 3: Demand) puts it clearly: “Substitute goods are those goods which can be used in place of each other for satisfaction of a particular want.” Tea and coffee, butter and margarine, petrol and diesel, a Maruti Suzuki and a Hyundai i10 — all are examples.

Why it matters: the cross-price elasticity

This is where the concept becomes a tool, not just a definition. Economists measure the strength of the substitution relationship using cross-price elasticity of demand. It answers the question: By how much does the demand for good Y change when the price of good X changes?

• $E_{XY}$ = cross-price elasticity of demand
• The numerator is the percentage change in quantity demanded of good Y
• The denominator is the percentage change in price of good X

For substitute goods, $E_{XY} > 0$. A positive sign tells you that when the price of X goes up, the demand for Y goes up (and vice versa). The larger the number, the closer the substitutes — if $E_{XY}$ is very high, consumers treat the two goods as almost identical.

A diagram in words

Imagine a standard demand curve for good Y (say, Pepsi). It slopes downward — price on the vertical axis, quantity on the horizontal. Now, the price of Coke (good X) rises. What happens to the demand curve for Pepsi? It shifts to the right — at every price, consumers now want more Pepsi than before. That rightward shift is the visual signature of substitute goods. If the price of Coke had fallen, the Pepsi demand curve would shift left.

The opposite: complementary goods …