IB DP Economics: Demand, Supply and Market Equilibrium (Unit 2.1-2.3)

This guide covers sub-topics 2.1 to 2.3 of Unit 2 Microeconomics — Demand, Supply, and Competitive Market Equilibrium — for IB Diploma Programme Economics, first assessment 2022, at SL-level content. For the full syllabus, including HL-only extensions, see the IB DP Economics syllabus guide.

Where this fits in the syllabus

These three sub-topics open Unit 2 Microeconomics (35 SL / 70 HL teaching hours) – not the largest unit in the syllabus: at Standard Level, Unit 4 (the global economy, 45 hours) is larger, and at Higher Level, Unit 3 (macroeconomics, 75 hours) is larger – and build directly on the scarcity-and-opportunity-cost toolkit introduced in Unit 1. They also set up the diagrams that Unit 2’s later sub-topics on elasticity (2.5, 2.6) build directly on top of: elasticity asks how much quantity demanded or supplied responds to a change in price or another determinant, a question that only makes sense once demand, supply and equilibrium — and specifically the distinction between a shift and a movement — are already secure. Weakness in 2.1–2.3 is one of the most common reasons students later struggle with elasticity calculations and diagrams.

Syllabus coverage

IB DP ECONOMICS — UNIT 2: MICROECONOMICS, SUB-TOPICS 2.1–2.3

• 2.1 Demand — the law of demand (as price rises, quantity demanded falls, ceteris paribus); a change in price causes a movement along the demand curve, while a change in any other determinant (income, tastes, price of substitutes/complements, population, expectations) causes the whole curve to shift
• 2.2 Supply — the law of supply (as price rises, quantity supplied rises, ceteris paribus); a change in price causes a movement along the supply curve, while a change in a non-price determinant (costs of production, technology, number of firms, taxes/subsidies, expectations, and the prices of related goods in joint or competitive supply) causes a shift
• 2.3 Competitive market equilibrium — equilibrium is where quantity demanded equals quantity supplied; a shortage (price below equilibrium) pushes price up, a surplus (price above equilibrium) pushes price down, and a competitive market self-corrects toward equilibrium absent intervention; the price mechanism performs three linked functions — signalling, incentive, and rationing; consumer surplus, producer surplus and social surplus, and allocative efficiency at the competitive equilibrium (where marginal benefit equals marginal cost)

How to approach it

The single most exam-rewarded precision in this content is distinguishing “change in demand” or “change in supply” (a shift of the whole curve) from “change in quantity demanded” or “change in quantity supplied” (a movement along the curve) — examiners specifically reward candidates who use this precise vocabulary rather than the vaguer phrase “demand changes” for both situations. When a determinant other than price changes, the whole curve shifts and a new equilibrium is established where the new curve intersects the other, unchanged curve: a rightward shift of demand with supply unchanged raises both equilibrium price and quantity; a rightward shift of supply with demand unchanged lowers equilibrium price but raises equilibrium quantity. If both curves shift simultaneously, the direction of the change in price or quantity can become ambiguous without more specific information about the relative size of each shift — a common higher-mark question tests exactly this ambiguity, so always check whether a described scenario affects demand, supply, or both before analysing it.

Worked example: the price mechanism’s three functions

A question asks how the price mechanism allocates a scarce good without central planning.

Signalling:   a rise in the price of a good signals to producers and
              consumers that something has changed in the market --
              for example, rising demand or falling supply

Incentive:    the higher price incentivises producers to supply more
              (higher revenue justifies the rising marginal cost of
              extra units) and incentivises consumers to demand less

Rationing:    price rations the now-scarcer good to those most
              willing and able to pay, without any central authority
              directly allocating output

Conclusion:   these three functions work together automatically in a
              competitive market -- no single participant plans them
              -- which is precisely the feature evaluative questions
              often ask you to assess against alternative allocation
              mechanisms, such as government-planned allocation

Naming all three functions explicitly, rather than describing price changes only in general terms, is what a strong evaluative answer on the price mechanism does.

Consumer surplus, producer surplus and allocative efficiency

Beyond the three functions above, the equilibrium diagram also shows how much value a competitive market creates. Consumer surplus is the difference between what consumers are willing and able to pay for each unit and what they actually pay – graphically, the area below the demand curve and above the market price. Producer surplus is the difference between what producers actually receive and the minimum price they would have accepted – the area above the supply curve and below the market price. Social (or community) surplus is the sum of the two, and it is at its maximum at the competitive equilibrium, where the demand and supply curves intersect. This is because, at that point, marginal benefit equals marginal cost: the value society places on the last unit consumed exactly equals the cost of producing it, so resources are allocated to their highest-valued use. Producing more or less than the equilibrium quantity destroys some of this total surplus – this is what is meant by allocative efficiency, and it is the key link between the demand-and-supply diagram and later evaluative questions about whether a market outcome is efficient.

Common mistakes

Confusing a shift of a curve with a movement along it, or using the vague phrase “demand changes” for both situations rather than the precise terminology examiners reward. Drawing demand and supply curves the wrong way round (demand slopes down, supply slopes up). Forgetting that every law of demand or supply statement assumes ceteris paribus — other factors held constant. Not explaining the direction of adjustment (shortage → price rises; surplus → price falls) when asked to analyse a market change. Answering an ambiguous “both curves shift” scenario as if only one curve moved.

Reading the equilibrium diagram

A standard demand-and-supply diagram plots price on the vertical axis and quantity on the horizontal axis, with the downward-sloping demand curve and upward-sloping supply curve intersecting at equilibrium price and quantity. Practise reading off both a shortage and a surplus directly from this diagram: at any price below equilibrium, the supply curve sits to the left of the demand curve at that price level, meaning quantity demanded exceeds quantity supplied – a shortage; at any price above equilibrium, the reverse holds, and quantity supplied exceeds quantity demanded – a surplus. Being able to identify these regions on a diagram, not just recite the definitions, is what exam-style short-answer questions on this content typically test.

Quick revision checklist

• Practise stating precisely whether a described change causes a shift or a movement, before analysing its effect.
• Know the direction of equilibrium change for a rightward shift of demand alone, supply alone, and both together.
• Learn the three functions of the price mechanism (signalling, incentive, rationing) as a named set, ready for evaluative essays.
• Practise diagrams showing shortage and surplus adjustment toward equilibrium.
• Treat 2.1–2.3 as a prerequisite for the elasticity sub-topics later in Unit 2, not standalone content.

Official syllabus

International Baccalaureate Organization, Economics guide, Diploma Programme, first assessment 2022 — the same source already cited by the full syllabus guide. Verified 2026-09-06.