What is the law of demand?

The law of demand states that, other things being equal, as the price of a good rises the quantity demanded falls, and as the price falls the quantity demanded rises. Price and quantity demanded move in opposite directions. The demand curve therefore slopes downward from left to right. The two reasons are the income effect (a lower price raises real purchasing power, so buyers can afford more) and the substitution effect (a lower price makes the good cheaper relative to substitutes, so buyers switch toward it).

What is the law of supply?

The law of supply states that, other things being equal, as the price of a good rises the quantity supplied rises, and as the price falls the quantity supplied falls. Price and quantity supplied move in the same direction, so the supply curve slopes upward from left to right. A higher price makes production more profitable, encouraging existing firms to supply more and new firms to enter, and it helps cover the rising marginal cost of producing extra units.

What factors shift the demand curve?

The demand curve shifts when something other than the good's own price changes. The main factors are income (for a normal good, higher income raises demand), the prices of substitutes and complements, consumer tastes and preferences, the size and structure of the population, expectations of future prices, and, in some cases, government policy. An increase in demand shifts the curve to the right, while a decrease shifts it to the left.

What factors shift the supply curve?

The supply curve shifts when something other than the good's own price changes. The main factors are the costs of production (such as wages and raw material prices), the state of technology, indirect taxes and subsidies, the number of firms in the market, the prices of other goods the firm could produce, and natural factors such as weather for agricultural goods. An increase in supply shifts the curve to the right, while a decrease shifts it to the left.

SingaporeEconomics

Demand and supply for Singapore O-Level Economics (2286): the law of demand and the law of supply, the factors that shift each curve, and the vital distinction between a movement along a curve and a shift of the whole curve

Q: What is the difference between a movement along a curve and a shift of the curve?

A change in the good's own price causes a movement along the demand or supply curve (a change in quantity demanded or quantity supplied). A change in any other determinant, such as income, the prices of related goods, costs of production or technology, shifts the whole curve to a new position (a change in demand or supply). This distinction is heavily tested: get the cause right (own price versus other factor) and you get the correct movement-or-shift answer.

A module overview for Singapore O-Level Economics (SEAB 2286) on demand and supply: the law of demand and why the demand curve slopes down, the law of supply and why the supply curve slopes up, the determinants that shift each curve, and the much-tested distinction between a movement along a curve and a shift of the whole curve.

Generated by Claude Opus 4.87 min readSEAB-2286Updated 2026-06-13

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

Why this module matters
The two laws
What shifts each curve
The distinction that wins marks
A worked shift analysis
How this module is examined
Check your knowledge

Why this module matters

Demand and supply is the engine of price theory and the most frequently used toolkit in the whole O-Level Economics course. Almost every later analysis, of market equilibrium, elasticity, market failure, price controls, indirect taxes and even exchange rates, is an application of demand and supply curves. If you can draw both curves accurately, explain why they slope as they do, and decide correctly whether an event causes a movement or a shift, you have the foundation for most of Paper 2.

This overview ties together the dot points below, each with its own worked answer and practice. The order matters: learn each curve, then its shift factors, then the distinction between movements and shifts, which is where most marks are won or lost.

The two laws

Start with the buyers. Demand and the law of demand defines effective demand (willing AND able to buy) and explains why the demand curve slopes downward, through the income effect and the substitution effect.

Then turn to the sellers. Supply and the law of supply defines supply and explains why the supply curve slopes upward: a higher price makes production more profitable and helps cover the rising cost of producing extra units.

What shifts each curve

A demand or supply curve is drawn holding everything except the good's own price constant. When one of those other things changes, the whole curve moves. The factors affecting demand include income, the prices of substitutes and complements, tastes, population and expectations. The factors affecting supply include costs of production, technology, indirect taxes and subsidies, the number of firms and weather.

The distinction that wins marks

The single most tested idea in this module is the difference between a movement along a curve and a shift of the whole curve. The rule is simple but must be applied carefully: the good's own price causes a movement along the curve; any other factor shifts the whole curve. Getting this wrong reverses the entire analysis, so it deserves deliberate practice.

A worked shift analysis

Deciding movement or shift, then tracing the effect

A sudden rise in the world price of cocoa raises the cost of making chocolate bars. Trace what happens in the market for chocolate bars.

step 1: identify what has changed

The trigger is a rise in a cost of production (cocoa), NOT a change in the price of chocolate bars themselves. So this is a supply-side determinant, which means the supply curve will shift, not move along.

step 2: decide the direction of the shift

Higher production costs make supplying chocolate bars less profitable at every price, so firms supply less at each price. The supply curve shifts to the LEFT (a decrease in supply), from $S_1$ to $S_2$ .

step 3: find the new equilibrium

With demand unchanged, the leftward supply shift creates a shortage at the old price, so price is bid up. The market moves up along the unchanged demand curve to a new equilibrium with a HIGHER price and a LOWER quantity.

S_1 \rightarrow S_2 \;\Rightarrow\; P \uparrow,\; Q \downarrow

step 4: separate the shift from the movement

Note the two distinct ideas: the cocoa cost shifted the supply curve (a change in supply), and the resulting higher price caused a movement ALONG the demand curve (a change in quantity demanded). Demand itself did not change; only the quantity demanded fell as price rose. Confusing these two would be the classic error.

How this module is examined

Define with willing and able. Use the precise wording for both demand and supply to bank definition marks.
Name the determinant first. Before drawing, decide whether the trigger is the good's own price (movement) or another factor (shift).
Label both curves and the new equilibrium. A clear before-and-after diagram, with $P$ and $Q$ axes and the shift arrow, carries the analysis.

Check your knowledge

Attempt these under timed conditions, then check the model solutions.

State the law of demand and the law of supply. (4 marks)
Explain why the demand curve slopes downward. (3 marks)
Give three factors that would shift the demand curve for coffee to the right. (3 marks)
Give three factors that would shift the supply curve for rice to the left. (3 marks)
A fall in the price of tea raises the amount of tea people buy. Is this a movement or a shift, and why? (3 marks)

Solutions

Q1: The law of demand states that, other things being equal, as price falls the quantity demanded rises and as price rises the quantity demanded falls. The law of supply states that, other things being equal, as price rises the quantity supplied rises and as price falls the quantity supplied falls.
Q2: The demand curve slopes downward because a fall in price raises the quantity demanded for two reasons. The income effect: a lower price raises real purchasing power, so buyers can afford more. The substitution effect: a lower price makes the good cheaper relative to its substitutes, so buyers switch toward it. Both push quantity demanded up as price falls.
Q3: Any three of: a rise in income (coffee being a normal good); a rise in the price of a substitute such as tea; a fall in the price of a complement such as sugar or milk; a change in tastes in favour of coffee; an increase in the population of coffee drinkers; or an expectation that coffee prices will rise soon.
Q4: Any three of: a rise in the cost of production such as fertiliser or fuel; an indirect tax placed on rice; a fall in the number of rice farmers; poor weather or a failed harvest; or a rise in the price of an alternative crop, drawing land away from rice.
Q5: This is a movement along the demand curve, not a shift. The cause is a change in tea's OWN price, which moves us down along the existing demand curve to a larger quantity demanded. Only a change in a factor other than tea's own price (such as income or the price of coffee) would shift the whole demand curve.

Sources & how we know this

Singapore-Cambridge GCE O-Level Economics (Syllabus 2286) — Singapore Examinations and Assessment Board (2026)