Reading an equilibrium concentration-time graph

A recurring graph-interpretation task, distinct from predicting a shift by name: explaining WHAT a graph shows about equilibrium, and how the system responds when it is disturbed mid-graph.

What to look for on the graph

FLAT/CONSTANT sections: all three concentration lines running horizontally (unchanging) indicate the system IS at equilibrium — forward and reverse rates are equal, so net concentrations do not change, even though both reactions continue.
A SUDDEN JUMP in one species' concentration (e.g. more reactant added) marks the point of disturbance — read the graph for exactly which species changed and by how much.
AFTER the disturbance, concentrations move again before flattening out at NEW, different equilibrium values — this second flat section is the NEW equilibrium position, reached via Le Chatelier's response to the disturbance.
The DIRECTION each line moves after the disturbance tells you which way the equilibrium shifted — reactant lines falling and product lines rising means the system shifted toward products (forward), and vice versa.

Worked example Explaining a graph after adding extra reactant mid-experiment A concentration-time graph for CO(g) + 2H₂(g) ⇌ CH₃OH(g) is flat, then extra CO is added, causing the CO line to jump up. Explain how the system responds to restore equilibrium, referring to the graph. BEFORE THE ADDITION: all three lines are flat, showing the forward and reverse reaction rates were equal — the system was at equilibrium. THE DISTURBANCE: adding CO increases [CO], shown as a sudden upward jump in the CO line on the graph. THE RESPONSE: Le Chatelier's principle predicts the system shifts to PARTIALLY OPPOSE the increase — the forward reaction rate temporarily exceeds the reverse rate, consuming some of the extra CO (and H₂) to produce more CH₃OH. THE NEW EQUILIBRIUM: the graph shows CO and H₂ concentrations falling somewhat from their new peak, and CH₃OH rising, before all three lines flatten again at a NEW equilibrium — with CO higher than before (since not ALL of the addition is consumed) but CH₃OH also higher than before. ✓ Reference the SPECIFIC graph features (flat sections, the jump, the direction of movement afterward) throughout — a purely theoretical Le Chatelier answer with no reference to what the graph actually shows misses the "refer to the graph" instruction. ✕

Explaining the shift without referencing what the graph actually shows

When a question explicitly says "refer to the graph in your answer", a theoretically correct Le Chatelier explanation that never mentions the flat sections, the jump, or the direction lines move afterward has not met the instruction — always describe the specific visual features supporting your explanation.

Graph feature	What it shows
All lines flat	System at equilibrium — forward rate = reverse rate
A sudden jump in one line	The point of external disturbance
Lines moving after the jump, then flattening again	The system shifting to a NEW equilibrium

Practice for this standard covers the whole topic — flashcards and quiz.