HS-PS1-6: Shifting Equilibrium to Make More Product
Also called NGSS HS-PS1-6 · Topic: Chemical Reactions · Physical Science
The Official Wording
Refine the design of a chemical system by specifying a change in conditions that would produce increased amounts of products at equilibrium.
Clarification statement: Emphasis is on the application of Le Chatelier’s Principle and on refining designs of chemical reaction systems, including descriptions of the connection between changes made at the macroscopic level and what happens at the molecular level. Examples of designs could include different ways to increase product formation including adding reactants or removing products.
Assessment boundary: Assessment is limited to specifying the change in only one variable at a time. Assessment does not include calculating equilibrium constants and concentrations.
NGSS Lead States, 2013 · Read it on nextgenscience.org · checked October 15, 2026
What HS-PS1-6 Means in Plain English
High school students act as chemical engineers. Given a reaction that runs both forward and backward, they choose one change in conditions, such as adding a reactant or removing a product, that pushes the system to make more product, and explain the change at the particle level.
Students change a single variable per trial and do not calculate equilibrium constants. The work builds on collision reasoning from HS-PS1-5 and on engineering design practices (HS-ETS1-3), and it underpins industrial processes like making ammonia.
HS-PS1-6 I Can Statements
- I can describe equilibrium as forward and reverse reactions at equal rates.
- I can choose one change that shifts a reaction toward products.
- I can explain the shift in terms of particle collisions.
HS-PS1-6 Worked Example
A design challenge in a well plate, where a deeper red color shows more product has formed.
Which one change makes the most red iron thiocyanate product at equilibrium?
Set It Up: A pale equilibrium mixture of iron(III) nitrate and potassium thiocyanate, a 12-well plate, droppers, and small amounts of each reactant.
- Fill four wells with the same pale orange-red mixture as controls and tests.Equal color in every well
- Add two drops of iron(III) nitrate to one well.Turns deep crimson
- Add two drops of potassium thiocyanate to another well.Also turns deep red
- Add a pinch of a salt that ties up iron ions to a third well.Color fades to pale yellow
- Rank the changes and pick the design that maximizes product.Adding a reactant wins
Adding either reactant deepened the red color, while removing iron ions faded it, so adding reactant raises the product yield.
Why it works: Extra reactant ions meet more frequently, speeding the forward reaction until a new balance holds with more product, which is Le Chatelier's principle.
Common Mistakes with HS-PS1-6
Thinking equilibrium means equal amounts
“At equilibrium there is half reactant and half product.”
Try this: Equilibrium means the forward and reverse rates match. The amounts can be very unequal and still hold steady.
Thinking the reaction stops at equilibrium
Saying nothing is happening once the color stops changing
Try this: Both reactions keep running at the same rate. The color is steady because product forms exactly as fast as it breaks apart.
Changing two things in one design
Adding reactant and heating the same well
Try this: Change one condition per trial so the result shows which change caused the shift. Combine changes only after testing each alone.
Where HS-PS1-6 Fits
Practice HS-PS1-6
Quick Check
- A product is removed as it forms. What happens to the equilibrium?
- Do the forward and reverse reactions stop at equilibrium?
- Why test only one change at a time?