Ions & pH
This ion is found in greater concentration in every acid
This substance is a common acid used in labs to lower pH and add H₃O⁺ ions.
HCl (hydrochloric acid)
Bromothymol blue turns this color in an acidic solution.
Yellow
This is the word for changing an acid into a base — or a base into an acid — by adding the opposite.
Neutralize / neutralization
A strong scientific claim must include three things: a conclusion, this type of support, and ion-level reasoning.
Evidence (observable data — such as an indicator color change)
This ion is found in greater concentration in every base
OH- (Hydroxide ion)
This substance is a common base used in labs to raise pH and add OH⁻ ions.
NaOH (sodium hydroxide)
Red cabbage indicator turns this color in a basic solution.
Green or green/yellow
When H₃O⁺ and OH⁻ react during neutralization, they produce this molecule.
Water (H₂O)
A student writes: 'The solution is an acid.' What is missing from this claim?
Evidence (what color the indicator showed) and ion reasoning (more H₃O⁺ than OH⁻).
A solution has a pH of 3. How does its H₃O⁺ concentration compare to a solution with pH 7?
pH 3 has a much higher concentration of H₃O⁺ ions — lower pH = more H₃O⁺.
A student says tap water is neutral. What does that mean about its H₃O⁺ and OH⁻ ions?
Neutral means H₃O⁺ and OH⁻ are balanced
A student tests a solution with BTB and gets a green color. What does this indicate about the ion balance?
The solution is neutral — H₃O⁺ ≈ OH⁻, pH ≈ 7. Ions are balanced.
A student wants to neutralize a basic solution. Should they add HCl or NaOH? Why?
HCl — it adds H₃O⁺ ions that react with and cancel out the excess OH⁻ ions.
Rewrite this claim to make it stronger: 'The mystery solution is a base.' Add evidence and ions.
Example: 'The solution is a base because BTB turned blue, indicating more OH⁻ ions than H₃O⁺ ions.'
At this pH, a solution has exactly equal H₃O⁺ and OH⁻ ions.
pH 7 (neutral)
Name one physical property that many acids share, and one that many bases share.
Acids: sour taste. Bases: slippery / soapy feel. (Never taste or touch unknowns!)
BTB turns blue. Red cabbage turns green/yellow. Do these results agree or disagree? Explain.
They agree — both indicate a basic solution with more OH⁻ than H₃O⁺.
A solution starts at pH 4. NaOH is added until BTB turns blue. Describe what happened to the ions in two steps.
Step 1: OH⁻ from NaOH reacted with excess H₃O⁺, moving toward neutral.
Step 2: additional OH⁻ exceeded H₃O⁺, making the solution basic.
A student's evidence is: 'The solution smelled like vinegar.' Why is this weak scientific evidence?
Smell is a sensory observation — it is not measurable or repeatable by others, and provides no ion-level data.
A solution has a pH of 12. Describe its ion balance and what type of substance it is.
It has far more OH⁻ ions than H₃O⁺ ions — it is a strongly basic solution.
A solution turns red cabbage indicator red/pink. A student adds NaOH. What happens to the H₃O⁺ concentration, and why?
H₃O⁺ decreases — the OH⁻ ions from NaOH react with and cancel out H₃O⁺ ions.
A scientist uses red cabbage indicator and gets a purple result. She then adds HCl drop by drop. Describe the expected color change and why.
The color shifts from purple toward red/pink as HCl adds H₃O⁺ ions, increasing acidity.
A student adds too much NaOH to an acidic solution. What happens to the pH and what would BTB show? Explain why.
pH rises above 7, BTB turns blue — excess OH⁻ now outnumbers H₃O⁺, making the solution basic instead of neutral.
Two students test the same solution. Student A gets red/pink (red cabbage). Student B gets yellow (BTB). Do these results agree? What claim do they support together?
Yes — both indicate an acid. Combined claim: the solution is an acid with more H₃O⁺ than OH⁻ ions, confirmed by two independent indicators.