The base in the burette has the same concentration, so the equivalence point stays at 25.0 mL. For weak acids the readout shows how much of the acid is ionized (A−) and how much is not (HA).
The pH scale shows how acidic or basic a water solution is, usually from 0 to 14. Pure water is neutral at pH 7. Lower numbers are acidic (lemon juice is about 2) and higher numbers are basic (bleach is about 12.6). Each step is a 10-fold change in hydrogen ions, so pH 3 has 10 times more than pH 4.
pH and pOH. pH = −log[H+] and pOH = −log[OH−], with concentrations in mol/L. Water ionizes a little: H2O ⇌ H+ + OH−, with Kw = [H+][OH−] = 1.0 × 10−14 at 25 °C, so pH + pOH = 14.00. Lemon juice at pH 2.2 has [H+] = 10−2.2 ≈ 6.3 × 10−3 mol/L, about 63,000 times more than pure water (10−7 mol/L).
Strong vs weak acids. A strong acid (HCl, HNO3) ionizes completely, so 0.10 M HCl has [H+] = 0.10 M and pH 1.00. A weak acid ionizes only partly: HA ⇌ H+ + A−, Ka = [H+][A−]/[HA]. For acetic acid Ka = 1.8 × 10−5 (pKa 4.74) (source: OpenStax Chemistry 2e, Appendix H). With an ICE table, Ka = x2/(C0 − x). For C0 = 0.10 M the quadratic gives x = 1.33 × 10−3 M, pH 2.88, and only 1.3 % of the molecules ionize. When C0/Ka > 100 the shortcut [H+] ≈ √(KaC0) is good enough. Vinegar (5 %, 0.833 M) gives pH 2.41, the 2.4 that OpenStax quotes.
Dilution. Diluting a strong acid 10× raises the pH by 1. Diluting a weak acid 10× raises it only by about 0.5, because the fraction that ionizes grows (α ≈ √(Ka/C0), Le Chatelier). Very dilute solutions approach pH 7 from one side: the simulator solves the full charge balance [H+] + [Na+] = [OH−] + [A−], so 10−8 M HCl gives pH 6.98, not 8.
Titration curves. 25.0 mL of 0.10 M HCl needs 25.0 mL of 0.10 M NaOH (moles acid = moles base). The pH stays low, then jumps from about 4 to 10 within ±0.05 mL of the equivalence point (pH 7.00). For acetic acid the curve starts higher (2.88), passes a flat buffer region, and the equivalence point is basic (pH 8.72) because acetate is a weak base (Kb = Kw/Ka = 5.6 × 10−10). Phenolphthalein (8.2–10) fits that jump; litmus changes too gradually to show it. Try other acids in Neutralize → Go deeper: make your own acid: weaker acids and more dilute solutions give a smaller jump (source: OpenStax Chemistry 2e, 14.7).
Buffers. A mix of a weak acid and its conjugate base resists pH change: pH = pKa + log([A−]/[HA]) (Henderson–Hasselbalch). At half-equivalence (12.5 mL) [A−] = [HA], so pH ≈ pKa (the simulator shows 4.75 with the exact solution). Blood is buffered by carbonic acid and bicarbonate at pH 7.35–7.45; going from 7.4 to 7.0 means 2.5 times more H+ — a medical emergency. See the buffers simulation.
Temperature and the limits of 0–14. Kw grows with temperature (water’s ionization absorbs heat): at 37 °C Kw ≈ 2.4 × 10−14, so neutral water there is pH 6.81, still neutral because [H+] = [OH−]. pH can also be below 0 or above 14 in very concentrated solutions (10 M HCl is about −1 on paper). The simulator works at 25 °C. Its indicator colors are typical chart colors; real ones depend on the brand and strength.
Typical values from the simulator. Real samples vary, so the usual range is given too. Never taste or mix cleaners: bleach and ammonia together make a toxic gas.
| Liquid | pH | Usual range | H+ vs pure water |
|---|---|---|---|
| Lemon juice | 2.2 (acidic) | 2.0–2.6 | about 63,000× more |
| Vinegar | 2.4 (acidic) | 2.4–3.0 | about 40,000× more |
| Orange juice | 3.5 (acidic) | 3.3–4.2 | about 3,200× more |
| Black coffee | 5.0 (acidic) | 4.8–5.1 | 100× more |
| Milk | 6.7 (slightly acidic) | 6.5–6.8 | about 2× more |
| Pure water | 7.0 (neutral) | 7.0 at 25 °C | the same |
| Blood | 7.4 (slightly basic) | 7.35–7.45 | about 2.5× fewer |
| Seawater | 8.1 (basic) | 7.9–8.2 | about 13× fewer |
| Baking soda in water | 8.3 (basic) | about 8.3 | about 20× fewer |
| Soapy water | 10.0 (basic) | 9–10.5 | 1,000× fewer |
| Ammonia cleaner | 11.6 (basic) | 11–12 | about 40,000× fewer |
| Bleach | 12.6 (basic) | 12.5–13 | about 400,000× fewer |
Vinegar 2.4 and the 10-fold rule: OpenStax Chemistry 2e 14.2 and USGS “pH and Water”. The other values are typical textbook values (illustrative; brands differ).
Pick one answer for each question, then press Check my answers to see the explanations. New questions picks another set.
A 45-minute lesson (or two 25-minute parts). Start in Test liquids on a projector (Full screen button): ask the class to sort five liquids into acid / neutral / base before you test them. Then pairs work through the seven missions; each asks for a prediction before the lab shows the result, and a certificate prints when all are done.
The pH values of everyday liquids are typical textbook values (illustrative; brands vary). The neutralization and dilution numbers are calculated exactly from Kw = 1.0 × 10−14 and Ka(acetic acid) = 1.8 × 10−5 at 25 °C (charge balance, no shortcuts). Indicator colors are typical chart colors. A drop is taken as 0.05 mL. In real bleach, indicators fade after a while because bleach destroys the dye, and phenolphthalein slowly loses its color above about pH 12–13; the simulator shows the color you see right after mixing.
Hands-on follow-up: red cabbage indicator with kitchen liquids (adult supervision for hot water; goggles; never mix bleach and ammonia). Go deeper covers pH = −log[H+], pOH, Kw, Ka and ICE tables, titration curves and buffers for high school (HS-PS1-2, HS-PS1-7) and AP Chemistry Unit 8. In Neutralize, the “Go deeper: make your own acid” box sets any pKa and concentration, with the titration curve and the share of acid ionized. Continue with the titration curves and buffers simulations.
Standards: NGSS MS-PS1-2 (properties of substances before and after they interact) · high school in Go deeper: HS-PS1-2, HS-PS1-7, AP Chemistry Unit 8 (Acids and Bases)
| Time | Step | What students do | Go to |
|---|---|---|---|
| 5 min | Hook | Predict acid / neutral / base for 5 liquids, then test them | Simulator |
| 10 min | Explore | Missions 1–2: cabbage colors, the 10× rule | Simulator |
| 10 min | Neutralize | Missions 3–5: how much base, one drop, strong vs weak | Simulator |
| 8 min | Dilute | Missions 6–7: water it down, vinegar | Simulator |
| 5 min | Order | Order liquids from acid to base | Order game |
| 7 min | Check | Quiz (10 questions) | Quiz |
Worksheet: a print-out for students and a separate answer key page for the teacher (opens the print dialog; choose "Save as PDF" to keep a file).
The pH scale tells how acidic or basic a water solution is. It usually runs from 0 to 14: below 7 is acidic, 7 is neutral and above 7 is basic. Lemon juice is about pH 2, pure water 7, baking soda in water about 8.3 and bleach about 12.6.
Key takeaway: low pH = acidic, 7 = neutral, high pH = basic.pH counts hydrogen ions on a log scale: each whole number is ten times more (or fewer) H+ than the next. pH 5 has 10 times more H+ than pH 6 and 100 times more than pH 7. This keeps huge numbers (from 1 to 0.00000000000001 mol/L) on a short 0–14 scale.
Key takeaway: one pH step = 10×, two steps = 100×.They neutralize each other: H+ from the acid joins OH− from the base to make water, and the other ions make a salt. With equal amounts of a strong acid and a strong base the result is neutral (pH 7). Near that point one drop can change the pH by 3 units.
Key takeaway: acid + base → salt + water.Red cabbage has purple pigments (anthocyanins) that change shape, and color, when they gain or lose H+. They are red in strong acids, pink in weak acids, purple near neutral, blue and green in bases and yellow in very strong bases. You can make it at home by soaking chopped cabbage in hot water.
Key takeaway: a natural indicator from your kitchen.It spreads the H+ ions out (fewer per liter), so the pH rises toward 7. Each 10× dilution of a strong acid raises the pH by about 1, but the pH can never pass 7 with water alone. To bring one cup of pH 2 acid to about pH 7 you would need about 25,000 liters of water.
Key takeaway: water moves pH toward 7, never past it.A strong acid such as HCl ionizes completely in water, so every molecule gives up its H+. A weak acid such as acetic acid ionizes only partly (Ka = 1.8 × 10−5): at 0.10 M only about 1.3 % of its molecules do, so its pH is 2.88 instead of 1.00. Strength is not the same as concentration.
Key takeaway: strong = fully ionized; weak = partly ionized.Yes. pH = −log[H+] has no limits, so a 10 M strong acid is about pH −1 on paper, and a 10 M strong base about 15. The 0–14 range covers most water solutions at 25 °C. In very concentrated solutions chemists use activities instead of concentrations, so these numbers are only rough.
Key takeaway: the scale is open at both ends.Neutral means [H+] = [OH−]. Water ionizes more when it is warmer, so Kw grows: at 37 °C it is about 2.4 × 10−14 and neutral water is pH 6.81. It is still neutral, not acidic. So pH + pOH = 14 is exactly true only at 25 °C.
Key takeaway: neutral = equal H+ and OH−, not always 7.00.