L40. Acids, Bases, and pH

States, Solutions, and Acids

R-report

L40. Acids, Bases, and pH

How does pH tell us whether a solution is acidic, basic, or neutral?

Central Idea:

Acids and bases are categories of aqueous solutions defined by how they change the balance of hydrogen and hydroxide ions in water: acids increase the concentration or activity of H+ (often present as H3O+), while bases increase OH–. The pH scale expresses hydrogen-ion activity on a logarithmic scale so each whole-number step is a tenfold change in H+ activity; lower pH values mean more acidic (higher H+), higher pH values mean more basic (higher OH–), and pure water near 25 °C is about neutral at pH 7. Example: lemon juice is about pH 2; mild soap is usually pH 10.

Procedure:

- Gather safe materials and tools (lemon juice, dilute vinegar, soap solution, pH paper or meter, goggles). - Name materials and write concentrations (e.g., 0.1 M HCl or household vinegar ~0.01–0.1 M acetic acid). - Record clear observations (color change, bubbling, slippery feel described, indicator color, pH reading). - Separate facts from interpretation (what you saw vs why it happened). - Write a simple chemical equation if a reaction occurred (include phases and coefficients). - Test your idea: try reversing steps, check for gas, precipitate, or mass change to identify a chemical change. - Wear gloves and goggles; never taste or touch unknown solutions in class, please.

Comparison:

When acids or bases react, some visible things may change while the underlying atoms are conserved. Compare what stays the same (same elements, total atom counts) with what changes (new arrangements, new molecules, gas, or solid). Ask: are the same atoms and the same chemical names present after the event, or have new substances formed? Check atom counts. - Measure mass before and after (units: g); a clear mass change suggests gas loss, gas release, or other mass change. - Check for gas (bubbles, test with indicator or trap CO2), and look for new solid (precipitate) or color change. - Try reversing steps or dilute the mixture; if you cannot reverse it and a new substance remains, it is likely a chemical change.

Synthesis:

Synthesis: chemical changes make new substances with new formulas and often gas or precipitate, while physical changes rearrange matter without new formulas; use vocabulary such as acid, base, neutralization, precipitate, pH, and indicator when you explain observations. Example equation: 1 HCl (aq) + 1 NaOH (aq) → 1 NaCl (aq) + 1 H2O (l). Count atoms: left H = 2 (1 from HCl, 1 from NaOH); right H = 2 (in H2O); Na = 1 left and right; Cl = 1 left and right; charges balance. Check units and temperature.

1. Check units and sample labels (concentrations in M, mass in g). 2. Tell the micro-level atoms/bonds story: list reactant formulas and predicted products and draw a simple particle sketch. 3. Confirm visible evidence fits the story: gas, precipitate, color, pH reading. 4. Write balanced equations with coefficients and check atoms and charge. 5. Repeat measurements and compare results; record uncertainties. 6. For the next problem: predict pH direction, plan tests, measure pH (with units), check mass if closed, draw equation, and decide if new substance was made, document outcomes.