Acids, Bases and pH

Chapter 7 promised that amounts of substance would reappear when acids and bases are neutralised, and this chapter delivers that. It defines acids and bases in two simple ways, explains what the pH number measures and why a change of one unit is a tenfold change, tabulates the pH of everyday substances, and treats neutralisation as a reaction you can balance (Chapter 6) and that makes a salt (Chapter 4). Each fact is attributed to the Wikipedia page that states it, and my own arithmetic or general knowledge is marked "(check)".

Acids and Bases: Two Simple Definitions

The Acid article gives two definitions. The older, Arrhenius, says: "An Arrhenius acid is a substance that, when added to water, increases the concentration of H⁺ ions in the water." The broader Brønsted–Lowry definition describes an acid as "a species that donates a proton to a Brønsted–Lowry base." The Base (chemistry) article mirrors these. An Arrhenius base is, in its words, "a substance which dissociates in aqueous solution to form hydroxide ions OH⁻" (credited there to Arrhenius, 1884), and a Brønsted–Lowry base "is a substance that can accept hydrogen cations (H⁺)" (1923). That page adds that this is the broader definition and that Arrhenius bases are a subset of it.

A hydrogen atom that has lost its one electron is a bare proton (Chapter 2's electron and proton counts), so "donates H⁺" and "donates a proton" mean the same thing here (my link to earlier chapters (check)). The Hydronium article says that the ion produced when water is protonated is H₃O⁺, but also that the aqueous proton is more accurately represented as H⁺(aq), and it suggests using the symbol H⁺(aq) where possible. This course writes H⁺ for short, as the pH formula pages do (my choice (check)).

On everyday properties, the Acid article says acids "form aqueous solutions with a sour taste" and "can turn blue litmus red". The Base (chemistry) article says bases are "slippery to the touch, can taste bitter", and "turn red litmus paper blue". Tasting is not a test you should use, as the safety box below explains.

Strong and Weak

I take only what the pages say. The Acid article: "A strong acid is one that completely dissociates in water", while a weak acid "only partially dissociates", so both the acid and its conjugate base are present at equilibrium. The Base (chemistry) article: "A strong base is a base that is quantitatively protonated upon exposure to water", while a weak base only partly accepts protons. That article notes that "strong" here names a category and does not describe a qualitative intensity. So strong and weak describe how fully a substance reacts with water, not how concentrated the solution is (my restatement of that point (check)). "Equilibrium" is a topic for Chapter 9.

What pH Measures

The PH article defines pH as "the decimal logarithm of the reciprocal of the hydrogen ion activity, a_H+", that is, pH = −log₁₀(a_H+). It says the range is "commonly given as 0 to 14". The Hydronium article gives a simpler working form, pH = −log([H₃O⁺]/M), where M is mol/L. That page also says that the molar concentration of hydronium or H⁺ ions determines a solution's pH. "Activity" is a refinement of concentration that I do not cover (check); this chapter uses concentration in mol/L, a unit built on Chapter 7's mole (my simplification (check)).

The key idea is the logarithm. The PH article states that a difference of one pH unit is equivalent to a tenfold difference in hydrogen ion concentration. The lower the pH, the higher the concentration. Here are some worked numbers, with every pH calculated in Python as −log₁₀ of the concentration (my arithmetic (check)):

Pure water is the middle of the scale: the PH article says "Pure water has a pH of 7 at 25 °C, meaning it is neutral." In the table that is a concentration of 10⁻⁷ mol/L. So pH 3 versus pH 6 is a factor of 10³ = 1000 in hydrogen-ion concentration (Python: 1000.0), not "twice as acidic" or "half as much". Values below 7 are acidic, above 7 basic (the neutral-point pairing follows from the page's table, which labels pure water as neutral and ammonia as a base).

The pH of Common Substances

The PH article lists these "average pH of common solutions", reproduced as the fetch tool returned them. Ranges are given as the page gives them, because real samples vary.

Using the logarithm rule, take gastric acid at the top of its listed range (pH 1.5) and orange juice at the top of its listed range (pH 4.2). The difference is 2.7 units, so the hydrogen-ion concentration of the gastric acid is about 10^2.7 times larger, which Python gave as 501.19 (roughly 500 times). This comparison is my own calculation and the page's ranges are wide, so treat it as an order of magnitude (check).

Neutralisation: Acid + Base → Salt + Water

The Neutralization (chemistry) article defines it as "a chemical reaction in which acid and a base react with an equivalent quantity of each other". It says the reaction was historically written as "acid + base (alkali) → salt + water", and its example is HCl + NaOH → NaCl + H₂O. That equation is already balanced, as Chapter 6's atom count shows: one each of H, Cl, Na and O on both sides, with H counted as 2 on each side (my check of the page's equation (check)). By Chapter 6's classification it also has the double displacement pattern (my reading (check)).

The salt is sodium chloride, Na⁺ and Cl⁻ in a lattice (Chapter 4). A "salt" in chemistry means an ionic compound of this kind, not only table salt (my general knowledge (check)). In the Brønsted–Lowry picture, the acid HCl donates a proton, H⁺, and the hydroxide ion takes it to make water (my fit of the definitions to the equation (check)).

acid + base  →  salt + water
HCl + NaOH  →  NaCl + H2O      (Neutralization article's example)
H2SO4 + 2NaOH  →  Na2SO4 + 2H2O   (my own example, balanced by atom count (check))

Chapter 7's mole ratios apply directly. In HCl + NaOH → NaCl + H₂O the coefficients are 1 : 1, so 0.0250 mol of HCl needs 0.0250 mol of NaOH. With M(NaOH) of about 22.99 + 16.00 + 1.01 = 40.00 g/mol (my sum of rounded masses (check)), that is 0.0250 × 40.00 = 1.00 g of NaOH (Python). For the second equation above, the ratio is 1 : 2, so 0.0250 mol of H₂SO₄ needs 0.0500 mol of NaOH (Python: 0.0500).

The page also says how the final pH depends on strength. For a strong acid with a strong base, "the pH of such a solution is close to a value of 7"; with a weak acid and a strong base, the pH at the end-point "is greater than 7". So neutral pH is not guaranteed after "neutralising", and I do not explain the reason here (check).

Indicators and Litmus

The pH indicator article describes an indicator as "a halochromic chemical compound added in small amounts to a solution" that lets you determine pH visually or spectroscopically. Litmus, it says, is "a naturally occurring pH indicator made from a mixture of lichen species". The Litmus article adds that litmus is "a water-soluble mixture of different dyes extracted from lichens", often absorbed into filter paper, and that "in an acidic medium, blue litmus paper turns red" while "in a basic or alkaline medium, red litmus paper turns blue". Both pages give the colour change as occurring over pH 4.5 to 8.3 (the Litmus article: "at 25 °C"). The indicator article's other transitions are below. Universal indicator, it says, is used "when only rough knowledge of pH is necessary".

A practical consequence of the ranges (my reasoning (check)): litmus changes across a wide band, so it tells you "acid" or "base" but not a precise value, while an indicator that changes at pH 8.3 to 10.0 only reveals that a solution is clearly basic.

Where to Read More

• Encyclopaedias. The Wikipedia articles above are the sources used here; the PH article also covers measurement and the activity idea this chapter skips.
• A school textbook. Look for titration, buffers and the acid dissociation constant, none of which are covered here.
• Practical. Ask a supervised science teacher or use a kit with litmus paper to test safe, dilute items such as those in the table; do not improvise with cleaning products (my suggestion (check)).

Exercises