Physics and chemistry Year 8: pH, acid–base reactions and precipitation
Physics and chemistry, Year 8 (Portugal): How to tell if a solution is acidic, neutral or basic, what happens to the pH when an acid meets a base, and how dissolved ions can form a solid salt.
The pH scale
The pH tells the chemical character of an aqueous solution. On the usual scale from 0 to 14 (at 25 °C): below 7 is acidic, 7 is neutral, above 7 is basic. You can find it with indicators, such as universal indicator, which change colour with the pH, and more precisely with a pH meter. Approximate values: lemon juice about 2, pure water 7, a baking soda solution about 8, household bleach above 11.
Two ways solutions react
When solutions are mixed, their ions can meet and react. In an acid–base reaction an acid and a base react and the pH moves toward 7: that is why an antacid eases heartburn and why lime is spread on soil that is too acidic. In a precipitation reaction, ions from two solutions form a salt that is only slightly soluble in water and appears as a solid, the precipitate. This is how stalactites, shells and corals of calcium carbonate build up.
Predicting the pH and writing the equations
Mix 50 mL of hydrochloric acid with 50 mL of sodium hydroxide of the same concentration: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l). Acid + base gives salt + water. With matching amounts, the pH ends near 7; with twice as much acid, some acid is left and the pH is below 7. Precipitation: CaCl₂(aq) + Na₂CO₃(aq) → CaCO₃(s) + 2 NaCl(aq). The (s) is the white, poorly soluble precipitate; NaCl stays dissolved. Atoms match: 1 Ca, 2 Cl, 2 Na, 1 C, 3 O.
Acid plus base is not always pH 7
«Mix an acid with a base and it turns neutral»: only if the amounts match. The reaction uses acid and base in the proportion given by the equation; if acid is left over, the final solution is acidic, and if base is left over, it is basic. So to predict the pH, compare the amounts. Another mistake: «acid means dangerous, base means harmless». A strong base such as caustic soda (sodium hydroxide) also irritates and burns.
Hard water, limescale and antacids
Water is hard when it carries a lot of dissolved calcium and magnesium ions. At home it leaves limescale in kettles and pipes, because calcium carbonate, only slightly soluble, precipitates; in industry it clogs boilers and pipes. It can be treated with softeners, which swap those ions, and limescale is removed with vinegar. To find how hard the water in your region is, look up the report published by the local water supplier.
Test what you learned
An aqueous solution has pH 9 at 25 °C. What is its chemical character?
- a) Acidic, because 9 is a high number
- b) Neutral
- c) Basic
Correct answer: c) Basic — Right: above 7 is basic. Baking soda in water is a familiar example.
Solutions of calcium chloride and sodium carbonate are mixed and a white solid forms. Which substance is that solid?
- a) Sodium chloride, NaCl, the familiar table salt
- b) Calcium carbonate, CaCO₃, a salt only slightly soluble in water
- c) Calcium chloride, CaCl₂, that did not dissolve
- d) No solid forms: the ions simply mix
Correct answer: b) Calcium carbonate, CaCO₃, a salt only slightly soluble in water — Right: Ca²⁺ and CO₃²⁻ meet and form CaCO₃(s), the precipitate.
A student mixes an acid solution with a basic solution. Which statement about the final pH is correct?
- a) It is always exactly 7
- b) It is the average of the two starting pH values
- c) It is always basic, because a base «wins» over an acid
- d) It depends on the amounts: leftover acid gives pH below 7, leftover base above 7, matching amounts about 7
Correct answer: d) It depends on the amounts: leftover acid gives pH below 7, leftover base above 7, matching amounts about 7 — Right: to predict the pH, compare the amounts and see which reactant is left over.
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