Acids and Bases: Properties of Acids and Bases
FoundationAcid solutions contain hydrogen ions: they release hydrogen with reactive metals and carbon dioxide with carbonates. Bases react with acids; soluble bases are alkalis. Indicators and the pH scale tell acid from alkali; strength means degree of dissociation, distinct from concentration.
- Acids release H⁺(aq) in water; alkalis release OH⁻(aq); bases react with acids and the soluble ones are alkalis
- Acid + metal above H → salt + H₂; acid + carbonate → salt + water + CO₂; acid + metal oxide/hydroxide → salt + water
- Test for CO₂: limewater turns milky
文體小知識：科學知識說明文——從廚房與實驗室的例子說明酸和鹽基的性質、三類典型反應、指示劑與 pH 標度，並分清強弱與濃稀。
§Text Passage
1Lemon is sour because of citric acid, vinegar because of ethanoic acid, and gastric juice contains hydrochloric acid that aids digestion; common laboratory acids are hydrochloric, sulfuric and nitric acids. Dissolved in water, acids dissociate into hydrogen ions — acid properties are the behaviour of these ions. Dry hydrogen chloride gas shows no acidity: only in water does it release hydrogen ions and turn blue litmus red, showing the essential role water plays in displaying acidity.
2Acids have three typical reactions. With reactive metals above hydrogen in the series, such as zinc and magnesium, they give a salt plus hydrogen. With carbonates such as marble, they give a salt, water and carbon dioxide — bubble the gas into limewater and it turns milky. With metal oxides or hydroxides, they give a salt plus water. Keep the products straight in answers: acid plus metal gives hydrogen; only acid plus carbonate gives carbon dioxide.
3Opposing acids are bases — substances that react with acids, commonly metal oxides, metal hydroxides and carbonates. Those that dissolve in water and release hydroxide ions are called alkali, such as sodium hydroxide and ammonia solution. Sodium hydroxide is the main ingredient of drain cleaners; dilute ammonia cleans glass. Concentrated acids and alkalis are corrosive: wear safety goggles when handling, and flush any splash on skin with plenty of water at once.
4Indicators tell acid from alkali: litmus turns red in acid and blue in alkali; phenolphthalein is colourless in acid and pink in alkali; methyl orange is red in acid and yellow in alkali. The pH scale runs from 0 to 14 — below 7 acidic, above 7 alkaline — measured with a pH meter or universal indicator. Distinguish strength from concentration: a strong acid dissociates completely in water, a weak acid such as ethanoic acid only partially; concentration is simply how much is dissolved, unrelated to strength.
1Summary 重點摘要
Acid solutions contain hydrogen ions: they release hydrogen with reactive metals and carbon dioxide with carbonates. Bases react with acids; soluble bases are alkalis. Indicators and the pH scale tell acid from alkali; strength means degree of dissociation, distinct from concentration.
2Vocabulary cards Vocabulary
A substance that releases hydrogen ions in water; its solution has pH below 7 and turns blue litmus red.
A substance that reacts with an acid — commonly metal oxides, hydroxides and carbonates.
A soluble base whose solution contains hydroxide ions and turns red litmus blue, e.g. sodium hydroxide.
The H⁺(aq) released by acids in water, the source of all acidic properties.
The OH⁻(aq) released by alkalis in water, the source of all alkaline properties.
A dye showing different colours in acidic and alkaline solutions, e.g. litmus, phenolphthalein, methyl orange.
A common indicator: red in acid, blue in alkali.
A 0–14 scale of acidity: below 7 acidic, above 7 alkaline, 7 neutral.
An acid that dissociates completely in water, e.g. hydrochloric acid, as opposed to partially dissociating weak acids.
The chemical name of baking soda, a base that releases carbon dioxide with acids, used in baking and neutralisation.
3Step by step Steps
- Step 1: The common identity of acids — hydrogen ionsHydrochloric, sulfuric, nitric and ethanoic acids are all acids because each dissociates into hydrogen ions in water. For example, dry hydrogen chloride gas leaves dry litmus paper untouched, yet the paper turns red at once once the gas dissolves in water — acidic properties appear only with water, the ions existing hydrated as H⁺(aq). The sour taste, corrosiveness and litmus turning red are all the work of hydrogen ions — the starting point of every reaction in this topic.
- Step 2: Acid plus reactive metal — salt and hydrogenMetals above hydrogen in the series push hydrogen ions out of acids as hydrogen gas. Zinc with dilute hydrochloric acid: Zn + 2HCl → ZnCl₂ + H₂, bubbles of hydrogen crowding the granules. Magnesium reacts even more briskly with dilute sulfuric acid: Mg + H₂SO₄ → MgSO₄ + H₂. Note that copper and silver, below hydrogen, do not react with dilute acids, so the laboratory never makes hydrogen with copper. The salt's name pairs the metal with the acid's anion — zinc plus hydrochloric acid gives zinc chloride.
- Step 3: Acid plus carbonate — salt, water and carbon dioxideEvery carbonate releases carbon dioxide with an acid. Marble (mainly calcium carbonate) with dilute hydrochloric acid: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂; the solid dissolves as bubbles stream off. The standard test is to bubble the gas into limewater, which turns milky — proof of carbon dioxide. For example, pouring vinegar onto baking soda (sodium hydrogencarbonate) in the kitchen fizzes with exactly this gas — a textbook acid-plus-carbonate reaction, and the raising of cakes relies on it too.
- Step 4: Acid plus metal oxide or hydroxide — salt and waterMetal oxides and hydroxides are bases; with acids they give a salt and water, with no gas. Dilute sulfuric acid removes rust: iron(III) oxide reacts to iron(III) sulfate and water, the rust vanishing and the solution turning yellow. Copper(II) hydroxide with dilute sulfuric acid: Cu(OH)₂ + H₂SO₄ → CuSO₄ + 2H₂O — the blue solid dissolves into a blue solution. These reactions belong to neutralisation, the subject of the next lesson; industrial descaling and derusting follow the same route.
- Step 5: Base versus alkaliBase is the umbrella term for substances that react with acids; an alkali is the soluble member that releases hydroxide ions in water. For example, sodium hydroxide dissolves and dissociates fully into Na⁺ and OH⁻ — a typical alkali and the main ingredient of drain cleaner; ammonia solution also produces hydroxide ions and cleans glass; copper(II) oxide, insoluble in water, reacts with acids but is a base, not an alkali. Alkali properties — soapy feel, bitter taste, red litmus turning blue, phenolphthalein turning pink — all stem from OH⁻(aq); state it explicitly to echo the H⁺(aq) of acids.
- Step 6: Indicators and the pH scaleIndicators are colour codes for acid and alkali: litmus red in acid, blue in alkali; phenolphthalein colourless in acid, pink in alkali — ideal for titration; methyl orange red in acid, yellow in alkali. To gauge the degree, read the pH scale: below 7 acid, 7 neutral, above 7 alkali. For example, lemon juice is about pH 2.3, pure water about 7, baking-soda solution about 8.5, drain cleaner up to 14. Measure with a pH meter or a universal indicator against its colour chart; exam questions often ask for both the reading and the colour change.
- Step 7: Strength and concentration are two rulersStrength measures how completely an acid dissociates; concentration measures how much is dissolved — independent of each other. Hydrochloric acid is strong, dissociating completely; ethanoic acid is weak, only partly dissociated even in water. For example, 0.1 M hydrochloric acid holds far more hydrogen ions than 2 M ethanoic acid — a dilute strong acid can out-acid a concentrated weak one. Likewise sodium hydroxide is a strong alkali, ammonia solution a weak one. DSE often compares a strong and a weak acid of the same pH: argue by degree of dissociation, never blur strength with concentration.
4Diagrams Diagrams
5Worked example Example
Inquiry context: from the kitchen a student brings lemon juice and baking soda (sodium hydrogencarbonate). In the laboratory a pH meter reads about 2.3 for the juice and about 8.5 for the soda solution. Answer: (1) classify both and name the type of substance each contains; (2) write the equation for zinc with dilute hydrochloric acid; (3) write the equation for calcium carbonate with dilute hydrochloric acid and state how to test the gas produced.
- Classify: juice pH 2.3 is below 7 — acidic, containing citric acid (an acid); soda solution pH 8.5 is above 7 — alkaline, sodium hydrogencarbonate being a base
- Acid plus metal: zinc with dilute HCl — Zn + 2HCl → ZnCl₂ + H₂, with Zn, H and Cl all balanced
- Acid plus carbonate: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
- Test the gas: bubble it into limewater; a milky colour proves carbon dioxide
- Answer: The lemon juice reads about pH 2.3, below 7, so it is acidic and contains citric acid; the baking-soda solution reads about pH 8.5, above 7, so it is alkaline and sodium hydrogencarbonate is a base. Zinc with dilute hydrochloric acid: Zn + 2HCl → ZnCl₂ + H₂, giving zinc chloride and hydrogen. Calcium carbonate with dilute hydrochloric acid: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂, giving calcium chloride, water and carbon dioxide; bubbling the gas into limewater turns it milky, proving the gas is carbon dioxide.
6Common mistakes Mistakes
7Quick check Quick check
Q1Which ion do all acidic solutions have in common?
Q2Write the equation for zinc reacting with dilute sulfuric acid.
Q3Name the three products when marble reacts with dilute hydrochloric acid.
Q4How do you test whether a jar of gas is carbon dioxide?
Q5A baking-soda solution has pH about 8.5 — acidic or alkaline, and what type of substance is it?
8Recap Recap
- Acids release H⁺(aq) in water; alkalis release OH⁻(aq); bases react with acids and the soluble ones are alkalis
- Acid + metal above H → salt + H₂; acid + carbonate → salt + water + CO₂; acid + metal oxide/hydroxide → salt + water
- Test for CO₂: limewater turns milky
- Indicators: litmus red/blue, phenolphthalein colourless/pink, methyl orange red/yellow; pH below 7 acid, above 7 alkali
- Strength = degree of dissociation (HCl strong, ethanoic acid weak); concentration is separate — two different rulers