Common Acids
FoundationLemons contain citric acid and vinegar contains ethanoic acid — acids are common in foods. Acids taste sour, can be corrosive, react with some metals to give hydrogen, and have a pH below 7.
- Common acids: citric (lemons), ethanoic (vinegar), lactic (yoghurt), carbonic (soft drinks).
- General acid properties: sour taste, corrosive, react with some metals to give hydrogen.
- Acids react with calcium carbonate (marble) to give carbon dioxide.
文體小知識：科普知識文章用生活實例解釋科學概念，先看現象再講原理，讀完就能解釋日常見到的東西。
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1Take a sip of fresh lemon water in the morning and a sour taste spreads across your tongue — that is citric acid. Vinegar tastes sour because of ethanoic acid, yoghurt owes its tang to lactic acid, and the fizz in soft drinks comes from carbonic acid. Acids have always been on our dining table.
2In chemistry, acids share a set of properties: their solutions taste sour, can be corrosive, react with reactive metals such as magnesium and zinc to give hydrogen gas, and react with marble (calcium carbonate) to give carbon dioxide. Scientists quantify acidity with the pH scale: below 7 is acidic, and the smaller the number, the stronger the acid.
3But acids are not automatically dangerous: dilute acids in food are safe to eat, while concentrated laboratory acids can burn skin. Handle them with safety goggles, and rinse any splash immediately with plenty of water.
4Once you know the properties of acids, the next question follows naturally: how can we quickly tell whether a solution is acidic? That is where indicators — the topic of the next lesson — come in.
1Summary 重點摘要
Lemons contain citric acid and vinegar contains ethanoic acid — acids are common in foods. Acids taste sour, can be corrosive, react with some metals to give hydrogen, and have a pH below 7.
2Vocabulary cards Vocabulary
A group of compounds whose solutions taste sour and have a pH below 7, e.g. citric acid and ethanoic acid.
The natural acid found in lemons, oranges and other citrus fruits.
The acid in vinegar that gives it its sour taste, commonly called acetic acid.
The natural acid that makes yoghurt slightly sour, produced by bacterial fermentation.
Able to eat away skin, metals and other materials; concentrated acids are highly corrosive.
A scale from 0 to 14 showing how acidic or alkaline a substance is: 7 is neutral, below 7 is acidic, above 7 is alkaline.
The gas given off when acids react with some metals; it burns with a pop when tested with a lit splint.
Adding plenty of water to make a solution less concentrated; dilute acids are far less corrosive than concentrated ones.
A mixture of indicators showing the whole pH range 0 to 14 through a series of colours.
3Step by step Steps
- Acids at the dinner tableAcids are common compounds, and many sit right on our dinner table: lemon juice contains citric acid, vinegar ethanoic acid, yoghurt lactic acid and soft drinks carbonic acid — most sour tastes in food come from these natural acids. Carbon dioxide is dissolved into soft drinks under pressure; when the bottle is opened the pressure drops, the gas escapes, the carbonic acid decreases and the taste slowly becomes less sour. Knowing these everyday acids is the first step in acid-alkali chemistry.
- Sour but also corrosiveHow dangerous an acid is depends on its type and concentration: dilute vinegar is safe to drink and used in seasoning and pickling, while concentrated sulphuric acid in a laboratory corrodes clothes, paper and skin, burning flesh on contact. Diluting the same acid greatly reduces its corrosiveness because fewer acid particles remain in each unit volume. So always know the concentration before using an acid, and in experiments use only the dilute acids your teacher specifies.
- Acids react with metalsAcids react with many reactive metals to give hydrogen. Drop a magnesium ribbon into dilute hydrochloric acid and the tube immediately fills with bubbles — that is hydrogen; hold a lit splint at the mouth of the tube and the gas burns with a pop, which is the test for hydrogen. Zinc reacts with the same acid too, but being less reactive it gives fewer, slower bubbles. Metal fittings are eaten away by acidic solutions, so leftover acid should never be poured casually into metal sinks.
- Acids react with marbleCalcium carbonate fizzes in acid, giving off carbon dioxide. Drip vinegar onto a piece of marble or a seashell and bubbles appear at once; test the gas with a burning splint and the flame goes out, proving it is carbon dioxide. Seashells, eggshells and marble all contain calcium carbonate, so they all react the same way with acid. Acid rain eating away marble statues and buildings is exactly this reaction causing damage in nature.
- A first look at pHHow acidic or alkaline something is can be expressed as a pH value: lemon juice has a pH of about 2, vinegar about 3, and pure water is 7 — the smaller the pH, the more acidic, and 7 is neutral. Measure with pH paper or universal indicator: dip the paper in the solution and read the value from the colour chart. Doctors and farmers both use pH — for example, testing soil acidity to decide which crops suit the land — showing how widely this indicator is applied.
- Safety rulesSafety rules for handling acids must be strictly followed: if dilute acid splashes on your skin, rinse at once with plenty of water and tell your teacher, because the water dilutes and washes the acid away, reducing corrosion. Always wear safety goggles to protect your eyes and never taste any chemical. Stand test tubes in a rack when measuring out acid, and if you spill any, report it immediately rather than cleaning up yourself, so the accident does not grow worse.
4Diagrams Diagrams
5Worked example Example
Example: John puts magnesium ribbon into dilute hydrochloric acid and into pure water. Only the acid gives bubbles. Explain why, and describe how to test the gas.
- Dilute hydrochloric acid is an acid and reacts with the reactive metal magnesium, releasing hydrogen.
- Pure water is not an acid, so no such reaction happens and no bubbles form.
- To test: hold a lit splint at the mouth of the tube.
- Hydrogen burns with a distinctive pop — hearing the pop confirms the gas is hydrogen.
- Answer: Dilute hydrochloric acid is an acid, so it reacts with magnesium, a reactive metal, releasing hydrogen and forming many bubbles; pure water is not an acid and does not react with magnesium, so no bubbles form. To test the gas, hold a lit splint at the mouth of the tube: hydrogen burns with a pop, which confirms the gas is hydrogen.
6Common mistakes Mistakes
7Quick check Quick check
Q1Which acid is found in vinegar?
Q2What gas is released when an acid reacts with magnesium, and how is it tested?
Q3Roughly what is the pH of lemon juice — acidic or alkaline?
Q4Why does acid rain damage marble statues?
Q5What should you do if dilute acid splashes on your skin?
8Recap Recap
- Common acids: citric (lemons), ethanoic (vinegar), lactic (yoghurt), carbonic (soft drinks).
- General acid properties: sour taste, corrosive, react with some metals to give hydrogen.
- Acids react with calcium carbonate (marble) to give carbon dioxide.
- pH below 7 is acidic and smaller means stronger; pH 7 is neutral.
- Lab safety: never taste chemicals; rinse spills with plenty of water immediately.