📚 The pH Scale and Indicators | pH 标度与指示剂
The pH scale is a simple number scale used in chemistry to show how acidic or alkaline a solution is. It runs from 0 to 14 and helps scientists, students and even farmers compare different substances safely. Indicators are special dyes that change colour depending on the pH of the solution they are added to. In this article, you will learn how to read the pH scale, use common indicators and understand why pH matters in everyday life.
pH 标度是化学中用来表示溶液酸性或碱性程度的简单数字标度。它的范围从 0 到 14，帮助科学家、学生甚至农民安全地比较不同物质。指示剂是一种特殊的染料，加入溶液后会根据 pH 值改变颜色。在本文中，你将学习如何读取 pH 标度、使用常见指示剂，并理解为什么 pH 在日常生活中很重要。
1. What Is the pH Scale? | 什么是 pH 标度？
The pH scale is a continuous scale from 0 to 14 that measures how acidic or alkaline a solution is. A pH of 7 is neutral, so pure water has a pH of 7. Solutions with a pH below 7 are acidic; solutions with a pH above 7 are alkaline. The scale is based on the concentration of hydrogen ions (H⁺) in solution: more H⁺ ions make a lower pH, while fewer H⁺ ions make a higher pH.
pH 标度是一个从 0 到 14 的连续标度，用来衡量溶液的酸性或碱性。pH 值为 7 是中性，所以纯水的 pH 值为 7。pH 值低于 7 的溶液呈酸性；pH 值高于 7 的溶液呈碱性。该标度基于溶液中氢离子（H⁺）的浓度：H⁺ 离子越多，pH 值越低；H⁺ 离子越少，pH 值越高。
It is important to remember that pH is not a measure of how strong a chemical is in the same way as concentration. A very dilute strong acid can have a similar pH to a weak acid, so the pH value tells you the balance of acidic or alkaline behaviour in that particular solution, not the total amount of acid present.
重要的是要记住，pH 并不等同于化学物质的浓度。非常稀的强酸可能与弱酸具有相近的 pH 值，因此 pH 值告诉你的是该特定溶液中酸性或碱性行为的平衡，而不是所含酸的总量。
2. Acids and Alkalis on the pH Scale | pH 标度上的酸和碱
Acids have pH values between 0 and 6. The closer the pH is to 0, the more acidic the solution is. Common laboratory acids include hydrochloric acid, sulfuric acid and citric acid. Alkalis have pH values between 8 and 14. The closer the pH is to 14, the more alkaline the solution is. Common alkalis include sodium hydroxide and potassium hydroxide. A neutral solution with pH 7 is neither acidic nor alkaline; pure water and sodium chloride solution are good examples.
酸的 pH 值在 0 到 6 之间。pH 值越接近 0，溶液酸性越强。实验室中常见的酸包括盐酸、硫酸和柠檬酸。碱的 pH 值在 8 到 14 之间。pH 值越接近 14，溶液碱性越强。常见的碱包括氢氧化钠和氢氧化钾。pH 值为 7 的中性溶液既不是酸性也不是碱性；纯水和氯化钠溶液都是很好的例子。
Many students confuse the terms 'acid' and 'alkali'. An alkali is a soluble base that dissolves in water to release hydroxide ions (OH⁻). Not all bases are alkalis, but at KS3 we usually meet soluble metal hydroxides such as sodium hydroxide and calcium hydroxide.
许多学生混淆 '酸' 和 '碱' 这两个术语。碱是一种可溶性碱，溶于水后释放氢氧根离子（OH⁻）。并非所有碱都可溶，但在 KS3 阶段我们通常遇到可溶的金属氢氧化物，如氢氧化钠和氢氧化钙。
3. Universal Indicator and Colour Changes | 通用指示剂与颜色变化
Universal indicator is a mixture of several dyes that turns a different colour at different pH values. It is used as a solution or on paper strips. The typical colour range is: red for pH 0-2, orange or yellow for pH 3-6, green for pH 7, blue for pH 8-11, and violet or purple for pH 12-14. By comparing the colour with a pH colour chart, you can estimate the pH of an unknown solution.
通用指示剂是几种染料的混合物，在不同的 pH 值下会变成不同的颜色。它可以作为溶液使用，也可以涂在纸条上。典型的颜色范围是：pH 0-2 呈红色，pH 3-6 呈橙色或黄色，pH 7 呈绿色，pH 8-11 呈蓝色，pH 12-14 呈紫色或紫罗兰色。通过将颜色与 pH 色卡进行比较，你可以估算未知溶液的 pH 值。
When using universal indicator solution, add only a few drops to a small sample of the solution in a test tube. Do not add the solution to the indicator because that wastes indicator and can make the colour harder to read.
使用通用指示剂溶液时，只需在试管中的少量样品里滴加几滴。不要把溶液加入指示剂中，因为这样会浪费指示剂，而且颜色更难读取。
4. Litmus Paper as an Indicator | 石蕊试纸指示剂
Litmus is a simple indicator that shows whether a solution is acidic or alkaline but does not give a precise pH value. Red litmus paper stays red in an acid and turns blue in an alkali. Blue litmus paper turns red in an acid and stays blue in an alkali. In a neutral solution, red litmus stays red and blue litmus stays blue.
石蕊是一种简单的指示剂，可以显示溶液是酸性还是碱性，但不能给出精确的 pH 值。红色石蕊试纸在酸中保持红色，在碱中变蓝。蓝色石蕊试纸在酸中变红，在碱中保持蓝色。在中性溶液中，红色石蕊保持红色，蓝色石蕊保持蓝色。
Litmus is very useful for a quick acid or alkali check. For example, if a drop of lemon juice is placed on blue litmus paper, the paper turns red, proving that lemon juice is acidic. If a drop of sodium hydroxide solution is placed on red litmus paper, the paper turns blue, proving it is alkaline.
石蕊对于快速检测酸或碱非常有用。例如，将一滴柠檬汁滴在蓝色石蕊试纸上，试纸变红，证明柠檬汁是酸性的。将一滴氢氧化钠溶液滴在红色石蕊试纸上，试纸变蓝，证明它是碱性的。
5. Other Common Indicators | 其他常见指示剂
There are many other indicators besides litmus and universal indicator. Phenolphthalein is colourless in acidic and neutral solutions but turns pink in alkaline solutions, usually above pH 8.2. Methyl orange is red in acidic solutions, yellow in neutral and alkaline solutions. These indicators
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