Why are lemons sour and soapy water slippery? We will learn about the acidic, neutral, and alkaline properties of aqueous solutions.
Basic Knowledge
Aqueous solutions are divided into three types according to their properties.
・Acidic: blue litmus paper→red, BTB solution→yellow, phenolphthalein→colorless, and produces hydrogen when magnesium is added.
・Neutral: no color change in litmus paper, BTB→green, phenolphthalein→colorless.
・Alkaline: red litmus paper→blue, BTB→blue, phenolphthalein→red, and feels slippery (it dissolves proteins).
Strength is expressed using pH on a scale of 0〜14: pH7 is neutral, below 7 is acidic, and above 7 is alkaline.
Examples of acidic substances: hydrochloric acid, sulfuric acid, acetic acid, and carbonic acid. Examples of alkaline substances: sodium hydroxide, calcium hydroxide, and ammonia.
Litmus paper（blue→red = acid, red→blue = alkali）
BTB solution（acidic = yellow, neutral = green, alkaline = blue）
Phenolphthalein（acidic and neutral = colorless, alkaline = red）
pH（the hydrogen-ion concentration index. 0〜14, with 7 being neutral）
Purple cabbage juice（acidic = red, neutral = purple, alkaline = green〜yellow）
pH test paper（universal test paper that indicates pH by color）
Further Exploration (Background and Significance)
The substances responsible for acidity and alkalinity are hydrogen ions (H+) and hydroxide ions (OH-), which are studied in the third year of middle school. In an aqueous solution, an abundance of H+ makes it acidic, while an abundance of OH- makes it alkaline. pH represents the “logarithm of hydrogen-ion concentration,” so a difference of 1 in pH means a tenfold difference in H+ concentration.
pH values of familiar liquids: lemon juice=2, vinegar=3, cola=2.5, rainwater=5.6 (slightly acidic because of CO2 in the air), tap water=7, seawater=8.2, soapy water=10, household bleach=12. Gastric juice has a pH of 1〜2 and is a very strong acid, which breaks down proteins.
- Acidic: blue→red, BTB yellow, pH＜7
- Alkaline: red→blue, BTB blue, phenolphthalein red, pH＞7
- Neutral: no change, BTB green, pH=7
- pH is a scale from 0〜14
- A difference of 1 in pH means a tenfold difference in H+ concentration
- Gastric juice pH 1〜2, rainwater pH 5.6
- Magnesium + acid → hydrogen is produced
Points to Note (Easily Confused)
① Be careful because the color changes for litmus paper (blue→red = acid) and BTB (acid = yellow) work in different directions. ② Phenolphthalein turns red only in alkaline solutions (it is colorless in acidic solutions). ③ The smaller the pH, the stronger the acid; the larger the pH, the stronger the alkali. ④ Alkaline and basic have nearly the same meaning (middle-school textbooks often use “alkaline”).
Practice
- What type of solution turns BTB solution yellow?
- Which is more strongly acidic, pH=3 or pH=4? Also, how many times different are their hydrogen-ion concentrations?
- What kind of aqueous solution turns phenolphthalein red?