Lemon juice and vinegar taste sour, and soapy water feels slippery on your hands. The difference comes from which ions appear when they dissolve in water.
An acid releases hydrogen ions (H⁺) in water and a base releases hydroxide ions (OH⁻), and how strong they are is given by pH, a number from 0 to 14.
Changing the solution in the first diagram changes the ions in the beaker; moving the pH in the second makes litmus, bromothymol blue (BTB) and phenolphthalein each change color at its own pH.
pH 7 is neutral; the further below 7, the more acidic, and the further above 7, the more basic.
Acidic and Basic
🧪 Behind the Sour Taste and the Slippery Feel
①The sour taste of lemon juice and vinegar comes from hydrogen ions (H⁺)
②The slippery feel of soapy water and drain cleaner comes from hydroxide ions (OH⁻)
③An acid releases H⁺ in water, and with metals such as magnesium it gives off hydrogen gas
④A base releases OH⁻ in water and dissolves proteins, which is why it feels slippery
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Ions in acidic, basic and neutral solutions and their place on the indicator strip
Acidic Solution (HCl)
📝 Ions in the Beaker
①Acidic: hydrochloric acid, HCl → H⁺ + Cl⁻
②Basic: sodium hydroxide, NaOH → Na⁺ + OH⁻
③Neutral: salt water contains only Na⁺ and Cl⁻, with neither H⁺ nor OH⁻ left over
Indicators and pH
🌈 Telling Acids from Bases with Indicators
①Litmus paper: blue turns red in acid, red turns blue in base
②BTB solution: yellow in acid, green when neutral, blue in base
③Phenolphthalein: colorless in acidic and neutral solutions, pink in basic ones (from about pH 8.2)
④Each indicator changes color over a different pH range, so BTB is handy for checking neutrality
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How litmus, BTB and phenolphthalein change color with pH
pH 3
The pH Scale
📏 What pH Means
①pH expresses how acidic or basic a solution is as a number from 0 to 14
②pH 7 is neutral (pure water)
③The lower the pH below 7, the more acidic: stomach acid is about 2, vinegar about 3
④The higher the pH above 7, the more basic: soapy water is about 10, drain cleaner about 13
Summary
Acid
Acid → releases H⁺ (hydrogen ions)
Example: HCl → H⁺ + Cl⁻
Base
Base → releases OH⁻ (hydroxide ions)
Example: NaOH → Na⁺ + OH⁻
pH scale
pH < 7 acidic | pH = 7 neutral | pH > 7 basic
The further from 7, the stronger the acidity or basicity
🎯 Common Test Points
①Acid: releases H⁺, tastes sour, gives hydrogen gas with metals
Different acids all taste sour because they all release H⁺.
Example 2
Is a solution of pH 2 acidic or basic? What about pH 12? What color does each turn with phenolphthalein?
1
Since pH 7 is neutral, pH 2 is acidic and pH 12 is basic.
2
Phenolphthalein turns pink only in basic solutions, so the pH 2 solution stays colorless and the pH 12 solution turns pink.
▸ pH 2 is acidic (colorless); pH 12 is basic (pink)
In the diagram above, raising the pH from 8 to 9 turns only phenolphthalein from colorless to pink.
Unit Test
Which statement about acids and bases is correct?
①An acid dissolves in water and releases OH⁻
②A pH greater than 7 is acidic
③In a basic solution phenolphthalein turns pink
④An acidic solution makes blue litmus paper even bluer
⑤The lower the pH, the weaker the acid
▸ ③ In a basic solution phenolphthalein turns pink
1
An acid releases H⁺ (① wrong, OH⁻ is from a base), and a pH greater than 7 is basic (② wrong).
2
An acidic solution turns blue litmus red (④ wrong), and a lower pH means a stronger acid (⑤ wrong); in a basic solution phenolphthalein turns pink (③ correct).