When acids meet bases
Heartburn medicines (antacids) contain bases such as magnesium hydroxide or aluminum hydroxide. Gardeners spread lime on soil that’s too acidic. Factories treat acidic wastewater before releasing it. All of these rely on the reaction between acids and bases.
Neutralization
The reaction of an acid with a base to form a salt and water is called neutralization中和反应:
A salt盐 is a compound made of metal ions (or ammonium ions) and the negative ions left over from an acid, such as sodium chloride, sodium sulfate, and calcium carbonate.
At the particle level, neutralization is the from the acid combining with the from the base to form water:
Mixing sodium hydroxide and hydrochloric acid produces no visible change, so an indicator is needed to show when the reaction is complete. Below, sodium hydroxide solution is added to hydrochloric acid containing phenolphthalein. Watch the pH meter and the color of the solution:
While only a little sodium hydroxide has been added, acid is left over and the solution is acidic. When exactly enough has been added, the pH is 7. Add more, and the solution becomes basic and the phenolphthalein turns pink. Notice how, near the point of exact neutralization, just a drop more base makes the pH jump.
The pH scale
An indicator tells you whether a solution is acidic or basic, but not how acidic or basic it is. That is described by pH, which usually ranges from 0 to 14:
- pH below 7: acidic; the lower the pH, the more acidic.
- pH exactly 7: neutral.
- pH above 7: basic; the higher the pH, the more basic.
Click a substance to see where it falls:
Measuring pH with pH paper: put a small strip of pH paper on a clean, dry tile or glass plate, touch a drop of the solution to it with a clean glass rod, and compare the color with the color chart. pH paper only gives whole numbers; a pH meter gives more precise readings.
Using neutralization
- Treating soil: adding lime (calcium hydroxide) neutralizes acid in soil.
- Treating wastewater: acidic wastewater from a sulfuric acid plant can be neutralized with lime before it’s released:
- Medicine: antacids neutralize excess stomach acid, for example:
- Rain: normal rain is slightly acidic (pH about 5.6) because it contains dissolved carbon dioxide. Rain with a pH below 5.6 is acid rain.
Example 1: When is neutralization complete?
Dilute hydrochloric acid is added drop by drop, with stirring, to sodium hydroxide solution containing phenolphthalein. How do the color and pH change? How can you tell when the reaction is exactly complete?
- At the start. Sodium hydroxide solution is basic (pH above 7), so the phenolphthalein is pink.
- Adding acid. The sodium hydroxide is gradually neutralized and the pH falls, but as long as any base is left, the solution stays pink.
- Exactly neutralized. Only sodium chloride and water remain, the pH is 7, and the pink color just disappears.
- Adding more. Acid is now in excess and the pH drops below 7, but phenolphthalein stays colorless in acid, so the color doesn’t change again. Add the acid drop by drop and stir to catch the exact end point.
Example 2: Wet pH paper
A student wets a strip of pH paper with distilled water before testing white vinegar, sodium hydroxide solution, and salt water. How do the readings compare with the true values?
- Wetting dilutes the sample. The solution being tested gets diluted, so it becomes less acidic or less basic.
- Vinegar. Less acidic means the pH moves toward 7: the reading is too high.
- Sodium hydroxide solution. Less basic means the pH moves toward 7: the reading is too low.
- Salt water. It’s already neutral, so diluting it leaves the pH at 7: no change. That’s why pH paper must be dry.
- A solution with pH 2
pH below 7 is acidic.
- A solution with pH 7
pH 7 is neutral.
- A solution with pH 12
pH above 7 is basic.
- A solution that turns litmus blue
Litmus turns blue in basic solutions.
- Normal rain (pH about 5.6)
Dissolved carbon dioxide forms carbonic acid, making rain slightly acidic.
- Human blood (pH about 7.4)
Just above 7, so slightly basic.
- A solution that turns phenolphthalein pink
Phenolphthalein turns pink in basic solutions.
- Lemon juice
Lemon juice contains citric acid; its pH is about 2–3.
- Sodium hydroxide solution exactly neutralized by hydrochloric acid
Only sodium chloride and water remain, so the pH is 7.
Common mistakes
Mistake 1: “Any reaction that forms a salt and water is neutralization.” Neutralization specifically means an acid reacting with a base. and also form a salt and water, but neither is a neutralization.
Mistake 2: “A higher pH means more acidic.” It’s the opposite: the lower the pH, the more acidic; the higher the pH, the more basic.
Mistake 3: Dipping pH paper into the sample, or wetting it first. Dipping contaminates the sample, and wetting dilutes it. Touch a drop of the solution to dry paper with a glass rod.
Quick check
Which solution is the most acidic?
Below 7 is acidic, and the lower the pH, the more acidic the solution.
Which of these uses neutralization?
Lime is a base that neutralizes acid in the soil. B is an acid reacting with a metal oxide, C is an acid reacting with a salt, and D is adsorption.
Summary
- Neutralization: acid + base → salt + water. At the particle level, . Not every reaction that forms a salt and water is a neutralization.
- Sodium hydroxide and hydrochloric acid react with no visible change; phenolphthalein shows the end point (the pink just disappears, or just appears).
- pH below 7 is acidic (lower is more acidic), 7 is neutral, and above 7 is basic (higher is more basic). Measure with dry pH paper and a color chart, reading whole numbers.
- Neutralization is used to treat soil and wastewater and to relieve heartburn. Normal rain has a pH of about 5.6; acid rain is lower.
On to AP (for later)
The particle-level equation is the net ionic equation of AP topic 4.2. The sudden pH jump you saw near the end point is the hallmark of the titration curves in topic 8.5. The math behind pH is logarithms; lesson 2.12.3 in Stage 2 gives the exact definition of pH and explains why a pH difference of 1 means a tenfold difference in concentration.