When acid meets base
They cancel each other and make salt, water and heat.
The ant bite
A red ant bit Keerthi. The bite left her skin red and stinging. Her brother rubbed moist baking soda on it, and the pain eased. The ant's bite injects an acid.
Why do you think the baking soda helped?
What this lesson covers
The idea
An acid and a base mixed in sufficient quantity neutralise each other, leaving a solution that is neither acidic nor basic: Acid + Base → Salt + Water + Heat.
The ant bite
A red ant bit Keerthi. The bite left her skin red and stinging. Her brother rubbed moist baking soda on it, and the pain eased. The ant's bite injects an acid.
Why do you think the baking soda helped?
- Baking soda is a base. It cancels the acid.
- Baking soda is cold, so it numbs the skin.
- It washes the ant away.
Drop by drop
The beaker has lemon juice (an acid) with litmus solution in it, so it is red. Add lime water (a base) drop by drop. Find the purple point, then add more to see what too much does.
Neutralisation
The colour went red → purple → blue. At purple the liquid was neither acidic nor basic. The base had used up the acid. A little heat was given out, too.
An acid and a base mixed in sufficient quantity neutralise each other, leaving a solution that is neither acidic nor basic: Acid + Base → Salt + Water + Heat.
- Everyday problem | Fix by neutralisation
- Red ant bite (an acid, formic acid) | moist baking soda (a base)
- Soil too acidic (too much chemical fertiliser) | lime (a base)
- Soil too basic | manure or compost (organic matter releases acids)
- Factory waste that would harm fish | add basic substances before it is released
Notes
An acid and a base mixed in sufficient quantity neutralise each other: Acid + Base → Salt + Water + Heat. The solution is neither acidic nor basic.
Check yourself
What do an acid and a base make when they neutralise each other?
A farmer's soil is too acidic after a lot of chemical fertiliser. What should be added?
Blue litmus solution turned red in lemon juice. After a lot of lime water was added it turned blue. What does blue show?
Why is moist baking soda rubbed on a red ant bite?
- Only an acid. The acid is used up; it does not just stay.
- Only water. Water forms, but salt forms too.
- Salt and water, with heat given out — correct. Yes! Acid + Base → Salt + Water + Heat.
- More acid. That would make it worse.
- Lime, which is a base — correct. Yes! A base neutralises the acid in the soil.
- Lemon juice. Lemon juice is an acid. It would make things worse.
- The mixture is acidic again. Blue is the colour for a base, not an acid.
- The mixture is neutral. Neutral looks purple. Blue shows too much base.
- The mixture is now basic: too much lime water was added — correct. Yes! Past the neutral point the extra base makes it basic.
- The sting is a base, and baking soda is an acid. It is the other way round: the sting has an acid.
- The sting is an acid, and baking soda is a base that neutralises it — correct. Yes! Acid + base cancel each other.
- Baking soda is a neutral liquid. It acts as a base here.