Grade 5 (age 10-11)

Acid, Base and Indicators

The liquids around us can be acidic, basic, or neutral. Sour things like vinegar and lemon are acidic, while slippery soap and toothpaste are basic. We show their strength with pH, where below 7 is acidic and above 7 is basic. Slide the pH from 0 to 14 and watch three indicators change color.

What are acidic and basic?

pH7

🧪 Acidic and Basic

①Acidic: tastes sour (vinegar, lemon)

②Basic: feels slippery (soap, detergent)

③Neutral: neither acidic nor basic (water)

④Lower pH = stronger acid; higher pH = stronger base

Let's check with indicators!

🔬 What is an indicator?

①Indicator: substance that changes color with acid/base

②Litmus: acid red, base blue

③Phenolphthalein: acid colorless, base pink

④BTB: acid yellow, neutral green, base blue

Acid & Base in Daily Life

In Daily Life

Acid: vinegar, soda, lemon / Base: soap, detergent, toothpaste

Acids and bases are everywhere around us

💡 Why know this?

①Bee sting is basic — neutralize with vinegar (acid)

②Acid rain damages plants and buildings

③Too much stomach acid → take antacid (base)

④Soap (base) cleans acidic grease

Summary

Acid & Base Core

Acid (pH < 7) / Neutral (pH = 7) / Base (pH > 7)

Indicators reveal acid vs. base by color change

🎯 Key Points

①Acid: sour, dissolves metals, pH < 7

②Base: slippery, pH > 7

③Indicators reveal acid/base

④Litmus is the simplest

⑤Acid + base → neutralization

Examples and Unit Test

Example 1

Phenolphthalein turns pink in a solution. Is the solution acidic, neutral, or basic?

Phenolphthalein stays colorless in acid and turns pink in base.

Since it turned pink, the solution is basic.

▸ Basic

Phenolphthalein: colorless in acid, pink in base. A handy way to spot bases.

Example 2

A solution has pH 2. What happens to blue litmus paper dipped in it, and give an example of such a solution.

pH 2 is below 7, so the solution is acidic.

An acid turns blue litmus red; examples are vinegar and lemon juice.

▸ Blue litmus turns red / e.g. vinegar, lemon juice

Lower pH is more acidic. Acids turn blue litmus red.

Unit-test style

Which statement about acids, bases, and indicators is correct?

①Phenolphthalein turns pink in acid

②A pH below 7 is acidic

③BTB solution turns yellow in base

④Soap is an acidic substance

⑤Litmus paper never changes color

▸ ② A pH below 7 is acidic

pH 7 is neutral; below 7 is acidic and above 7 is basic.

Phenolphthalein is pink in base (①), BTB is blue in base (③), soap is basic (④), and litmus changes color with acids and bases (⑤).

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