What is the pH scale? Explain the importance of pH in everyday life with three examples.

1. 1 · Define in one line. pH is a scale from 0 to 14 that shows how acidic or basic a solution is; 7 is neutral.
2. 2 · Explain the scale. Below 7 is acidic and above 7 is basic. More H⁺ ions means a lower pH. Universal indicator shows the pH by its colour.
3. 3 · Draw the pH scale. Draw a bar from 0 to 14 and mark 7 as neutral. Label "acidity increases" towards 0 and "basicity increases" towards 14. Mark gastric juice (about 1.2), lemon juice (about 2.2), water (7), blood (7.4) and NaOH solution (14).
4. 4 · Give the examples.
   • Digestion: an antacid such as milk of magnesia neutralises extra stomach acid.
   • Tooth decay: starts below pH 5.5; basic toothpaste neutralises the acid.
   • Soil: plants need a suitable pH; lime is added to acidic soil.
   • Stings: baking soda, a mild base, eases the pain of a bee sting, which leaves an acid.
5. 5 · Link to real life. Our body works only between pH 7.0 and 7.8, and rain below pH 5.6 is acid rain, which harms river life.
6. 6 · Close in one line. So controlling pH keeps our body, teeth, soil and rivers healthy.

Scoring tip: Give each example with its number (5.5, 5.6, 7.0 to 7.8). Numbers make the answer look exact.