Acids, Bases, and pH
Grade 7 · Science · Study Guide
Learn to identify acids and bases by their properties, understand the pH scale, and see how they neutralize each other.
Acids and bases are two of the most important categories of chemicals in everyday life and in your own body. You encounter them constantly — orange juice is an acid, baking soda is a base, and your stomach uses a powerful acid to digest food. Understanding the difference between them and how to measure their strength is a core part of chemistry.
An acid is a substance that donates hydrogen ions (H⁺) when dissolved in water. Acids taste sour (like lemon juice or vinegar), can corrode metals, and turn blue litmus paper red. Common acids include hydrochloric acid (HCl, found in your stomach), sulfuric acid (H₂SO₄, used in car batteries), and citric acid (in oranges and lemons). A base (or alkali) is a substance that accepts hydrogen ions or releases hydroxide ions (OH⁻) in water. Bases taste bitter, feel slippery (like soap), and turn red litmus paper blue. Common bases include sodium hydroxide (NaOH, used in drain cleaner), baking soda (sodium bicarbonate), and ammonia.
The pH scale measures how acidic or basic a solution is, running from 0 to 14. A pH of 7 is neutral (pure water). Values below 7 are acidic — the lower the number, the stronger the acid. Values above 7 are basic (alkaline) — the higher the number, the stronger the base. The pH scale is logarithmic: a pH of 5 is 10 times more acidic than a pH of 6, and 100 times more acidic than a pH of 7.
When an acid and a base react, they neutralize each other to produce a salt and water. This neutralization reaction is why antacid tablets (bases) relieve stomach acid — they literally neutralize excess HCl. Indicators like litmus paper and pH paper allow us to test solutions. Living things are highly sensitive to pH — your blood must stay between 7.35 and 7.45 or serious problems occur.
| Substance | Approximate pH | Acidic, Neutral, or Basic |
|---|---|---|
| Battery acid | 0–1 | Very acidic |
| Lemon juice | 2 | Acidic |
| Orange juice | 3–4 | Acidic |
| Pure water | 7 | Neutral |
| Baking soda | 8–9 | Basic |
| Bleach | 12–13 | Very basic |
Key terms
- Acid
- A substance that releases hydrogen ions (H⁺) in water; has a pH below 7.
- Base
- A substance that accepts H⁺ ions or releases OH⁻ ions in water; has a pH above 7.
- pH scale
- A logarithmic scale from 0–14 that measures the acidity or alkalinity of a solution.
- Neutral
- A solution that is neither acidic nor basic; pH of exactly 7 (pure water).
- Neutralization
- The reaction between an acid and a base that produces a salt and water.
- Indicator
- A substance (like litmus paper) that changes color to show whether a solution is acidic or basic.
Flashcards
- Acid
- Releases H⁺ ions in water; pH below 7; tastes sour; turns blue litmus red.
- Base
- Accepts H⁺ or releases OH⁻ in water; pH above 7; tastes bitter; turns red litmus blue.
- pH scale
- Scale from 0–14 measuring acidity (below 7) or alkalinity (above 7). 7 = neutral.
- Neutralization
- Acid + Base → Salt + Water. Each cancels the other's acidic/basic properties.
- Litmus paper
- An indicator paper: turns red in acid, turns blue in base.
- Logarithmic scale
- Each pH unit represents a 10× change in acidity (e.g., pH 4 is 10× more acidic than pH 5).
Practice questions
What pH value represents a neutral solution?
- 0
- 7
- 10
- 14
Show answer
Answer: 7
A pH of 7 is neutral — equal concentrations of H⁺ and OH⁻ ions. Pure water has a pH of exactly 7 at room temperature.
Acids turn red litmus paper blue.
Show answer
Answer: False
Acids turn BLUE litmus paper RED. Bases turn red litmus paper blue. A simple mnemonic: A(cid) → R(ed), B(ase) → B(lue to red is the opposite).
When an acid and a base react, they produce a salt and ___.
Show answer
Answer: water
Neutralization: Acid + Base → Salt + Water. For example: HCl + NaOH → NaCl (table salt) + H₂O. The products are typically far less reactive than the reactants.
A solution with a pH of 3 is:
- Neutral
- Basic
- Acidic
- Slightly basic
Show answer
Answer: Acidic
Any pH below 7 is acidic. pH 3 (like vinegar or cola) is significantly acidic. The closer to 0, the stronger the acid.
The pH scale is logarithmic, so a pH of 5 is 10 times more acidic than a pH of 6.
Show answer
Answer: True
The pH scale is logarithmic (base 10). Each whole number change = 10× difference in acidity. pH 4 is 10× more acidic than pH 5, and 100× more acidic than pH 6.
Fun facts
- Your stomach produces hydrochloric acid with a pH of about 1.5 to 2 — strong enough to dissolve metal given enough time.
- The pH of the ocean has dropped from 8.2 to about 8.1 since the Industrial Revolution — that 0.1 difference represents a 26% increase in acidity.
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