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Acids and Bases
We use many everyday items like lemon, tamarind, salt, sugar, and vinegar. Do they all taste the same? Let’s explore their tastes!
Tastes of Substances
- Some substances are sour, like lemon.
- Others are bitter, sweet, or salty.
Acids and Bases
- Acids:
- Found in sour-tasting foods like curd, lemon juice, orange juice, and vinegar.
- The word “acid” comes from the Latin word acere, meaning sour.
- Examples of natural acids:
- Acetic acid in vinegar.
- Formic acid in ant stings.
- Citric acid in citrus fruits (oranges, lemons).
- Lactic acid in curd.
- Oxalic acid in spinach.
- Ascorbic acid in amla, citrus fruits (Vitamin C).
- Tartaric acid in tamarind, grapes, unripe mangoes.
- Bases:
- Bitter in taste and feel soapy e.g. baking soda.
- Examples of bases:
- Calcium hydroxide in lime water.
- Ammonium hydroxide in window cleaner.
- Sodium hydroxide in soap.
- Potassium hydroxide.
- Magnesium hydroxide in milk of magnesia.
Testing Acids and Bases: Indicators
Special substances called indicators help us test if something is acidic or basic.
- Litmus:
- A natural dye from lichens.
- Purple in distilled water.
- Turns red in acidic solutions.
- Turns blue in basic solutions.
- Available as litmus paper (red and blue).
Activity 4.1: Testing with Litmus Paper (click here)
- Mix water with lemon juice.
- Put a drop on red litmus paper. Note the color change.
- Repeat with blue litmus paper.
- Try with other substances like tap water, detergent solution, vinegar, etc.
- Some substances won’t change color; these are neutral.
- Neutral Solutions:
- Do not change the color of red or blue litmus paper.
More to Know:
- Lime Water:
- Mix water with lime (chuna), stir, and let it settle. The top clear part is lime water.
Other Natural Indicators
- Turmeric:
- Turns red with basic solutions.
Activity 4.2: Testing with Turmeric Paper
- Make turmeric paste and apply it to blotting paper to make turmeric paper.
- Test with soap solution and other substances.
- China Rose (Gudhal):
- Turns acidic solutions dark pink (magenta).
- Turns basic solutions green.
Activity 4.3: Testing with China Rose
- Make China rose indicator by soaking petals in warm water.
- Test with various solutions.
More Activities
Activity 4.4: Using Lab Chemicals
Test hydrochloric acid, sulphuric acid, nitric acid, acetic acid, sodium hydroxide, ammonium hydroxide, and calcium hydroxide with litmus, turmeric, and China rose indicators.
Acid Rain
- Acid Rain:
- Caused by pollutants like carbon dioxide, sulphur dioxide, and nitrogen dioxide.
- These gases dissolve in rainwater to form acids (carbonic acid, sulphuric acid, nitric acid).
- Acid rain damages buildings, historical monuments, plants, and animals.
Neutralisation
We know that acids turn blue litmus red and bases turn red litmus blue. Let’s see what happens when we mix an acid with a base using phenolphthalein as an indicator.
Activity 4.5: Demonstration by Teacher
- Materials: Dilute hydrochloric acid, phenolphthalein solution, sodium hydroxide solution.
- Steps:
- Fill one-fourth of a test tube with dilute hydrochloric acid and note its color.
- Add 2-3 drops of phenolphthalein and observe the color.
- Add sodium hydroxide solution drop by drop until the solution turns pink.
- Add one more drop of hydrochloric acid and observe the color change to colorless.
- Add one drop of sodium hydroxide again to see the solution turn pink.
Observations:
- Phenolphthalein turns pink in a basic solution and stays colorless in an acidic solution.
- When an acid is mixed with a base, they neutralize each other, forming salt and water.
- Heat is produced during neutralisation, making the test tube warm.
Neutralisation Reaction:
- Definition: The reaction between an acid and a base that produces salt and water with the evolution of heat.
- Example:
- Hydrochloric acid (HCl) + Sodium hydroxide (NaOH) → Sodium chloride (NaCl) + Water (H2O)
Neutralisation in Everyday Life
Indigestion
- Our stomach contains hydrochloric acid to digest food.
- Too much acid causes indigestion.
- Solution: Antacids like milk of magnesia (magnesium hydroxide) neutralize excess acid.
Ant Bite
- Ant bites inject formic acid into the skin.
- Solution: Rubbing moist baking soda (sodium hydrogencarbonate) or calamine solution (zinc carbonate) neutralizes the acid.
Soil Treatment
- Excessive chemical fertilizers make soil acidic.
- Plants do not grow well in too acidic or too basic soil.
- Solution:
- Acidic soil: Treated with bases like quick lime (calcium oxide) or slaked lime (calcium hydroxide).
- Basic soil: Treated with organic matter (compost) which releases acids to neutralize the basic nature.
Factory Wastes
- Factory wastes often contain acids that can harm water bodies.
- Solution: Neutralizing factory wastes with basic substances before disposal.
Key Points
- Neutralisation involves mixing an acid with a base to produce salt and water with heat.
- It is useful in treating indigestion, ant bites, acidic soil, and factory waste management.
Chapter Summary:
- Acids are sour in taste.
- Bases are generally bitter in taste and soapy to touch.
- Acid turns blue litmus red.
- Bases turn red litmus blue.
- Substances which are neither acidic nor basic are called neutral.
- Solutions of substances that show different colors in acidic, basic, and neutral solutions are called indicators.
- An acid and a base neutralize each other and form a salt.
- A salt may be acidic, basic, or neutral in nature.
Keywords
| Number | Keyword | Number | Keyword |
|---|---|---|---|
| 1 | Acid | 5 | Indicator |
| 2 | Acidic | 6 | Neutral |
| 3 | Base | 7 | Neutralisation |
| 4 | Basic | 8 | Salt |
