TOPIC 1: ACIDS, BASES AND SALTS

ACIDS, BASES AND SALTS

Acids have a sour taste. For example, lime juice (from local limes) and tamarind are acidic in nature.
Acids react with certain metals like zinc and magnesium to produce hydrogen gas. For example, when tamarind (which contains tartaric acid) is mixed with zinc, hydrogen gas is produced.
Acids react with bases to form water and salt in a process known as neutralization. For instance, when vinegar (acetic acid) is mixed with baking soda (sodium bicarbonate), a neutralization reaction occurs, producing carbon dioxide, water, and sodium acetate.
Acids turn blue litmus paper red.
Acids conduct electricity because they contain free hydrogen ions (H⁺) that move in solution.

Alkalis (Bases that dissolve in water)
Alkalis have a bitter taste. A local example is baking soda (sodium bicarbonate), which has a bitter taste.
Alkalis have a slippery feel. A local example is soap solution, which contains sodium hydroxide (NaOH), a strong alkali.
Alkalis neutralize acids to form water and salt. For example, limewater (calcium hydroxide) neutralizes acids like vinegar.
Alkalis turn red litmus paper blue.
Alkalis conduct electricity because they contain free hydroxide ions (OH⁻) in solution.

Bases (Not Soluble in Water)
A base is any substance that can neutralize an acid to form water and salt. Bases do not necessarily dissolve in water. A local example of a base is copper(II) oxide (a black powder), which is a base but does not dissolve in water.

Key Difference: All alkalis are bases, but not all bases are alkalis because not all bases dissolve in water to release hydroxide ions.

2. Classifying Locally Available Substances as Acids or Alkalis

Acids

• Tamarind (contains tartaric acid)
• Lemon juice (contains citric acid)
• Vinegar (contains acetic acid)
• Orange juice (contains citric acid)
• Milk (contains lactic acid)

Alkalis

• Baking soda (sodium bicarbonate, NaHCO₃)
• Soap solution (sodium hydroxide, NaOH)
• Limewater (calcium hydroxide, Ca(OH)₂)
• Washing soda (sodium carbonate, Na₂CO₃)
• Ammonia solution (NH₃ in water)

3. Differences Between Bases and Alkalis

Bases
A base is any substance that can neutralize an acid to form water and salt.
Example: Copper(II) oxide is a base but does not dissolve in water.

Alkalis
An alkali is a base that dissolves in water to form hydroxide ions (OH⁻).
Example: Sodium hydroxide (NaOH) is an alkali because it dissolves in water and forms hydroxide ions.

Key Difference: All alkalis are bases, but not all bases are alkalis, since alkalis must dissolve in water to release hydroxide ions.

4. Preparing Indicators from Locally Available Materials

An indicator is a substance that changes color in the presence of an acid or an alkali. You can easily make natural indicators from locally available materials:

Red Cabbage Indicator
Cut some red cabbage leaves and boil them in water for about 10 minutes.
Strain the liquid to obtain a purple solution.
In an acidic solution (e.g., vinegar), the color changes to red. In an alkaline solution (e.g., soap solution), the color changes to green or blue.

Turmeric Indicator
Take some turmeric powder, mix it with water, and you have an indicator.
In an acidic solution, turmeric remains yellow, but in an alkaline solution, it changes to red or brown.

Hibiscus Flower Indicator
Hibiscus petals (locally available) can be used to prepare an indicator.
The color of hibiscus extract changes in acidic or alkaline solutions (e.g., it turns red in acidic solutions and green in alkaline solutions).

5. The pH Scale: Understanding Acid and Alkaline Strength

The pH scale is used to measure how strong or weak an acid or alkali is. The scale ranges from 0 to 14:

• Acids have a pH less than 7. The stronger the acid, the closer the pH is to 0.
• Neutral substances have a pH of 7 (like pure water).
• Alkalis have a pH greater than 7. The stronger the alkali, the closer the pH is to 14.

Examples of pH values

• Strong Acid: Hydrochloric acid (pH 0-1)
• Weak Acid: Vinegar (pH 3)
• Neutral: Water (pH 7)
• Weak Alkali: Ammonia solution (pH 10)
• Strong Alkali: Sodium hydroxide (pH 14)

6. Importance and Influence of pH in Real Life

The strength of an acid or alkali (based on its pH value) has many practical applications:

Agriculture
The pH of soil affects plant growth. Some plants thrive in acidic soil (like pine trees), while others prefer alkaline soil (like cabbages).
Lime (calcium carbonate) can be added to acidic soils to make them more alkaline, improving plant growth.

Food and Beverages
Foods such as lemons and tamarind are acidic and contribute to the sour taste. On the other hand, baking soda is an alkali that can be used to reduce acidity in food.

Cleaning and Disinfection
Soaps and detergents are alkaline and help in cleaning because they neutralize acidic dirt and grease.

Environmental Impact
Acid rain (with pH lower than 5.6) results from industrial pollution and harms ecosystems. For example, acidic lakes and rivers can kill aquatic life.

Health and Medicine
The human stomach produces hydrochloric acid (pH 1-2) for digestion. Antacids are used to neutralize excess stomach acid.

7. Using the pH Scale to Classify Substances

Using the pH scale, we can easily classify substances as acidic, neutral, or alkaline:

Acidic Solutions: Substances with a pH less than 7 are acids (e.g., vinegar, lemon juice, tamarind).
Neutral Solutions: Substances with a pH of 7 are neutral (e.g., pure water).
Alkaline Solutions: Substances with a pH greater than 7 are alkalis (e.g., baking soda solution, soap solution).

The pH of a substance helps determine its potential uses and how it reacts in different situations, from food preparation to agriculture and cleaning.