Ch 02 - Exploring Substances: Acidic, Basic, and Neutral7th Science

• Acids: Taste sour and do not feel slippery (e.g., Lemon juice, Vinegar).
• Bases: Taste bitter and feel soapy/slippery to the touch (e.g., Baking soda, Soap solution).

1. Vinegar: Acetic Acid
2. Ant Sting: Formic Acid
3. Citrus Fruits (Lemon/Orange): Citric Acid
4. Curd: Lactic Acid
5. Tamarind/Grapes: Tartaric Acid
6. Spinach: Oxalic Acid

• Lime Water: Calcium Hydroxide [$Ca(OH)_2$]
• Window Cleaner: Ammonium Hydroxide [$NH_4OH$]
• Soap: Sodium Hydroxide [$NaOH$] or Potassium Hydroxide [$KOH$]

(Note: The presence of the Hydroxide (OH⁻) ion is the primary chemical signature of these bases).

A neutral substance is one that is neither acidic nor basic and does not change the colour of any indicators (like litmus).

Examples:

1. Pure/Distilled Water [$H_2O$]
2. Sugar Solution
3. Common Salt Solution [$NaCl$]

Chemists use special testing substances called Indicators.
An indicator physically changes colour when brought into contact with an acidic or basic medium. Common natural examples include Litmus, Turmeric, and China Rose.

Litmus is extracted from Lichens (it has a mauve/purple colour in distilled water).

Biology Connection: Lichens are a symbiotic association between two organisms:

• Alga: Contains chlorophyll and synthesizes food via photosynthesis.
• Fungus: Provides structural shelter, water, and essential minerals to the alga.

• Acids: Turn Blue litmus Red. (Does not affect Red litmus).
• Bases: Turn Red litmus Blue. (Does not affect Blue litmus).
• Neutral: No colour change in either strip.

Notes:

• Salt: Can be acidic, basic, or neutral.
• Heat: Energy is always evolved (exothermic), raising the temperature of the reaction mixture.

Reaction: Turmeric (naturally yellow) turns Deep Red in the presence of a Base (like alkaline soap).

Limitation: It cannot distinguish between an acid and a neutral substance because it remains Yellow in BOTH acidic and neutral solutions.

• In Acid: Turns Dark Pink / Magenta.
• In Base: Turns Green.

The petal colour of Hydrangea flowers changes based on the surrounding soil chemistry:

• Acidic Soil: Produces Blue flowers.
• Basic (Alkaline) Soil: Produces Pink or Red flowers.

1. Purple Cabbage extract
2. Beetroot extract
3. Indian Blackberry (Jamun)

By utilizing Olfactory Indicators, which change their odour in different mediums.

• In a Base: The characteristic strong smell of onion or vanilla is destroyed (cannot be detected).
• In an Acid: The characteristic smell remains unchanged.

During neutralisation, the acidic and basic properties mutually destroy each other.
The reaction forms a new ionic compound called a Salt, produces Water, and always releases energy in the form of Heat.

No. The nature of the salt depends entirely on the relative strengths of the reacting acid and base.

• Strong Acid + Strong Base $\rightarrow$ Neutral Salt
• Strong Acid + Weak Base $\rightarrow$ Acidic Salt
• Weak Acid + Strong Base $\rightarrow$ Basic Salt

• Eggshell: Made of Calcium Carbonate ($CaCO_3$).
• Vinegar: Contains Acetic Acid.
• Reaction: Acid + Carbonate $\rightarrow$ Carbon Dioxide Gas ($CO_2$).
• The bubbles are the $CO_2$ gas escaping.

• Chemical Injected: Formic Acid (causes the burning pain).
• Remedy: Apply mild bases such as Moist Baking Soda (Sodium Hydrogen Carbonate, $NaHCO_3$) or Calamine Solution (Zinc Carbonate, $ZnCO_3$).
• Mechanism: The base neutralizes the injected acid via a localized neutralization reaction.

• If Too Acidic (from fertilizers): Farmers add strong bases like Quick Lime (Calcium Oxide, $CaO$) or Slaked Lime (Calcium Hydroxide, $Ca(OH)_2$).
• If Too Basic (Alkaline): Farmers add Organic Matter (Compost/Manure). As organic matter decomposes, it naturally releases acids that neutralize the excess alkalinity.

Industrial effluents often contain highly concentrated acids that can cause mass mortality in aquatic ecosystems (killing fish and flora).
Rectification: Before discharge, factories must deliberately add basic substances to the waste vats to neutralize the corrosive acids.

Variables:

• $CaCO_3$: Calcium Carbonate (Marble/Eggshell)
• $CH_3COOH$: Acetic Acid (Vinegar)
• $CO_2$: Carbon Dioxide gas (The bubbles)

Note:

• This reaction is characteristic of Carbonates reacting with Acids.

Primary airborne pollutants combine with moisture in clouds to form corrosive precipitation:

• Carbon Dioxide ($CO_2$) $\rightarrow$ forms Carbonic Acid ($H_2CO_3$).
• Sulphur Dioxide ($SO_2$) $\rightarrow$ forms Sulphuric Acid ($H_2SO_4$).
• Nitrogen Dioxide ($NO_2$) $\rightarrow$ forms Nitric Acid ($HNO_3$).
  (This acidic rainfall severely degrades soil, plants, and historical marble monuments).

Mineral acids (like Sulphuric or Hydrochloric acid) are extremely Corrosive.
They cause rapid and severe chemical burns to biological tissue and materials. They must be handled with appropriate protective equipment and should never be touched with bare skin or tasted under any circumstances.

Acharya P.C. Ray is honored as the Father of Modern Indian Chemistry.

• Industry: He founded Bengal Chemicals in 1901, India's first pharmaceutical enterprise.
• Education: He was a strong advocate for empirical scientific research and insisted that foundational science be taught using students' mother tongue.

The temperature rises.

• Neutralisation is an Exothermic Reaction.
• Heat is always evolved (produced) during the formation of salt and water.