Exploring Substances: Acidic, Basic, and Neutral - Comprehensive Science Notes

Introduction: The Science Fair Adventure

  • Event Information: On 28 February, a school hosted a science fair to celebrate National Science Day.
  • The Entry Mystery: Siblings Ashwin and Keerthi received a white sheet of paper at the gate. A volunteer sprayed a liquid on the sheets, causing the words ‘Welcome to the Wonderful World of Science’ to appear (Fig. 2.1).
  • The Colourful World of Substances Stall: The siblings explored activities showing colour changes upon mixing different substances, leading to a learning adventure regarding the nature of substances.

Nature — Our Science Laboratory

2.1.1 Litmus as an Indicator

  • Litmus Definition: Litmus is a natural substance obtained from lichens. It is available as a solution or as paper strips in two colours: blue and red (Fig. 2.3).
  • Classification of Substances:
    • Acidic: Substances that turn blue litmus paper to red.
    • Basic: Substances that turn red litmus paper to blue.
    • Neutral: Substances that do not affect the colour of either red or blue litmus paper.
  • Lichen Biology (Holistic Lens): Lichens are formed by the association of a fungus and an alga. They grow on rocks and trees in regions with abundant rainfall and clean air.
  • Indicator Definition: A substance that shows different colours in acidic and basic solutions is called an acid-base indicator.
Activity 2.1: Exploring Samples with Litmus
  • Samples Collected: Lemon juice, soap solution, amla juice, tamarind water, vinegar, baking soda solution, lime water, tap water, washing powder solution, sugar solution, and salt solution.
  • Procedure: Strips of blue and red litmus are placed on a clean white tile. Using a dropper, one drop of each sample is applied to check for colour changes (Fig. 2.2a and 2.2b).
  • Sorting Results (Table 2.2):
    • Group A (Acidic): Lemon juice, amla juice, tamarind water, vinegar.
    • Group B (Basic): Soap solution, baking soda solution, lime water, washing powder solution.
    • Group C (Neutral): Tap water, sugar solution, salt solution.
Preparation of Lime Water
  • Definition: Lime water is a solution of calcium hydroxide in water. It is not the same as the fruit "lime."
  • Preparation Steps: Mix lime (chuna/calcium oxide) in water. Leave it undisturbed for approximately one hour. Filter the liquid into another container to use as lime water.

Properties of Acids and Bases

Acids

  • Taste: Edible acidic substances taste sour.
  • Examples and Acid Names (Fig. 2.4):
    • Tamarind: Contains Tartaric acid.
    • Amla: Contains Ascorbic acid (Vitamin C) and Citric acid.
    • Citrate/Oxalate Sources: Fig. 2.4 lists Citric acid and Oxalic acid.
    • Others: Lemon contains Citric acid; Curd contains Lactic acid (implied by Fig 2.4 prompts).
  • Caution: Students are warned not to taste unknown substances or anything unless specifically asked.

Bases

  • Physical Properties: Basic substances generally feel soapy or slippery to the touch (e.g., baking soda solution rubbed between fingers).
  • Taste: Bases generally taste bitter.
  • Exceptions: Everything that tastes bitter is not necessarily basic. For example, bitter gourd (karela) is bitter but not basic.

Natural Indicators

2.1.2 Red Rose as an Indicator

  • Preparation (Activity 2.3): Collect fallen red rose petals. Wash and crush them using a mortar and pestle. Immerse them in hot water for 5–10 minutes until the water becomes coloured (Fig 2.6). Filter to obtain the extract.
  • Indicator Behavior:
    • Acidic Solutions: Turn the extract red (e.g., lemon juice).
    • Basic Solutions: Turn the extract green (e.g., soap solution).
  • Other Examples: Beetroot, purple cabbage, turmeric, Indian blackberry (jamun), and red hibiscus (gudhal).
  • Fascinating Fact (Hydrangea): This plant grows in the Himalayan region and North-eastern states. Flower colour depends on soil nature:
    • Acidic soil: Produces blue flowers.
    • Basic soil: Produces pink or red flowers.

2.1.3 Turmeric as an Indicator

  • Preparation (Activity 2.5): Mix turmeric (haldi) with a little water to make a paste. Dip filter paper in the paste to turn it yellow. Dry and cut into strips (Fig 2.9).
  • Indicator Behavior:
    • Basic Substances: Turn yellow turmeric paper to red.
    • Acidic/Neutral Substances: Leave the yellow colour unchanged.
  • Application Case: Ashwin created a Guru Purnima card by writing in Odia on turmeric paper using a basic solution (e.g., soap solution), causing the words to appear red.
  • Historical Note: Turmeric is a member of the ginger family. In Ayurveda, it is considered to have numerous health benefits and is known as the "Golden" spice.

2.1.4 Olfactory Indicators

  • Definition: Substances whose odours change in an acidic or basic medium.
  • Activity 2.6: Finely chopped onions are stored overnight with cotton cloth strips in a closed container.
    • Testing with tamarind water (acidic) vs. baking soda solution (basic) shows a change in the characteristic smell of the onion.

Acharya Prafulla Chandra Ray

  • Title: Known as the ‘Father of Modern Indian Chemistry’.
  • Education: Earned a doctorate in chemistry from the United Kingdom.
  • Career: Established India’s first pharmaceutical company in 1901.
  • Contributions: Highlighted the achievements of ancient Indian scientists through writings on the history of chemistry. He advocated for the use of the mother tongue in education.

Neutralisation Reactions

2.2 Concept and Experimentation

  • Reaction Mechanics: When an acid solution is mixed with a basic solution in sufficient quantity, the resulting solution is neither acidic nor basic.
  • Experiment (Activity 2.7):
    1. Add blue litmus to lemon juice (it turns red).
    2. Slowly add lime water (base).
    3. The solution eventually changes from red to blue, indicating it is no longer acidic.
  • Chemical Equation:     Acid+Base→Salt+Water+Heat\text{Acid} + \text{Base} \rightarrow \text{Salt} + \text{Water} + \text{Heat}
  • Observations: The reaction involves the evolution (release) of heat.

2.3 Neutralisation in Daily Life

  • Situation 1: Ant Bites: Red ants inject formic acid into the skin, causing stinging pain. Moist baking soda (a base) is applied to neutralise the acid.
  • Situation 2: Soil Treatment:
    • Acidic Soil: Caused by excessive chemical fertilisers; treated with lime (a base) to help plant growth.
    • Basic Soil: Treated with organic matter (manure/compost), which releases acids to neutralise the soil.
  • Situation 3: Industrial Waste: Factory waste released into lakes can be acidic and harmful to fish. It must be neutralised with basic substances before being discharged.

Let Us Enhance Our Learning (Review & Analysis)

  • Indicator Logic:
    • If a solution turns red litmus blue, it is basic. Adding an acid (like vinegar) in excess reverses the change.
    • Amla juice (acidic) added to a basic solution (green in rose extract) will turn it toward the red spectrum due to neutralisation or acidification.
  • Laboratory Scenarios:
    • Manya (Blindfolded testing): Manya should use olfactory indicators (like onion strips) because she cannot see colour changes but can detect changes in scent.
    • Bubbling Observation: Aman saw bubbles when vinegar (acid) hit eggshells or marble (calcium carbonate). No bubbles occurred with soap because soap is a base and does not react with carbonates to release gas (carbon dioxide) in the same way.
  • Secret Messages:
    • Orange juice (acidic) used as ink can be revealed using an indicator extract (like red rose or red cabbage) that changes colour when it reacts with the acid on the paper.
    • The science fair entry paper used a turmeric spray (yellow) reacting with a soap solution (base) used to write the message, causing the red "Welcome" words to appear.