This presentation takes you through every page of the chapter — with all questions, answers, hints, and key points to help you revise confidently!
Jinu's teacher poured a pink liquid from one tumbler into an empty tumbler — and it turned yellow!
Q: What could be the secret behind the experiment? Why did the pink liquid turn yellow?
Simple version: Why did the colour of the liquid change when poured into another glass?
The pink liquid was pathimugam water (a natural indicator). The second tumbler had a substance like baking soda or ash suspension (a base) which changed the colour of pathimugam water from pink to yellow.
Pathimugam water is a natural indicator — it changes colour in the presence of acids or bases.
Q: What materials are kept in the Science Kit for this chapter?
Answer: Transparent glass tumblers, pink-coloured pathimugam water, vinegar, tamarind water, lemon juice, salt, ash, lime, baking soda, and buttermilk.
Many of these are common kitchen items — science is all around us!
Answer: Litmus is a dye made from the extract of lichens (plants that grow on trees and rocks). It helps identify acids and bases by changing colour.
Answer: Blue litmus paper and Red litmus paper. Both are available in school laboratories. Litmus can also be dissolved in water to make litmus solution.
Dip blue and red litmus papers in various liquids. What colour changes do you observe?
Q: Which liquids turned blue litmus into red?
Answer: Lemon juice, vinegar, tamarind water, buttermilk — all acids turn blue litmus red.
Q: Which liquids turned red litmus into blue?
Answer: Lime water, soap water, baking soda solution, ash suspension — all bases turn red litmus blue.
What happens when you rub a red Hibiscus flower on white paper and dip it in an acidic liquid?
Answer: The paper (originally pink/red from hibiscus) changes colour in acidic liquids — it can be used like blue litmus paper to detect acids.
What happens when the hibiscus paper turns red and is dipped in a base?
Answer: It can then be used like red litmus paper to detect bases.
💡 Hint: Hibiscus flower contains a natural pigment that acts as an indicator — just like litmus!
Answer: Indicators are substances that help identify acids and bases by changing their colour. Litmus paper is an indicator.
Answer: Phenolphthalein and Methyl Orange.
For Acids: Methyl Orange, Blue litmus paper, Hibiscus paper, Pathimugam water.
For Bases: Phenolphthalein, Red litmus paper, Turmeric.
Add Phenolphthalein and Methyl Orange to various liquids. What colour changes do you observe?
Answer: It is a mixture of many indicators that can identify both acids and bases. It gives different colours depending on the nature and concentration of the substance.
Answer: By comparing the colour produced with the colour chart on the bottle.
💡 Hint: Universal Indicator is more detailed than litmus — it can tell you HOW acidic or basic something is.
Answer: They all taste sour. All acids have a sour taste.
Answer: Soap tastes alkaline (bitter/soapy). Soap is basic in nature. All bases have an alkaline taste.
Answer: Soap water, baking soda solution, lime water, ash suspension — all bases feel slippery.
Answer: Bases are slippery to touch.
Q: Which of these can turn blue litmus red — Orange juice, Rice soup, Black tea, Bilimbi juice, Grape juice, Tomato juice, Coconut water?
Answer: Orange juice, Black tea, Bilimbi juice, Grape juice, Tomato juice — these are acidic and will turn blue litmus red.
Rice soup and Coconut water are generally neutral or mildly alkaline — they may NOT turn blue litmus red.
💡 Hint: If a food item tastes sour, it likely contains acid and will turn blue litmus red!
- Q1: Acid in buttermilk and curd? → Lactic acid
- Q2: Acid in vinegar? → Acetic acid
- Q3: Acid in lemon? → Citric acid
- Q4: Acid in gooseberry? → Ascorbic acid (Vitamin C)
Answer: Curd contains a bacteria called Lactobacillus. These bacteria produce lactic acid when they feed on milk — this gives curd its sour taste.
Answer: A little curd (containing Lactobacillus bacteria) is added to boiled and cooled milk. The bacteria multiply and produce lactic acid, turning the milk into curd.
💡 Hint: Sour taste = acid present. Lactobacillus bacteria are the "workers" that make curd sour!
Answer: Ants contain formic acid in their bodies. When they bite, this acid enters our body and reacts with human tissue, causing pain.
Answer: Formic acid
💡 Hint: "Formic" comes from "Formica" — the Latin word for ant!
Answer: Hydrochloric acid (HCl) is produced in the stomach to help digest food.
Answer: When too much hydrochloric acid is produced in the stomach, it causes acidity. Symptoms: abdominal pain, heartburn, nausea, constipation.
Answer: Antacids — medicines that neutralize the excess acid in the stomach. "Anti" = against, "acid" = acid. Antacids work AGAINST excess acid!
Answer: Bubbles are produced. A gas is released that pushes at your thumb when you close the test tube.
Answer: Hydrogen gas — it burns with a pop sound when a burning matchstick is brought near it.
Answer: Yes! Hydrogen gas is produced in both cases. This is a general property of acids.
💡 Hint: Acid + Metal → Salt + Hydrogen gas. The pop sound confirms hydrogen!
Answer: Henry Cavendish, a British scientist.
Answer: "Producing water" — hydrogen reacts with oxygen to form water.
Answer: Used as fuel in rockets, hydrogen-powered vehicles, and hydrogen-filled balloons (lightest gas). In September 2023, a hydrogen bus was introduced in New Delhi.
Answer: Caustic soda (180g), coconut oil (1 kg), water (350 ml), sodium silicate (100g), stone powder (100g), colour and perfume.
Answer: A large amount of heat is liberated — so the solution must cool before mixing with oil.
Answer: Caustic soda (sodium hydroxide) is a strong base that can cause burns on skin.
💡 Hint: Soap is made by mixing a base (caustic soda) with oil — this is called saponification!
Q1: Which plant parts work as natural indicators?
Red spinach, Blue clitoria (Sanghupushpam), Red cabbage, Beetroot — and others you can explore!
Q2: How do you prepare natural indicator paper strips?
Rub the coloured part of the plant on paper strips, OR use the juice, OR boil in water and use the coloured liquid. Dry the strips and test with acids and bases.
💡 Hint: Different plants give different colours in acids and bases — that's what makes them indicators!
Basic Science | Guided Revision Presentation