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SCIENCE | Class 7 (Startup Science) Solutions; Chapter 5: Different kinds of Materials



 A. Answer in Detail


 1. With the help of an experiment, explain the neutralization reaction.

Answer:  

Objective: To demonstrate the reaction between an acid and a base to form salt and water.  

Materials: Hydrochloric acid (HCl), sodium hydroxide (NaOH), phenolphthalein indicator, beaker, dropper, glass rod, evaporating dish.  

Procedure:  

1.  First we pour 10 mL of NaOH into a beaker.  

2. Add 2-3 drops of phenolphthalein. The solution turns pink (basic).  

3. Slowly add HCl while stirring. The pink color fades as neutralization occurs.  

4. Stop when the solution becomes colorless (pH 7).  

5. Evaporate the solution to obtain NaCl crystals.  

Conclusion: Acid + Base → Salt + Water.  



 2. How to identify NaOH and HCl using turmeric powder?

Answer: Turmeric acts as a natural indicator:  

  •  Yellow in acidic/neutral solutions.  
  •  Red in basic solutions.  

Steps:  

1.  First we add turmeric to both test tubes.  

2. Red color → NaOH (base).  

3. No color change (yellow) → HCl (acid).  



 3. Account for the following:

(a) Baking soda on ant bites

Answer:  Ant venom contains formic acid (HCOOH). Rubbing baking soda (NaHCO₃), a base, neutralizes the acid and relieves pain by reducing acidity.  


(b) Quicklime in acidic soil

Answer:  Quicklime (CaO) reacts with water to form Ca(OH)₂ , neutralizing acidic soil (H⁺ ions) and raises soil pH for better crop growth.  


(c) Antacid for acidity

Answer:  Excess stomach acid (HCl) causes heartburn (acidity).  

Antacids like Mg(OH)₂ neutralizes HCl and reduces stomach pH and hence relieving discomfort.  


B. Answer Briefly



 1. Differentiate between acidic and basic substances.

Answer:  

| **Property** | **Acids**    | **Bases**              |  

|----------------------|--------------------|----------------------------|  

| pH.      | <7 (e.g., lemon juice)  | >7 (e.g., soap)    |  

| Taste    | Sour                  | Bitter                             |  

| Litmus | Blue → Red     | Red → Blue                  |  



2. What are indicators? Give examples.

Answer:  

The Substances that changes color with pH are indicators.

- Natural: Turmeric, litmus, China rose.  

- Synthetic: Phenolphthalein (colorless→pink), methyl orange (red→yellow).  



 3. Window cleaning solution turns red litmus blue. Explain.

Answer:  

Window cleaning solution turns red litmus blue because it contains ammonium hydroxide (NH₄OH) which is a base. Bases turns red litmus blue.  



4. Why do you need indicators?

Answer:  We need indicators to visually identify acidic/basic nature using color changes without lab equipment.  



 5. How is China rose indicator prepared? 

Answer:  China rose indicator is prepared by the steps:

1. First we soak China rose petals in warm water.  

2. And then filtering the extract. The extract is the indicator.

When it reacts with -

- Acid: Dark pink.  

- Base: Green.  



 6. Why is NaCl safe despite corrosive reactants?  

Answer:  HCl (acid) and NaOH (base) are corrosive due to extreme pH.  

Neutralization removes H⁺/OH⁻, leaving NaCl (pH 7), which is non-corrosive.  That's why NaCl is safe despite corrosive reactants.


7. Why does a vegetable stain turn reddish-brown with soap?

Answer: A vegetable stain contains anthocyanins (pH-sensitive pigments). And the soap is basic so it raises pH value, changing pigment color to reddish-brown.  


8. Methyl orange in a bitter, soapy solution?

Answer: The Bitter/soapy solution is base. Methyl orange turns yellow in bases, if a drop of methyl orange is added to it.


 9. Chemical nature of distilled water?

Answer: Chemical nature of distilled water is neutral (pH 7). Contains equal H⁺ and OH⁻ ions.


C1. State whether the following statements are True (T) or False (F).

1. Question: Bases turn blue litmus red.  

   Answer: **F**  

   *Explanation:* Bases turn red litmus blue, not blue litmus red.  

2. Question: Phenolphthalein is colourless in basic medium.  

   Answer:**F**  

   *Explanation:* Phenolphthalein turns **pink** in basic medium.  

3. Question: If soil is too acidic, quicklime is added to it.  

   Answer: **T**  

   *Explanation:* Quicklime (calcium oxide) neutralizes acidic soil by reacting with acidity.  

4. Question: Sulphuric acid is present in the stomach.  

   Answer: **F**  

   *Explanation:* The stomach contains hydrochloric acid (HCl), not sulfuric acid.  

5. Question: China rose has different colours in acidic, basic, and neutral medium.  

   Answer: **T**  

   *Explanation:* China rose turns magenta (acidic), green (basic), and retains its original color (e.g., pink) in neutral medium.  


C2. Fill in the Blanks

1. When acid and base react, ______ and ______ are formed.  

   **Answer:** Salt, water  

   *Explanation:* Acid-base neutralization produces salt and water.  


2. China rose is ______ in acidic medium and ______ in basic medium.  

   **Answer:** Magenta, Green  

   *Explanation:* China rose turns magenta in acidic solutions and green in basic solutions.  


3. ______ are used to test acidic and basic nature of solution.  

   **Answer:** Indicators  

   *Explanation:* Indicators like litmus or phenolphthalein detect pH changes.  


4.  Calamine lotion contains ______.  

   **Answer:** Zinc carbonate  

   *Explanation:* Zinc carbonate neutralizes acidic substances (e.g., ant bites).  


5.  ______ indicator is reddish-brown in basic medium.  

   **Answer:** Turmeric  

   *Explanation:* Turmeric turns reddish-brown in basic conditions.  

C3. Matching Columns

1. Dilute hydrochloric acid→ (i) China rose → (c) Red.  

   *Reason:* Acids turn China rose indicator red/magenta.  

2. Vinegar → (ii) Blue litmus → (c) Red.  

   *Reason:* Acids turn blue litmus red.  

3. Dilute calcium hydroxide → (iii) Phenolphthalein → (c) Red.  

   *Reason:* Phenolphthalein turns pink in basic solutions; "red" is the closest option provided.  

4. Curd → (iv) Turmeric → (a) Yellow.  

   *Reason:* Turmeric remains yellow in acidic curd.  

5. Soap → (v) Red litmus → (b) Blue.  

   *Reason:* Bases turn red litmus blue.  


C4. Multiple Choice Questions

1. Question: Atul was given a colorless solution in a test tube. He put a drop of this solution on blue litmus paper. It remained blue in color. The colorless solution is  

   Answer: (d) nothing can be said.  

   *Explanation:* Blue litmus stays blue in neutral or basic solutions; no conclusion without additional tests.  


2. Question: A solution turns red litmus paper blue. If a drop of phenolphthalein is added to it,  

   Answer: (a) it turns pink.  

   *Explanation:* Red litmus turning blue confirms a basic solution; phenolphthalein turns pink in bases.  


3. Question: Which of the following statement is true?  

   Answer: (d) Bases are bitter in taste and soapy to touch.  

   *Explanation:* Bases are bitter (e.g., soap) while acids are sour (e.g., lemon).  


4. Question: Orange juice is sour in taste due to the presence of  

   Answer: (b) citric acid.  

   *Explanation:* Citrus fruits like oranges contain citric acid.  


5. Question: Which of the following is added to the soil if it is too basic?  

   Answer: (c) Organic matter  

   *Explanation:* Organic matter (e.g., compost) releases acids to neutralize basic soil.  

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