Grade 7 Ch 2 Exploring Substances: Acidic, Basic & Neutral

 

Exploring Substances: Acidic, Basic, and Neutral

1. Nature – Our Science Laboratory

Indicator: The substances that change their colour on coming in contact with other substances, especially acids or bases, are called indicators.

Natural Indicator: Natural indicators are made from plants or other natural sources. For example, litmus (from lichens), red rose extract, and turmeric are all natural indicators.

Synthetic Indicator: Synthetic indicators are man-made chemicals prepared in laboratories.

Olfactory Indicators: These are the substances that change their smell when they come in contact with acids or bases. For example, vanilla extract and onion are olfactory indicators.

Acidic Substances: The substances that taste sour because they contain acids, are said to be acidic in nature. You may have tasted some edible substances that taste sour. For example, grapes, unripe mango, curd, vinegar, tamarind and lemon all contain acids.

Basic Substances: The substances which are bitter in taste and feel soapy to touch are said to be basic in nature.

Neutral Substances: Substances that are neither acidic nor basic are called neutral substances.

Litmus as an Indicator

1. What is Litmus?

• Litmus is a natural substance from lichens (fungus and alga combination).
• Available as blue and red paper strips or solution.
• It’s an acid-base indicator that changes colour in acidic or basic solutions.

2. How it Works:

• Acidic substances (e.g., lemon juice, vinegar) turn blue litmus red.
• Basic substances (e.g., soap solution, lime water) turn red litmus blue.
• Neutral substances (e.g., tap water, sugar solution) don’t change litmus colour.

Indicators and Their Role:

• Identify acids and bases by changing colour.
• Can be natural or synthetic (made in labs).

Acids:

• Sour-tasting substances contain acids.
• Common acids in edible items are:
• Lemon            Citric acid
• Curd                Lactic acid
• Tamarind        Tartaric acid
• Vinegar           Acetic acid
• Raw Mango    Tartaric acid
• Tomato           Citric acid & Oxalic acid

Bases:

• Usually taste bitter.
• But not all bitter substances are bases.
• Example: Bitter gourd (karela) is bitter but not basic.
• Feel slippery when touched.

3. Key Substances:

• Acidic: Lemon juice, amla juice, tamarind water, vinegar.
• Basic: Soap solution, baking soda solution, lime water, washing powder solution.
• Neutral: Tap water, sugar solution, salt solution.

4. Lime Water: Made by mixing calcium oxide (lime) with water, letting it sit, and filtering.

2. Red Rose as an Indicator

Steps to prepare red rose indicator:

1. Fallen red rose petals are washed..

2. Petals are crushed using a mortar and pestle..

3. Hot water is poured into a glass tumbler with petals.

4. The tumbler is covered & left for 5-10 minutes..

5. The liquid is filtered to obtain the flower extract indicator.

Activity: Testing samples with red rose indicator

1. 10-20 drops of red rose extract are added to 2 test tubes.

2. They are labeled A & B..

3. Lemon juice (20-30 drops) is added to test tube A

4. Soap solution (20-30 drops) is added to test tube B.

5. Colour changes are observed & recorded.

6. The test is repeated with other samples.

Name of the sample       Colour of the red rose extract        Nature

1. Lemon juice                        Turns red                                 Acidic

2. Soap solution                       Turns green                             Basic

3. Amla juice                           Turns red                                 Acidic

4. Tamarind water                  Turns red                                 Acidic

5. Vinegar                                Turns red                                 Acidic

6. Baking soda solution           Turns green                             Basic

7. Lime water                          Turns green                             Basic

8. Tap water                            No change                               Neutral

9. Washing powder solution   Turns green                             Basic

10. Sugar solution                   No change                               Neutral

11. Salt solution                      No change                               Neutral

                                Observations of Red rose extract

                    Acid                                                    Base                                    Neutral substances

                     ↓                                                              ↓                                                     ↓

Turns red rose extract red                 Turns red rose extract green.            No colour change

* Hence, red rose extract is an acid-base indicator.

Other Natural Indicators: Beetroot, purple cabbage, turmeric Indian blackberry (jamun), red hibiscus (gudhal).   

 Fascinating Fact:

• Hydrangea flowers change colour based on soil:
• Acidic soil: Blue flowers.
• Basic soil: Pink or red flowers.

3. Turmeric as an Indicator

Turmeric:

• Acts as an indicator for basic substances.
• Turns red in basic solutions (e.g., soap solution).
• Stays yellow in acidic or neutral solutions.

Activity: Steps to Prepare and Use Turmeric Paper

1. Turmeric (haldi) is mixed with water to form a paste.

2. Filter paper is dipped into the paste until it turns yellow.

3. The paper is dried..

4. It is cut into thin strips (turmeric paper").

5. A drop of each sample is placed on strips using a dropper.

6. Changes are observed & recorded.

Name of the sample               Colour of turmeric paper                  Nature

1. Lemon juice                        Yellow (No. Change)              Cannot be determined

2. Soap solution.                      Turns red                                 Basic

3. Amla juice                           Yellow (No Change)             Cannot be determined                                   

4. Tamarind water                  Yellow (No Change)              Cannot be determined

5. Vinegar                                Yellow (No Change)             Cannot be determined

6. Baking soda solution.          Turns red                                 Basic

7. Lime water                          Turns red                                 Basic

8. Tap water                            Yellow (No Change)              Cannot be determined

9. Washing powder solution   Turns red                                 Basic

10. Sugar solution                   Yellow (No Change)              Cannot be determined

11. Salt solution.                     Yellow (No Change)              Cannot be determined

 

 Fascinating Fact:

• Turmeric is a “Golden” spice used in food, Ayurvedic medicine, and known for health benefits.

Olfactory Indicators: Onion Experiment

1. Chopped onions & cotton/filter paper strips are placed in a container.

2. The container is sealed overnight.

3. Two strips are taken out & their odour is checked.

4. Tamarind water (acidic) is added to one strip, & baking soda solution (basic) to the other.

5. After the liquid spreads, odour is checked again.

6. Changes are observed & recorded.

7. The test is repeated with other samples

Name of the sample               Effect of onion Odour             Nature

1. Lemon juice                        No Change                               Cannot be determined

2. Soap solution                       Odour disappears.                   Basic

3. Amla juice                           No Change                               Cannot be determined

4. Tamarind water                  No Change                               Cannot be determined

5. Vinegar                                No Change                               Cannot be determined

6. Baking soda solution.          Odour disappears.                   Basic

7. Lime water.                         Odour disappears.                   Basic

8. Tap water                            No change                               Cannot be determined

9. Washing powder solution   Odour disappears.                   Basic

10. Sugar solution                   No change                               Cannot be determined

11. Salt solution.                     No change                               Cannot be determined

                               Observations of Onion Experiment

          Acid                                         Base                               Neutral Substances

            ↓                                                 ↓                                               ↓

No change in odour                 Removes onion odour             No change in odour

Olfactory Indicators

• Substances like onions whose smell changes in acidic or basic conditions.
• Example: Onion smell changes differently with tamarind water (acidic) or baking soda (basic).

4. What Happens When Acidic Substances Mix with Basic Substances?

Activity: Litmus Reaction in Acidic and Basic Solutions

1. One drop of lemon juice is added to a test tube.

2. 20 drops of water are added to it

3. The colour of the solution is observed (pale. yellow/colourless)

4. One drop of blue litmus solution is added.

5. The solution turns red

6. Lime water drops are slowly added while swirling

7. At a certain stage, the solution turns blue.

8. One more drop of lemon juice is added.

9. The solution turns red again.

Observation of Litmus Reaction:

   §  Acids turn blue litmus red, showing acidity.

   §  Lemon juice is acidic, so it turns blue litmus red.

   §  Adding lime water (a base) to it neutralizes the acid, turning the solution blue again..

   §  Neutralization occurs when an acid and base mix in the right amount.

   §  The result is a neutral solution, neither acidic nor basic.

   §  Neutralization reaction produces salt, water, and heat.

   §  These reactions occur in daily life.

   §      Reaction Formula:       Acid + Base     →   Salt + Water + Heat 

           5 Neutralisation in Daily Life.

Ant Bite Relief:

   Ø  Ant bite injects formic acid, causing irritation.

   Ø  Moist baking soda (base) is applied to neutralize the acid

   Ø  It provides relief.

Soil Treatment:

   Ø  Acidic Soil:

§  Caused by chemical fertilisers, affects plant growth.

§  Lime (a base) is applied to neutralize acidity.

   Ø  Basic Soil:

§  Treated with manure & compost

§  They release acids to restore balance.

   Ø  Neutral Soil:

§  May lack nutrients, affecting plant growth.

• Protecting Aquatic Life:
•           Factory waste is released into lakes.
•           Acids in waste affect the fish population.
•           It must be neutralized with a base.

 

Common Acids in Edible Substances

• Lemon: Citric acid
• Curd: Lactic acid
• Tamarind: Tartaric acid
• Vinegar: Acetic acid

Know a Scientist

Acharya Prafulla Chandra Ray:

• “Father of Modern Indian Chemistry”.
• Earned a UK doctorate, advanced Indian scientific research.
• Founded India’s first pharmaceutical company (1901).
• Promoted Indian culture and mother tongue in education.

Question & Answers

1. A solution turns the red litmus paper to blue. Excess addition of which of the following solution would reverse the change?

(i) Lime water (ii) Baking soda           (iii) Vinegar     (iv) Common salt solution.

Answer: (iii) Vinegar

Explanation: A solution that turns red litmus paper blue is basic in nature. To reverse this change, an acidic solution must be added, because only acids can turn blue litmus back to red. Vinegar contains acetic acid.

2. You are provided with three unknown solutions labelled A, B, and C, but you do not know which of these are acidic, basic, or neutral. Upon adding a few drops of red litmus solution to solution A, it turns blue. When a few drops of turmeric solution are added to solution B, it turns red. Finally, after adding a few drops of red rose extract to solution C, it turns green. Based on the observations, which of the following is the correct sequence for the nature of solutions A, B, and C?

(i) Acidic, acidic, and acidic   (ii) Neutral, basic, and basic 

(ii) Basic, basic, and acidic     (iv) Basic, basic, and basic

Answer: (iv) Basic, basic, and basic.

Explanation:

Solution A turns red litmus blue This happens only in a base. So A is basic.

Solution B turns turmeric red This also happens only in a base. So B is also basic.

Solution C turns red rose extract green This also happens only in a base. So C is also basic.

3. Observe and analyse Figs. 2.13, 2.14, and 2.15, in which red rose extract paper strips are used. Label the nature of solutions present in each of the containers.

Answer: Based on the observations in Figs. 213, 214, and 215, the red rose extract paper strips change colour according to the nature of the solutions:

Fig. 2.13: The red rose extract paper turns green This happens in a base. So, nature of solution is basic

Fig. 214: The red rose extract paper remains pink This happens in neutral solution. So, nature of solution is neutral.

Fig. 2.15: The red rose extract paper turns red This happens in an acid. So, nature of solution is acidic.

4. A liquid sample from the laboratory was tested using various indicators:

Indicator         Red litmus                   Blue litmus                 Turmeric

Change            No change                  Turned red                  No change in colour

    Based on the bests, identify the acidic or basic nature of the liquid and justify your answer.

 Answer:

Ø  Red litmus shows no change This happens in an acidic or neutral solution. So, the liquid may be acidic or neutral.

Ø  Blue litmus turns red This happens only in an acidic solution. So, this confirms the liquid is acidic

Ø  Turmeric shows no change This means the liquid is not basic, because bases turn turmeric to red colour.

So, the liquid is acidic  it turns blue litmus red, and shows no reaction with red litmus or turmeric.

5. Manya is blindfolded. She is given two unknown solutions to test and determine whether they 

     are  acidic or basic. Which indicator should Manya use to test the solutions and why?

Answer: Monya should use an olfactory indicator, like onion juice. She is blindfolded and cannot see colour changes. She can identify the solution by its odour change. Olfactory indicators change their smell in acidic and basic solutions.

6. Could you suggest various materials which can be used for writing the message on the white sheet of paper (given at the beginning of the chapter) and what could be in the spray bottle? Make a table of various possible combinations and the colour of the writing obtained.

Answer:

Writing Material                     Spray Bottle Content               Colour of Writing

Baking Soda Solution               Turmeric Water                       Red

Vinegar                                    Red Rose Extract                     Red

Lime Water                             Red Rose Extract                     Green

Soap solution                           Turmeric Water                      Red

Soap solution                           Red Rose Extract                     Green

Soop solution                          Red Litmus Solution                Blue

7. Grape juice was mixed with red rose extract; the mixture got a link of red colour. What will happen if baking soda is added. to this mixture? Justify your answer.

Answer: A mixture of grape juice and red rose extract, which has a bink of red, is acidic. And baking soda is basic. When baking soda is added to the mixture, the solution becomes basic. So, this would cause the colour to change to green.

8. Keerthi wrote a secret message to her grandmother on her birthday using orange juice. Can you assist her grandmother in revealing the message? Which indicator would you use to make it visible?

Answer: Keerthi's message, written with orange juice, can be revealed using red cabbage juice, orange juice is acidic and it reacts with red cabbage juice. This reaction turns the message. red/pink and makes it visible.

9. How can natural indicators be prepared? Explain by giving an example.

Answer: Some plants have substances that change colour in acids or bases. Natural indicators are made from such plants Eg: Red rose Red rose extract is made by boiling petals, straining, and storing the cooled liquid. It acts as a natural indicator and turns red acid and green in base.

10. Three liquids are given to you. One is vinegar, another is a baking soda solution, and the third is a sugar solution. Con you identify them only using turmeric paper? Explain.

Answer:

Baking soda solution (basic) Turmeric paper turns red

Vinegar (acidic) No change in turmeric paper (remains yellow).

Sugar solution (neutral) No change in turmeric paper (remains yellow).

Thus, turmeric paper identifies baking soda. However, it cannot distinguish between vinegar and sugar solution, as both keep it yellow.

11. The extract of red rose turns the liquid X to green. What will the nature of liquid X be? What will happen when excess of amla juice is added to liquid X?

Answer:

   §  Red rose extract turns liquid X green This happens in a basic solution. So, the liquid X is a base

   §  Amla juice is acidic. Acid neutralizes, the base. Adding excess amla juice makes liquid X acidic. So, the colour changes back to red as liquid X shifts from base to acid.

        12. Observe and analyse the information given in the following flowchart Complete the missing information.

Imagine a garden with plants showing signs of poor health.

The soil can be acidic, in nature.

The soil can be basic, in nature.

                                                          

Which indicator can be used to test the nature of the soil? Litmus Solution

The acidic soil can be treated. with lime.

The basic soil can be treated with manure & compost

 

Dive Deeper

Aman accidentally spilt vinegar on some pieces of an eggshell or marble and noticed bubbling. He then poured a soap solution on another piece of eggshell or marble, but no bubbles appeared.

Why did bubbles occur with vinegar but not with soap solution?

Answer:

   §  Vinegar is acidic. It reacts with calcium carbonate (base) in eggshells or marble. This reaction produces carbon dioxide gas. The gas forms bubbles..

   §  Soap solution is a base. Since bases do not react with other bases, it does not react with calcium carbonate. So, no bubbles appear.