Towards a just, equitable, humane and sustainable society

Identifying the strength of acids and bases with pH paper, and introduction of Arrhenius theory of Acids and Bases

0
No votes yet
0
Post a comment

1. Objective – Use experiments to help students understand Arrhenius theory of acids and bases, and using the experimental observation/results to analyse the strength of an acid/base (concept of pH).

2. Learning Outcome

  • Students will be able to (SWBAT) differentiate acids and bases through their properties (Litmus Test).
  • SWBAT to differentiate the strength of acids and bases using pH paper.
  • SWBAT to explain Arrhenius theory on the basis of pH paper.

3. Pre requisite knowledge

  • Understand the term ion (H+ & -OH) and how ions are formed in a solution (should be covered in class IX Chemical Bonding Chapter).
  • Litmus Test: how the litmus changes in different mediums (Class VII Acids and bases).

4. Apparatus and Chemical required (class size of 30)

  • Indicators - litmus paper (36 strips (18 Blue litmus & 18 red litmus)), pH paper (36 pH paper strips)
  • 6 pH indicator charts for reference.
  • Acids – lemon juice (50 ml), vinegar (50 ml), HCl (0.1N, 20 ml), H2SO4 (0.1N, 20 ml).
  • Bases - baking soda (50 ml), NaOH (0.1N, 20 ml), detergent solution (50 ml), antacid (ENO™; 20 ml).
  • Neutral- Water (50 ml), Common salt solution (50 ml).
  • 10 beakers (100 ml) (to make the stock solution of the above chemicals), 18 test tubes (10 ml), 6 test tube stands, 2 glass rods, 6 droppers.

Activity 1

5. Objective: To understand the properties of acids and bases using litmus paper.

6. Learning Outcome

SWBAT differentiate the acids and bases through their properties (Litmus Test).

7. Note for Teachers (Pre-activity)

  • The basic idea of Activity 1 is to differentiate between acids and bases using litmus paper and that of Activity 2 is to compare the strengths of the acids using the data from Activity 1. The reagents employed are an easily available mix of both natural and synthetic chemicals.
  • Students have to focus on colour change happening to litmus paper and have to record the changes in their observation Table(s). In order to avoid inconsistencies, it is better to use different test tubes for different solutions.
  • Based on the strength of the class, each group can be given multiple samples or different groups can be given same samples to check for consistency in their observations (Check  Smp.1, below)
  • Teacher can also give mineral acids and bases (HCl, NaOH etc.) to students if she or he feels confident on the ability of their class to safely handle strong acids and bases. If not, the teacher can demonstrate the testing of the solutions listed in Sec. 7 , Smp. 1.
  • If the children have no experience of handling glassware, it is better for the teacher to initially demonstrate the process of testing the solutions, and then guide the students in their group work.

Smp.1.    Samples of chemicals to be given (Example)

Note: Divide the classroom into 6 groups, each group gets three samples to test along with 6 litmus papers, 3 red and blue litmus papers each.

Samples to be tested by the teacher:

  • Hydrochloric Acid (HCl)
  • Sodium hydroxide (NaOH)
  • Sulphuric Acid (H2SO4)
  • Antacid (ENO™)

Group A: Lemon juice, water & Baking soda

Group B: Lemon juice, water & detergent

Group C: Vinegar, water & detergent

Group D: vinegar, salt solution, baking soda

Group E: Lemon juice, Salt solution, Baking soda

Group F: Vinegar, salt solution, detergent

Table.1 Observation Table Outline

S.No Sample Name Red Litmus Blue Litmus pH Scale (To be filled after Activity2)
1        
2        
3        
4        
5        
6        
7        
8        
9        
10        

8. Procedure (30 students in a class):

  • Divide the classroom into 6 groups(5 in each group), each group gets three samples to test  and litmus paper, 3 red and blue litmus papers each. (Smp.1).
  • The students and the teacher are asked to make the above mentioned table (Table. 1)
  • Children are asked to use the dropper and put the solution on both the litmus paper OR as an alternative; they can dip the litmus paper in the solution. Students will observe the colour change in the litmus paper and tabulate the results in the observation table (Table. 2). While using the dropper the amount of sample that can be given will be minimal, but when dipping the litmus paper in a test tube, the sample volume given should be more.
  • After Step 8(c) teacher compiles the information from the class and notes it down on the board and asks the students to do the same (Table. 3)
  • Step 8(d) is followed by the teacher testing solutions of HCl, H2SO4, NaOH & KOH. The students are asked to write down these observations as well. (Table.4)

9. Note for the teacher (Post Activity):

  • After filling up the table, ask the students to group results in the table based on change in colour of blue and red litmus papers. After grouping, the students are told that materials that turn blue litmus to red are acids, and those which turn red litmus blue are bases. (Learning Outcome 1)

Table. 2 Illustrative table filled by a student (from Group A) after Activity 1.

S.No Sample Name Red Litmus Blue Litmus pH Scale (To be filled after Activity2)
1 Lemon Juice No Change Turns Red  
2 Water No Change No Change  
3 Baking Soda Turns Blue No Change  

Table. 3 Illustrative table filled by a student (from Group A) after teacher compiles the information and displays on the board (first 6 of 10 displayed).

S.No Sampel Name Red Litmus Blue Litmus pH Scale (To be filled after Activity2)
1 Lemon Juice No Change Turns Red  
2 Water No Change No Change  
3 Baking Soda Turns Blue No Change  
4        
5        
6        
7        
8        
9        
10        

*for illustrative purpose only

Table. 4 Illustrative table (for Group A) filled by the student after teacher demonstration

S.No Sample Name Red Litmus Blue Litmus pH Scale (To be filled after Activity 2)
1 Lemon Juice No Change Turns Red  
2 Water No Change No Change  
3 Baking Soda Turns Blue No Change  
4 Vinegar No Change Turns Red  
5 Detergent Turns Blue No Change  
6 Salt Solution No Change No Change  
         
7 Hydrochloric Acid No Change Turns Red  
8 Sulphuric Acid No Change Turns Red  
9 Sodium Hydroxide Turns Blue No Change  
10 Antacid Turn Blue No Change  

*for illustrative purpose only

Activity 2

10. Objective: To introduce Arrhenius theory of acid and base by using pH

11. Learning Outcome:

  • SWBAT to differentiate the strength of acids and bases using pH paper
  • SWBAT to explain Arrhenius theory on the basis of pH paper

12. Note for teachers (Pre-activity):

  • The idea of this activiy is to make sure that children understand pH as a scale and based on that understanding teach them the Arrhenious theory of acids and bases.
  • The students have to focus on the colour change happening to pH paper and have to record the changes in their observation table.
  • Teacher has to make sure that the students are instructed to retain their pH paper after testing their samples to ensure that conflicts (if any) can be resolved when later on students compare their results.
  • Every group has to get a pH scale chart (Fig.1) so that they can compare the colour and infer the pH value by themselves.

13. Procedure

  • Ask the students to test the three solutions originally given during the Activity 1 using the pH paper. The students will be asked to observe the colour, and write down the corresponding number using the pH scale provided by the teacher to each group (Table 5).
  • The teacher compiles the student findings and then tests her initially used chemicals with a pH paper and writes the findings on the board (Table 6).

Fig. 1 pH scale to be given to each student group.

Table. 5 Illustrative table (for group A) filled by the student after Activity 2

S.No Sample Name Red Litmus Blue Litmus pH Scale (To be filled after Activity 2*)
1 Lemon Juice No Change Turns Red 3
2 Water No Change No Change 7
3 Baking Soda Turns Blue No Change 11
4 Vinegar No Change Turns Red  
5 Detergent Turns Blue No Change  
6 Salt Solution No Change No Change  
         
7 Hydrochloric Acid No Change Turns Red  
8 Sulphuric Acid No Change Turns Red  
9 Solidum Hydroxise Turns Blue No Change  
10 Antacid Turns Blue No Change  

*Values are for illustrative purposes and are NOT exact.

Table 6. Illustrative table filled by the student after teacher has demonstrated & compiled the data of Activity 2.

S.No Sample Name Red Litmus Blue Litmus pH Scale (To be filled after Activity 2*)
1 Lemon Juice No Change Turns Red 3
2 Water No Change No Change 7
3 Baking Soda Turns Blue No Change 11
4 Vinegar No Change Turns Red 3
5 Detergent Turns Blue No Change 9
6 Salt Solution No Change No Change 7
         
7 Hydrochloric Acid No Change Turns Red 1
8 Sulphuric Acid No Change Turns Red 1
9 Sodium Hydroxide Turns Blue No Change 14
10 Antacid Turns Blue No Change 9

*Values are for illustrative purposes and are NOT exact.

14. Note for Teacher (Post Activity) :

  • Now based on the knowledge from litmus test ask the students to separate the acids and write the corresponding pH number similarly for bases.
  • Ask questions to show what the scale stands for, what information does it give the students about two substances? How are the litmus and pH tests different?

Note: The idea behind asking these questions should be to elicit from students the difference between a strong acid and a weak acid, i.e., a concentrated form of a weak acid will not necessarily have a pH value of close to 1, no matter how much the acid is concentrated. Similarly, a weak base in concentrated form (no matter how much it is "concentrated") will not show a pH value of 14 in solution. Also, the idea is to show that there is differentiation in concentration in a given acid (with dilution for example) which does not become evident from litmus test.

Q. We have all agreed that acetic acid and hydrochloric acid are acidic in nature from the litmus test. But then why is it showing different pH?

We encourage students to come up with different reasons for the observation. The idea behind the question and the student discussion (directed by further teacher questioning) is to show that pH acts like a scale that tells us the strength of an acid or a base. From 0-7 it is acidic and 7-14 it is basic. 0 is highly acidic and 14 is highly basic and 7 is neutral. (Learning Outcome 2)

Teacher Explanation

We explain the Arrhenius theory by using the pH scale, an acid is a substance that releases H+ ions in aqueous solution, and base is a substance that releases -OH ions in the aqueous solution (Fig. 2; Learning outcome 3)

The difference in the strength of acetic acid and hydrochloric acid is because of the variation in the concentration of H+ ions.

Note: The concept of ions can be refreshed by using a video:

https://www.youtube.com/watch?v=EIAaGHK5pjA&t=12s

Towards the end of this video there are questions being asked which the students must be encouraged to answer. .

Fig. 2: Diagram illustrating that pH can be used as a scale to compare strengths of acids and bases.

 

 

 

Grade: 
9

Term: Term 2

Subject: 
Science

0
No votes yet
0
Post a comment