CHEMISTRY

ACIDS,BASES AND SALTS
Introduction
Activities
Exercise

ACIDS,BASES AND SALTS

LEARNERS ACTIVITIES


Activity 1:  Class experiments

Methodology: Guided discovery and class discussions

Instructional materials

Test tubes, dilute hydrochloric acid (or citric acid from lemon), dilute sodium hydroxide (or solution from ashes from banana peelings), water.

Procedure: (learners in groups of 5 )

  • Measure 2cm3 each of a solution of acid, base and water in three separate test-tubes.

  • Using a dropper or a straw, add two drops of litmus solution (use litmus papers) to each of the test tubes in (a) above.

  • Record the observation made in the table below.

  • Now repeat procedure (b) with methyl orange and phenolphthalein indicator. Record your observation in the table below.

Indicator

Colour in

 

 

 

Acid

Base (alkali)

Water (neutral)

Litmus

 

 

 

Methyl orange

 

 

 

phenolphthalein

 

 

 


Figure 1 Colours of indicators in different solutions

Question: What do you observe?

Activity 2

Extraction of indicators from local materials

Methodology: Group experiment, brain storming and guided discussion.

Instructional materials:

Mortar and pestle, test tubes, measuring cylinder. Glass rod or straw, filter funnel and papers, glass beakers, or plastic cups, red, blue, or yellow flower petals, lemon juice, dilute hydrochloric acid, solution or ashes, (banana peelings).
Soap solution, ethanol, or waragi, red flowers can be got from rose or hibiscus flower and blue petals from sweet potatoes or morning glory flower.

Procedure

  1. Place red flowers petals in a mortar and grind them using a pestle.

  2. Now add little of a solvent (ethanol or waragi) together with water and continue grinding until the extract is deep in colour as possible.

  3. Decant or filter about 10 cm3 of the extract into a test tube.

  4. Repeat procedure (a)-(c) with blue and yellow flowers and with other flowers.

  5. Using the flowers extracts, (indicators) above test the following solutions, lemon or orange juice, dilute hydrochloric acid soap solutions, dilute sodium hydroxide, solution of ashes.

  6. Record your observations in the table below

Plant extract

Colour of solution

Lemon or orange juice

Hydrochloric acid

Soap solution

Sodium hydroxide

Solution of ashes

1. Red flower

 

 

 

 

 

2. Blue flower

 

 

 

 

 

3. Yellow flower

 

 

 

 

 


Activity 3:   

Use of universal indicator to determine the pH of solutions.

Methodology: demonstration and group experiment.

Instructional materials:

Flowers extract as in activity 2 test tubes, lemon or orange juice, water, milk, soap solution, dilute sulphuric acid, dilute hydroxide, urea, fertilizer, commercial universal indicators.

Procedure

  1. Place a bout 2 cm3 each of orange juice, water, milk, soap solution, sulphuric acid, ashes solution, and sodium hydroxide in different test tubes.

  2. Using a dropper or straw place about 3 drops each of the universal indicator in each of the test tube in (a).

  3. Note the colour shown and record the corresponding pH value in the table below.

  4. Prepare a local universal indicator by mixing all the flower extracts.

  5. Now place about 1 cm3 of universal indicator in (d) above and repeat procedure (a), (b) and (c).
  •       (i) Chop red cabbage.
          (ii) Boil the chopped cabbage with  water.
          (iii) Decant the solution formed and add about 2 cm3 of solution into 6 test-tubes.
          (iv) Add the solutions above to each of the test tubes.
  • Question: what do you observe in each test-tube?

    pH

    Colour of universal indicator

     

    pH

    Colour of indicator

    1 – 2

    Red

     

    6 – 8

    Green

    3

    Pink

     

    9 – 10

    Blue

    4

    Brown

     

    11 – 12

    Indigo

    5

    Yellow

     

    13 – 14

    Violet

    Universal indicator

    PH value

    Orange juice

    Milk

    Water

    Soap solution

    Sulphuric acid

    Ashes solution

    Sodium hydroxide

    Commercial or laboratory

     

     

     

     

     

     

     

    Locally manufactured

     

     

     

     

     

     

     


    Figure 2 universal chart

    Activity 4:

    Use of indicators to prepare a soluble salt.

    Methodology: Demonstration, group experiment, and brain storming.

    Instructional materials:    Pipettes, burettes, 2M hydrochloric acid, 2M sodium hydroxide, methyl orange,  evaporating, basin, burner, conical flask, beakers.

    Procedure

    1. Pipette 25.0 cm3 of 2 M hydrochloric acid into a clean conical flask. Add 2-3 drops methyl orange indicator.

    2. Fill the burette with sodium hydroxide solution and adjust the reading to give the initial reading, record the initial reading.

    3. Now carefully run the sodium hydroxide solution from the burette until the colour changes from orange to yellow.

    4. Note the final reading on the burette and hence determine the volume of sodium hydroxide needed to react with the 25.0 cm3 of the acid pipetted. Let y cm3 = final reading – initial reading.

    Table of results for activity 4           

     

    Burette reading (cm3)

    Final burette reading (cm3)

     

    Initial burette reading (cm3)

     

    Volume of base (alkali), y (cm3)

     

    1. Now pipette 25.0 cm3 of the acid and repeat the titration but this time without the indicator. Add exactly the volume of solution hydroxide ‘y’ determined in (d).

    2. Place the resulting solution in an evaporating dish and carefully heat the solution until crystals start to form and allow to cool to form crystals.

    3. Weigh the salt to determine the mass formed and calculate the mass of the salt expected by mixing 1 dm3 of the acid and the sodium hydroxide solution.

     

    Figure 3 Titration and crystallisation of common salt

    Activity 5

    Materials: Test-tubes, test-tube racks, water and saliva.

    Procedure

    1. Learners should go into groups of 5 or 6 and one of them puts some of his/her saliva into a test-tube.

    2. A little water should be added to dilute the saliva.

    3. Add a drop or two of universal indicator to the solution in (b). Shake the mixture and observe. What is the pH of the saliva from this student?

    4. Repeat the activity three or four times.

    Follow up activity

    1. Biology, agriculture or chemistry department should be encouraged to grow plants which help the teaching of indicators.

    1. Learners should prepare universal indicator using flower extract by mixing orange, pink, yellow, green, blue, violet, indigo, flowers and use it to test for the nature of some solutions of salts including water from different sources, (e.g. bore hole, wells, rain water, tap water etc)

    2. Learners should also use the universal indicator to test for pH of different types of soils since acidity and alkalinity is very important in determining how well different plants grow.

    Learners should be able to relate the knowledge in chemistry to biology in relation to dissection of animals (rats) and insects (cockroaches).

    Importance of soil pH to farmers 

    The acidity or alkalinity of soil is very important in determining how well plants grow in particular soil. The sample of soil must be taken from about 10 cm below the surface.

    1. Plant                                                   Preferred pH range

    2. 1 Beans                                                          6.0 – 7.5

    3. 2 Cabbage                                                      6.0 – 7.5

    4. 3 Onions                                                        6.0 – 7.0

    5. 4 Potatoes                                                      5.5 – 7.0

    6. 5. Rose Flower                                               5.0 – 6.5

    7. 6. Sunflower                                                   6.0 – 7.5                    

    click to get answers

    Teachers' guide
    Scheme of work
    Lesson plan
    Exercise
    Useful web links
    www.about.com.chemistry
    www.answers.com
    www.csudh.edu