Sci 9.2 Distinguishing Acids and Alkalis PDF

## Distinguishing Acids and Alkalis ### 9.2 Distinguishing Acids and Alkalis #### Let's Begin * Look at the butterfly pea flower drink on the right. * Is it beautiful? * Do you know how different layers of color in the drink are made? #### A Natural Acid-Alkali Indicators * Some substanc...

## Distinguishing Acids and Alkalis ### 9.2 Distinguishing Acids and Alkalis #### Let's Begin * Look at the butterfly pea flower drink on the right. * Is it beautiful? * Do you know how different layers of color in the drink are made? #### A Natural Acid-Alkali Indicators * Some substances in Nature show different colors when they come into contact with acids or alkalis. * They can be used as acid-alkali indicators. * Let us study one of these natural indicators in Practical 9.1. ### **Practical 9.1** #### Aim * To study the color of red cabbage extract in acidic and alkaline solutions. #### Apparatus and Materials Per Group | Item | Quantity | |-----------------|----------| | mortar and pestle | 1 set | | test tube rack | 1 | | beaker (glass or plastic) (250 cm³) | 2 | | measuring cylinder (100 cm³) | 1 | | test tube | 3 | | dropper | 3 | | distilled water | 1 | | lemon juice | 3 | | baking soda solution | 1 | | red cabbage leaves| 3 | | warm water | 3 | #### Part I Preparing Red Cabbage Extract 1. Put small pieces of red cabbage leaves in a mortar. 2. Crush the leaves using a pestle. 3. Pour 50 cm³ of warm water into the mortar. 4. Continue to crush the leaves with the pestle. 5. Pour the liquid into a beaker. 6. This liquid is the red cabbage extract. 7. Record the color of the extract in the table on the next page. #### Part II Comparing the Color of Red Cabbage Extract in Acidic and Alkaline Solutions 1. Label three test tubes A, B and C. 2. Pour the red cabbage extract prepared in Part I into each of the test tubes until they are about one-third full. 3. Using a new dropper each time, add the following liquids to each of the tubes. | Test Tube | Substances | |-------------|------------------------------------------------------------| | Tube A | three drops of distilled water (neutral) | | Tube B | three drops of lemon juice (acidic) | | Tube C | three drops of baking soda solution (alkaline) | 4. Shake each tube gently to mix the solutions well. 5. Observe the color of the extract in each tube. 6. Record the results in the table on the next page. ##### Results | Original Color of Extract | Distilled Water (neutral) | Lemon Juice (acidic) | Baking Soda Solution (alkaline) | |----------|----------|----------|----------| | Purple | Purple | Red | Blue | #### Discussion 1. What is the purpose of setting up tube A? * To act as a control setup to compare the color when there is no acid or alkali. 2. Can the red cabbage extract be used as an acid-alkali indicator? Why? * Yes, this is because the extract shows different colors in acidic and alkaline solutions. * The red cabbage leaves used in Practical 9.1 contain a pigment. * The pigment shows different colors in acidic and alkaline solutions. * Therefore, red cabbage extract can be used as an indicator to distinguish between acids and alkalis. * Besides red cabbage leaves, extracts of some other plants can also be used as acid-alkali indicators. Examples are given below. **Butterfly Pea Flower Extract** | Substance | Color in Distilled water | Color in Acidic Solutions | Color in Alkaline Solutions | |---------------------------|-------------|----------------------------|-----------------------------| | Butterfly Pea Flower Extract | blue | purple | blue-green | **Red Onion Skin Extract** | Substance | Color in Distilled water | Color in Acidic Solutions | Color in Alkaline Solutions | |---------------------------|-------------|----------------------------|-----------------------------| | Red Onion Skin Extract | light brown | orange | brown-green | ### 9.2 Distinguishing Acids and Alkalis #### After-Class Activity * Search for more information about plant extracts that can be used as indicators to distinguish between acids and alkalis. #### Language Tips * 'Acid' an 'alkali' are nouns. 'Acidic' and 'alkaline' are adjectives. * e.g., Vinegar contains an acid. Vinegar is acidic. Soap solution contains an alkali. Soap solutions is alkaline. #### Do You Know? * Do you remember the butterfly pea flower drink in Let's Begin (page 9). * It is prepared using a solution of butterfly pea flower extract. * Syrup containing lemon juice is then added to the solution. * As lemon juice is acidic, the butterfly pea flower extract at the bottom of the drink turns purple. * Different layers of color eventually form in the drink. ### Key Point Some plants contain pigments which show different colors in acidic and alkaline solutions. Their extracts can be used as indicators to distinguish between acids and alkalis. #### Did You Know? * The use of plant extracts as natural indicators to distinguish between acids and alkalis was first discovered by Robert Boyle in the 17th century. * Robert Boyle observed things around him carefully and made this discovery. Careful observation is always the first step in making important scientific discoveries. #### Did You Know? * Hydrangea produces flowers of different colors depending on whether it grows in acidic or alkaline soil. * When it grows in acidic soil, it produces blue flowers. * When it grows in alkaline soil, it produces pink flowers. ### Home Lab 9.1 #### Aim * Revealing the Secret Message #### Materials * baking soda solution * blueberries * blueberry extract #### Procedure 1. Dip a cotton swab into baking soda solution. Then write a message on a piece of paper with the cotton swab. Wait until the piece of paper is dry. The message becomes invisible. 2. Crush 50g of blueberries and add warm water to them. Filter the mixture. The filtrate is the blueberry extract. 3. Ask another person to paint some blueberry extract across the paper using a sponge. The message appears again! Can you explain why? ##### Caution! * Do not taste the solutions! #### **B Litmus** * Litmus is an acid-alkali indicator commonly used in the laboratory. * It is a mixture of pigments extracted from several kinds of lichens. * There are two kinds of litmus: blue litmus and red litmus (fig. 9.15). * Litmus is available in solution form. To make it more convenient to use, litmus solution is added to paper strips and dried to become litmus paper. | Substance | Litmus Color | |----------------------------------------------|-------------------| | In acidic substances | Blue litmus turns red. | | | Red litmus remains red. | | In alkaline substances | Blue litmus remains blue. | | | Red litmus turns blue. | | In neutral substances | Both blue and red litmus show no color change. | ### **Practical 9.2** #### Aim * Using Litmus Paper to Classify Solutions as Acidic, Alkaline, or Neutral #### Apparatus and Materials per Group | Item | Quantity | |-----------------------|----------| | blue litmus paper | several | | red litmus paper | several | | spot plate | 1 | | forceps | 1 pair | | dropper | 1 | | unknown solutions (A to D) | 4 | #### Procedure 1. Use a pair of forceps to put four small pieces of blue litmus paper and four small pieces of red litmus paper in the cavities of a spot plate. 2. Add a drop of solution A to a piece of blue litmus paper. Add another drop to a piece of red litmus paper. Observe any color changes of the litmus paper. Record the results in the table on the next page. ##### Caution! * Wear safety spectacles! * Wear disposable. * Wash your hands thoroughly after the practical. 3. Repeat step 2 with solutions B to D. Use a new dropper for each of the different solutions. ##### Results | Litmus Paper | A | B | C | D | |----------|----|---- |---- |---- | | Blue Litmus Paper | From blue to red | Remaining blue | From blue to red | Remaining blue | | Red Litmus Paper | Remaining red | From red to blue | Remaining red | Remaining red | #### Discussion 1. Which solution(s) is/are acidic? **A, C** 2. Which solution(s) is/are alkaline? **B** 3. Which solution(s) is/are neutral? **D** ### Key Point * Litmus is an indicator commonly used to distinguish between acids and alkalis. * In acidic substances, blue litmus turns red. * In alkaline substances, red litmus turns blue. * In neutral substances (e.g., distilled water), both blue litmus and red litmus do not change color. ### Quick Check 1. Red cabbage extract can be used as a natural indicator to distinguish between acids and alkalis because: * A: it contains a pigment. * B: it shows different colors in acidic and alkaline solutions. * C: it turns red in acidic solutions. * D: it is not corrosive. * **B** 2. Which of the following substances turn(s) blue litmus solution red? * (1) Baking soda solution (Alkaline) * (2) Vinegar (Acidic) * (3) Distilled water (Neutral) * **C** ### C Universal Indicator and pH Scale. * Look at the picture below. Two students are discussing whether lemon juice or apple juice is more acidic without tasting them. Do you think that they can find out which one is more acidic using litmus paper? * The answer is no because litmus can only show if a substance is acidic, alkaline, or neutral. It cannot tell how acidic or alkaline a substance is. To compare the acidity of lemon juice and apple juice, we can find out their pH values using solutions or instruments. #### pH Scale * The pH scale consists of a range of pH values. It is used to describe the relative acidity or alkalinity of substances. In general, pH values range from 0 to 14. - Acidic substances have pH values lower than 7. The lower the pH value of an acidic substance, the more acidic it is. - Neutral substances have a pH value of 7 (i.e., pH 7) - Alkaline substances have pH values higher than 7. The higher the pH value of an alkaline substance, the more alkaline it is. * The pH value of a substance can be measured using universal indicator (Fig. 9.16 on the next page). Universal indicator is a solution made up of several indicators. It shows a range of colors depending on the acidity or alkalinity of the substances being tested. * Universal indicator is also available in paper form. Its paper form is called pH paper (fig. 9.17). * We can find a pH color chart on the packaging of universal indicator and pH paper. * To find out the pH value of a solution, we can add a few drops of universal indicator into the solution (fig. 9.19), or add a few drops of the solution to a piece of pH paper. A color will develop in the indicator or on the pH paper. We can match the color with the color chart on the packaging and find out the pH value of the solution (fig. 9.20) ### **Practical 9.3** #### Aim * Using pH Paper and Universal Indicator to Measure the pH Values of Some Substances #### Apparatus And Materials per Group | Item | Quantity | |---------------------------|----------| | pH paper | several | | dropper | 5 | | spot plate | 2 | | forceps | 1 pair | | pH color chart | 1 | | universal indicator | 1 | | sample substances (distilled water, oven cleaner, baking soda solution, lemon juice, apple juice)| - | ##### Caution! * Wear safety spectacles! * Wear disposable gloves. * Wash your hands thoroughly after the practical. #### Procedure ##### Part I Using pH Paper 1. Use a pair of forceps to put five pieces of pH paper in the cavities of a spot plate. 2. Add a drop of distilled water to a piece of pH paper. 3. Match the color of the pH paper with the pH color chart to find out the pH value of distilled water. Record the result in the table on the next page. 4. Repeat steps 2 and 3 using the other sample substances. Use a new dropper for each different substance. ##### Part II Using Universal Indicator 1. Use a dropper to add two drops of distilled water into a cavity on a spot plate. 2. Add two drops of universal indicator to the distilled water. 3. Observe the color of the mixture carefully. Match the color of the mixture with the pH color chart to find out the pH value of distilled water. Record the result. 4. Repeat steps 1 to 3 using the other sample substances. ##### Results | Sample | pH value measured using pH paper (Part I) | pH value measured using universal indicator (Part II)| |---------------------------|--------------------------------------|----------------------------------------------------| | Distilled Water | 7 | 7 | | Oven Cleaner | 14 | 14 | | Baking Soda Solution | 10 | 10 | | Lemon Juice | 2 | 2 | | Apple Juice | 2 | 2 | #### Discussion 1. Which substance(s) is/are neutral? **Distilled water** 2. Which substances are acidic? Write down the less acidic one first, then the more acidic one (i.e., in an order of increasing acidity). **Apple juice, Lemon juice** 3. Which substances are alkaline? Write them down in an order of increasing alkalinity. **Baking soda solution, Oven cleaner** ### Recall the case on p. 17. In Practical 9.3, we should find out that the pH value of lemon juice is lower than that of apple juice. In other words, lemon juice is more acidic than apple juice ### Key Point * The pH scale is used to describe the relative acidity or alkalinity of a substance. * Acidic substances have pH values lower than 7. * Neutral substances have a pH value of 7. Alkaline substances have pH values higher than 7. * Universal indicator and pH paper can be used to measure the pH values of substances. ### Different substances in our everyday life have different pH values. Some examples are shown below. | Substance | pH Value| | ---------------------------|----------| | Black Coffee | 5 | | Milk | 6 | | Baking Soda Solution | 9 | | Tomato Juice | 4 | | Orange Juice | 3 | | Distilled Water | 7 | | Seawater | 8 | | Lemon Juice | 2 | | Acid in Stomach | 2 | | Acid in Battery | 1 | | Soap Solution | 10 | | Drain Cleaner | 12 | | Antacid* | - | | Bleach | - | | Glass Cleaner | - | ### Quick Check 1. The pH value of a solution indicates * (1) whether the solution is acidic, alkaline, or neural. * (2) The relative acidity and alkalinity of the solution. * (3) The color of the solution. * **A** 2. Which of the following substances has a pH value higher than 7? * A: Lemon Juice * B: Tomato Juice * C: Soap solution * D: Distilled Water * **C** ### **D pH Meter** * A pH meter is an electronic instrument used to measure the pH values of solutions. * We have to calibrate it before use. After calibration, we can put the probe of the meter into the solution to be tested. * The pH value of the solution is displayed on the screen (fig. 9.22). ### **Practical 9.4** #### Aim * Studying the Advantages and Disadvantages of Using Universal Indicator and a pH Meter to Measure pH Values #### Apparatus and Materials Per Group | Item | Quantity | |---------------------------|----------| | beaker (glass or plastic) (250 cm³) | 1 | | beaker (glass or plastic) (100 cm³) | 1 | | spot plate | 1 | | dropper | 2 | | pH meter | 1 | | pH color chart | 1 | | universal indicator | 1 | | distilled water | 1 | | white vinegar | 1 | | soy sauce | 1 | #### Procedure ##### Part I Using Universal Indicator 1. Use a dropper to add two drops of white vinegar to a cavity on a spot plate. 2. Add two drops of universal indicator to the white vinegar. 3. Observe the color of the mixture carefully. Match the color of the mixture with the pH color chart to find out the pH value of the white vinegar. Record the result in the table on the next page. 4. Repeat steps 1 to 3 with soy sauce. ##### Part II Using A pH Meter 1. Add white vinegar into a 100 cm³ beaker until it is about one-third full. 2. Hold the probe of a calibrated pH meter above a 250 cm³ beaker. Rinse the probe thoroughly with distilled water. 3. Put the probe of the pH meter into the beaker of white vinegar. Stir gently. Wait until the reading of the meter becomes stable. Record the result. ##### Caution! * Do not rub the probe with a paper towel! * Do not rinse the probe with tap water. ##### Caution! * Do not stir the sample quickly with the probe! * Make sure the probe does not touch the bottom or the side of the beaker! 4. Rinse the probe with distilled water. 5. Repeat steps 1 to 4 with soy sauce. ##### Results | Sample | pH value measured using universal indicator (Part I) | Reading shown on the pH meter (Part II)| |-----------------|--------------------------------------|----------------------------------------------------| | White Vinegar | 1 | 2.12 | | Soy Sauce | Cannot read | 4.46 | ##### Note: * If you cannot read the pH value write "cannot read" in the table. #### Discussion * Suggest the advantages and disadvantages of using universal indicator and a pH meter to measure the pH values. | | Using Universal Indicator | Using a pH Meter | |---|---|---| | **Advantages** | * convenient (calibration is not needed) * less solution is needed | * less solution is needed | | **Disadvantages** | * not suitable for dark-colored solutions * cannot be reused * measurement is subjective and less accurate | * calibration is needed * more solution is needed * suitable for solutions of any color * can be reused * reading error is small due to the small interval between readings; with proper calibration, the measured pH value is more accurate. | ### Key Point * **A pH meter** is an electronic instrument used to measure the pH values of solutions. * A pH meter can measure the pH value of a solution more accurately than universal indicator. #### Did You Know? * Using pH Meters in Daily Life * The pH value of water affects the health of aquatic life. People who keep an aquarium usually use a pH meter to measure the pH value of the water in the aquarium. * They can adjust the pH value of the water using certain chemicals if the pH value of the water is not suitable for the plants and animals inside. ### Section Summary 9.2 1. The extracts of red cabbage and some other plants can be used as indicators to distinguish between acids and alkalis. 2. Litmus is a common acid-alkali indicator. | | In acidic substances | In alkaline substances | In neutral substances | |---|---|---|---| | Blue litmus | Turns red | Remains blue | Does not change color | | Red litmus | Remains red | Turns blue | Does not change color | 3. The pH scale: * acidic substances have pH values lower than 7. * neutral substances have a pH value of 7. * alkaline substances have pH values higher than 7. | pH Value | | |---|---| | 0 | more acidic | | 1 | more acidic | | 2 | more acidic | | 3 | more acidic | | 4 | more acidic | | 5 | more acidic | | 6 | more acidic | | 7 | neutral | | 8 | more alkaline | | 9 | more alkaline | | 10 | more alkaline | | 11 | more alkaline | | 12 | more alkaline | | 13 | more alkaline | | 14 | more alkaline | 4. Universal indicator and pH paper can be used to measure the pH values of substances. 5. A pH meter can measure the pH value of a solution more accurately than universal indicator. ### Section Exercise 9.2 #### Level 1 1. When three drops of universal indicator are added to an unknown colourless solution, the solution turns blue. Which of the following statements about the unknown solution is/are correct? * (1) The solution is alkaline. * (2) The solution is neutral. * (3) The solution will turn red litmus blue. * A: (1) only * B: (2) only * C: (3) only * D: (1) and (2) only * **A** 2. A student tests an unknown solution with a piece of red litmus paper. The litmus paper remains red. She then tests the solution with a piece of pH paper. Which of the following could be the color of the pH paper? * (1) Yellow * (2) Green * (3) Blue * A: (1) only * B: (2) only * C: (3) only * D: (1) and (2) only * **D** #### Level 2 3 A student wants to find out the pH values of green grape juice and red grape juice. * a. The student uses universal indicator to measure the pH values of the green grape juice and the red grape juice. However, the results are not very clear. Suggest a reason for this. * The color of the grape juice may be interfering with the color of the universal indicator, making it difficult to determine the exact pH value. * b. Suggest an electronic instrument the student can use to measure the pH values of the green grape juice and the red grape juice. * A pH meter is an ideal instrument for measuring the pH values of solutions like grape juice as it provides a more accurate and precise measurement compared to universal indicators. * c. State ONE advantage and ONE disadvantage of using the instrument suggested in b compared to using universal indicator. * **Advantages:** * pH meters provide a more accurate and precise measurement, reducing the chances of error due to subjective interpretation of colors. * **Disadvantages:** * pH meters require calibration before use, adding an extra step to the measurement process.