The grade 5 English science unit, Acids and Bases, meets the academic content standards set in the Korean curriculum, which state students should:

Transcription

1 This area addresses ph among different characteristics of solutions. It will be interesting for students to classify a variety of solutions into acids and bases by using the characteristics of the solutions. Students observe changes after placing indicators into a variety of solutions and classify them as acidic and alkaline. Students learn how the characteristics of the solution change when acidic and alkaline solutions are mixed. Students are to conceive and explain examples of how acidic and alkaline solutions are used in everyday life.

2 The grade 5 English science unit, Acids and Bases, meets the academic content standards set in the Korean curriculum, which state students should: a) Observe changes after placing indicators into a variety of solutions and then classify the solutions as acids and bases. b) Recognize the characteristics of acid and base solutions. c) Be aware through color change of indicators that acidity and alkalinity weaken when the two types of solutions are mixed. d) Be able to find examples in everyday life in which the characteristics of a solution change when acidic and alkaline solutions are mixed.

3 Have you ever eaten fruit that was still unripe? How did it taste? This is how an acid tastes. It is sour. This is an example of a natural acid. More powerful acids can also be made artificially. They can exist as a liquid, solid or gas. Acids are generally most useful as solutions of water. All acids contain hydrogen. Lemon juice Water Cup Teaspoon Baking soda Sugar Bases are the opposite of acids. They have a bitter taste. Bases are very good as cleaning agents. Weaker bases feel soapy when you touch them. All bases contain hydroxides. Many bases do not dissolve in water. The bases that do dissolve create an alkaline solution. 1. Fill a quarter of the cup with lemon juice. Add an equal amount of water. Taste a little bit of the liquid. 2. Add a teaspoon of baking soda into the cup with the lemon juice and water. Taste a little bit of the lemon soda. 3. Add sugar to the lemon soda to make it sweeter. Draw a picture to show what happened when you added the lemon juice with the baking soda. Lemon Juice Lemon Juice + Baking Soda Taste 1. What do most acids taste like? Give an example of an acid. 2. What do most bases taste like? Give an example of a base. 1

4 Do you remember what an acid and base taste like? They taste different, but you should never taste any chemical as it is extremely dangerous. You could make yourself really sick or even die. If we can't taste them, what is an easy way to tell an acid and base apart? We can use indicators. The most common indicator is litmus paper. This paper turns red in acids, blue in bases, or purple in neutral solutions. Neutral means that it is not an acid or base. Blue litmus paper Red litmus paper 6 unknown samples Well plate Red litmus paper turns blue in alkaline solutions. 1. The teacher will pour 6 samples into your well plate. 2. Dip a different strip of the red litmus paper into each sample for about one second and then remove. Note the color. 3. Dip a different strip of the blue litmus paper into each sample for about one second and then remove. Note the color. Sample Red Litmus Paper Color Blue Litmus Paper Color Which samples were acids? Which samples were bases? Which were neutral? 2. What would happen if you put blue litmus paper in an acid? 2

5 Some indicators, such as universal indicator, have a huge range of colors when they are added to different solutions. If a strong acid is added to the indicator, it will turn red. If a strong base is added to the indicator, it will turn blue. Each color has been given a number which helps us to tell if the solution is a strong acid, weak acid, strong base or weak base. Universal indicator Well plate 6 unknown samples Strong acids have a ph of 1-3. Weak acids have a ph of 4-6. Weak bases have a ph of Strong bases have a ph of If the color is green and has a ph number of 7, the solution is neutral. 1. The teacher will pour 6 samples into your well plate. 2. Dip a different strip of the indicator paper into each sample for about one second and then remove. 3. Compare the color with the color chart within 5 seconds. Sample Universal Indicator Paper Color ph (number) What Universal Indicator paper colors indicate a base? 2. What ph numbers show that a test sample is a strong acid? 3

6 Red cabbage juice also has a huge range of colors when it is added to different solutions. The purple color in red cabbage is due to pigments called anthocyanins. They are able to change colors when they are put in acids and bases. This is because both acids and bases slightly change the molecular structure of the anthocyanins. These new structures absorb and reflect light differently which changes the colors we see. The red cabbage juice changes to red in acids and blue-green in bases. It stays purple in neutral solutions. Red cabbage juice acidbase indicator ph chart. Red cabbage juice Well plate 6 known samples 6 pipettes 1. Pour a little of the red cabbage juice into 6 of the holes on the well plate. 2. Use a pipette to add the test chemicals to a different hole on the well plate containing the red cabbage juice. 3. Compare the color with the chart above. Sample Red Cabbage Juice Color ph (number) Dilute Hydrochloric Acid Sprite Bottled Water Baking Soda Solution Laundry Detergent Pipe cleaner 1. Which samples were acids? 2. Which samples were bases? 4

7 Dentists have recently started to warn people about the dangers of drinking acidic drinks. They have found that acidic drinks dissolve the enamel covering of the tooth. Drinking acidic drinks over a long period of time means that the enamel disappears and the tooth starts to crumble at the edges. This is known as erosion. What drinks should you stop drinking so that this doesn't happen to you? Dentists say that if the ph level of the drink is below 5.5, then tooth erosion will occur. The safe ph level for drinks is 5.5 and above. An example of tooth erosion. 6 different drinks Universal Indicator or Red cabbage juice Well plate Pipettes Poster paper 1. Decide which indicator you will use to determine the ph s of the drinks. Inform your teacher. 2. Depending on the indicator you chose, refer to the previous experiments to refresh your memory on how to complete the experiment. 3. Use the results from your chart to create a poster that warns people about the dangers of drinking acidic drinks. Drink Indicator Color ph 1. Which drinks had a ph lower than the safe ph of 5.5? 2. What happens to people who drink acidic drinks over a long period of time? 5

8 Acid rain is made when smoke from factories and motor vehicles, such as cars, mix with the water in the air. It is the sulfur dioxide and nitrogen oxides in the smoke that helps to make the acid rain. The ph of this rain is below 5. In some cases, the ph has been lower than mL vinegar Teaspoon Salt Cup Dull 10 yen coin Many structures are made from metal. Most bridges and motor vehicles are made from steel. Copper can be used to make roofs and other building parts. Acid rain destroys these metals little by little. This is known as corrosion. A power plant sending smoke into the air. 1. Pour 30mL of vinegar into a cup and add a teaspoon of salt. 2. Add a dull 10 yen coin and swirl the cup. Wait for 5 minutes and look at the 10 yen coin. 3. Put the 10 yen coin back in the cup with vinegar and leave it in there for a month or so. Draw pictures to show before and after shots of the 10 yen coin. 1. How is acid rain made? 2. What damage does acid rain do to metal objects? 6

9 Many statues are made from limestone which is largely made from calcium carbonate. Acid rain reacts with the calcium carbonate to dissolve the statues. Acid rain + Calcium carbonate = Salt + Carbon dioxide + Water Pipette Chalk Petri dish Vinegar Glue Eggshells and chalk are other examples of calcium carbonate. People should do their best to use renewable sources of energy, such as solar and wind power. These forms of energy produce much less pollution and can help reduce acid rain. Car companies are also trying to make cars that produce less harmful gases. A statue damaged by acid rain. 1. Make a statue with two pieces of white chalk. 2. Put the chalk statue into a petri dish. 3. Use a pipette to put slowly drop vinegar onto the chalk statue. Draw a picture to show what happened to the chalk. 1. What happens when acid rain falls on limestone statues? 2. What can people do to reduce acid rain? 7

10 Acids will react with bases to make salts and water. This reaction is called neutralization. Acid + Base = Salt + Water If you add just the right amount of acid to a base, you can make the liquid neutral. This means it has a ph of 7. Ziploc bag Vinegar Paper towel Baking soda Measuring cylinder ½ tablespoon Neutralization is not only limited to solutions, but also when an acid reacts with a carbonate. Acid + Base (carbonate) = Salt + Water + Carbon dioxide 1. Fill a Ziploc back with 125mL of vinegar. 2. Put 1½ tablespoons of baking soda on a paper towel and wrap it up. 3. Put the wrapped up paper towel into the Ziploc bag and quickly seal it up. Give it a little shake and put it on the ground. Draw a picture to show what happened when you added an acid and a base (carbonate). 1. What happens if you add an acid and a base together? 2. What is the ph of a neutral solution? 8

11 If you keep adding an acid to a base or a base to an acid, you will get to point where the solution is not acidic or alkaline. The substance will be neutral. This means the ph will be 7. When people eat too much food, sometimes their stomach makes too much acid. This acid rises into their throat and gives a burning sensation. This is called heartburn. Antacid tablets can be taken to help neutralize the extra acid the stomach is making. Antacids are bases. Baking soda Scales Beaker ½ teaspoon Vinegar Well plate Universal indicator Sodium bicarbonate (baking soda) is sometimes used as an antacid. 1. Dissolve 5g of baking soda in 200mL of water. Amount of antacid solution added to the vinegar. 0mL 2.5mL 5.0mL 7.5mL 10.0mL 2. Add ½ teaspoon of vinegar to 5 of the holes in the well plate. The acid in your stomach is very dilute hydrochloric acid. In this experiment, you will use vinegar instead. Universal Indicator Paper Color 3. Add the amount of baking soda solution as stated in the chart below. Use ph paper to determine the ph of the final solution. ph 1. What causes heartburn? 2. How does taking antacids help? 9

12 Alkaline solutions are used as cleaning products. The soap you use to wash your hands with is a base. The washing detergent for dishes and the laundry detergent for cleaning clothes are all bases. Even baking soda can be used to clean stains. Soaps, detergents and stain removers clean up messes and help to remove stains. They all do so in slightly different ways. Soaps are made of particles that have two ends. One end sticks to the water, while the other end sticks to the dirt. Detergents contain surfactants, which are substances that reduce the surface tension and helps dirt to mix with the water. Coffee Large bowl White cloth 4 clothespins Teaspoon Water Dishwashing liquid Laundry detergent Bleach 1. Use a dry cloth that has been stained with coffee and peg it above a bowl full of water using clothespins 2. Divide the cloth into 4 parts. Pour 5mL of water into the center of part 1, 5mL of dishwashing liquid in part 2, 5mL of laundry detergent in part 3 and 5mL of bleach in part Wait 5 minutes. Then, wash each part of the cloth in a bowl of water. Draw a picture to show what each part of the cloth looked like after you completed step What are many bases good at doing? 2. Which of the above cleaners removed the stain best? 10