Cabbage Experiment. Finding out What Substance Is in a Substance (Chemical Analysis)

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Cabbage experiment. Finding out what substance is in a substance (chemical analysis)

Objectives:

At the end of the experiment the students will learn:

1.  What chemical analysis is.

2.  The purpose of chemical analysis

3.  What are chemical indicators

4.  How it is made or its sources

5.  How indicators are used for determining what substance is inside a substance

6.  What are Acids and bases

7.  What is a pH scale

8.  Using cabbage as indicator to determine if a substance is acidic, basic or neutral.

9.  Role of CO2 in acidifying rain and water system (optional)

10.  Apply acidity and basicity in gardening. (optional)

Materials:

·  Red cabbage (NOTE: if you have hard time getting red cabbage, tumeric and also grape juice have distinct color difference in acidic and basic conditions. You need to find out yourself what it is before the experiment using all the substances to be used).

·  Clear plastic cups or glasses. (test tubes if you have them and a rack)

·  Plastic spoons

·  Straws

·  Bottle of water

·  Ammonia

·  Baking soda dissolved in water

·  Clear soft drink like sprite or 7 up

·  Calamansi (cut fresh) or prepared previously

·  Detergent (crushed and dissolved in water previously)

·  Antacid (crushed and dissolved in water)

·  Paper bags (or any paper to cover original containers)

NOTE: save the original containers and cover them with paper bags and reveal after the demo.

·  Periodic table (on power point if none in the classroom)

·  pH scale (on power point or draw on blackboard)

Engage

We will act as detectives .

What do we do at home when we want to know if a glass of clear liquid in the refrigerator is water or sprite? (we taste it)

How about if a plate of meat is adobo or mechado? (we smell it, we look at it and also we taste it)

How about if a solution is just water or soap solution? ( we look at it, smell and touch)

How about if a purse has coins or not without opening? ( shake it and listen to sound)

Which part of our body did we use in each case? Tongue to taste, eyes to see, nose to smell etc

We also use them even in the lab to observe what is happening. Warn them again: In the lab we do not taste or drink or even touch. BAWAL.

But what if we have to determine what substances are inside these substances. For example we want to know if they are acids or bases. What are acids and bases in the first place.

In chemistry, we love to classify things. Whether they are acids or bases is one of them.

There are many reasons why. (one of them is that acids and bases give different reactions. Also some acids are corrosive and also some bases are caustic. Acids can be destructive. For example when you have just eaten an orange, you can cut a guitar string. It can also dissolve egg shells. Some bases are strong enough to hurt your skin.

[ OPTIONAL those marked in red.

Show them the periodic table. Point to hydrogen.

Draw neutral hydrogen atom (planetary way). Show hydrogen has one electron(-) and one proton (+). Note hydrogen is the only atom with no neutrons. All other elements have neutrons. Since it has one positive charge and one negative charge , they cancel out and it has no net charge.

Hydrogen gives an electron when it reacts. When the electron is given off, it becomes all proton so it is now written as H+. This is now called hydrogen ion. (also hydronium ion). There is no negative charge to balance off the positive charge. Draw the atom and they will see the lack of balance or excess positive charge.

The acids give off H+ when dissolved in water. (draw a beaker with lots of H+). Bases have little H+ (draw beaker with little H+) (bases actually give another species called hydroxide ion in water. OH- ) ]

SHOW this. Show the following and discuss. Need power point.

pH is a better way to show how acidic or basic a substance is. Show pH scale (whole number on left). 1, 2, 3, 14. Show some substances indicated above pH scale.

In the middle of scale is water, pH 7. This is the neutral pH. Neither acidic nor basic.

Below ph 7, acidic

Above ph 7, basic.

The question: Is there a way to know if a substance is acidic or basic? Yes use so called indicators which has different color if solution is acidic or basis.

RED CABBAGE pH scale.

A lot of things in nature can be used. Show cabbage. We will use red cabbage to make a color indicator. It has different color depending on the pH. Show figure below. For example pH 2 dark purple. pH 7 is blue. Ph 10 blue.

Explore:

Preparing the cabbage indicator: You can bring a blender and filter and make the cabbage indicator right there and then or have a prepared one. You need only a piece of cabbage (2 inch square and 500 mL of distilled water or boiled water). It takes a few seconds to make it with a blender. Filter using paper towel or coffee filter or cloth and strainer. If you do not have a blender, just heat a piece in hot water and boil and filter. Just describe how you made it if you bring prepared ones. Cabbage juice can be refrigerated and used later even for weeks. Stale ones will stink so throw then away.

Procedure: (do together by calling out directions or write on a piece of paper and place on table).

In each station, have three clear glasses or shot glasses (or you can use white artist paint container with several little cubby holes), a small container of cabbage juice (at least 3 tbsps per group), cut calamansi (1/2 per group) (or 2 tbsps sprite or 7up), water in a bottle, 2 tbsps per group prepared baking soda solution in a bottle.

Ask students to add 1 tbsp of cabbage juice in each of the three glasses or shot glasses.

Then Tell them to add the juice of ½ cut calamansi to the first glass. What color did they observe.

Then to the second one, add 2 tbsp plain water from water bottle. Color?

Then to the third one, tell them to add 2 tbsps of baking soda solution or sprite or 7up. Color?

Using the pH scale of cabbage shown previously in engage part, ask them what is the pH of each solution judging from the color it has. Also ask if it is acidic or basic. They also told about the fact that pH 7 is neutral. Less than 7 is acidic. Above 7 is basic.

(note they know from pH scale that was shown to them during engage part that pink or red is acidic and has ph of around 2 or less than 7) When it is green or blue it is basic with ph of around 10 or greater than 7 and when it is neutral it is purple with pH of 7.

Extension:

Magic show (demo):

Add the basic solution (third cup) to the acidic solution (first cup) and observe the color. It should be blue since it is now neutral.

Bases reacts with acids and you form a neutral compound.

Now add this cup with which has neutral color of blue to a cup with vinegar in it. (it should turn pink since vinegar is acidic.

Now add this cup which is pink to a cup with solution with laundry detergentt in it. ( it should turn green since detergent is basic.

Optional (When I used baking soda solution, there was no change to pink even after blowing for 5 minutes yesterday. Two persons alternated doing this.

Alternative is using ammonia. It will work in the sense it will take relatively less time for color to change. However it gives very strong fumes.

Procedure:

Give them each a straw.

Give them a glass to which is added ammonia (tell them to be careful of fumes) and cabbage juice. It should be greenish.

Tell them to take turns to blow with their straw into glass. Tell them this is a contest with the group which makes the color change first winning.

Ask them after all groups are done, what is happening? (The CO2 (which they have met before in previous activity 1) is dissolving in water and making the solution what? Acidic. And acidic is what color? Pink to red.

Ask them what gases are in the atmosphere? (oxygen, nitrogen, water vapor and carbon dioxide).

Ask them why should we be concerned with too much carbon dioxide in the atmosphere? (some might answer global warming. Hopefully they will connect the previous observation and say it is because it will make the river and stream acidic.

Ask them what is bad about that? It will kill the fish and plants in rivers . ( a lab with erosion of egg shells can be done before this and then they will have a better feel of the destructive effect of acids. However you can give this info in the engage part. Other destructive effect is erosion of metal in houses or building or even sculpture (picture in internet of erosion of statues with acid rain).

Evaluate:

I have here several substances that I will dissolve in water. (note: If you have test tubes and rack or tons of glasses, you can ask them to this as part of explore. I am using this instead as a demo to evaluate their knowledge. Prepare a worksheet or do a call out.

Use magic show result instead as review.

*Have the actual containers or bottles with labels covered with paper bag)

I will pour them in glasses with cabbage indicators. Tell me if they are acidic, basic or neutral and whether their pH is 7, below 7 or above 7.

Substances:

Sprite, water, baking soda in water, alka seltzer in water, vinegar.

Optional: What chemical species has acid a lot of?

a)  H

b) H+

What is the use of cabbage in today’s experiment? (color indicator to determine if substance is acid or base)

Optional:

Problem. What can a gardener use to change the color of a blue hydrangeas to pink hydrangeas? Add lime. Calcium hydroxide (base) . Get picture from internet.

The main influence that affects flower hydrangea color is soil pH, which is a measure of the soil’s acidity. The pH scale is from 0 to 14. Seven is the midpoint and is neutral. PH levels above 7 indicate alkalinity, while those below 7 represent increasing degrees of acidity. The average garden soil is slightly acidic in the range from 5.5 to 6.5. The pH range of 4.5 to 7.5 is not uncommon in garden soils. Blue hydrangeas require a highly acid soil, while pink hydrangeas are produced in a neutral or slightly acid soil

Flowers also are common source of acid base indicators. They have substances in them which is sensitive to pH changes like cabbage which will turn colors in different conditions.