Science Lab: What Happens to the ph of Milk if you leave it out of the Refrigerator for Five Days? CONCEPTS: What s a chemical reaction (change)? What happens during a chemical reaction (change)? Do all substances have a chemical change when mixed together? What s the difference between chemical and physical changes? 1. Chemical reactions are different from physical reactions. 2. Chemicals may be classified in many different groups, according to the way they behave. One group of chemicals are known as acids and bases. Acids and bases are the opposite of each other. In between acids and bases are the neutrals, which are neither acidic or basic in nature. Orange juice is an acid (citric acid), drinking water is neutral, and Maalox is a base. 3. ph test paper is one way to measure the acidity or basic nature of a molecule with out having to taste it! 4. Milk has bacteria in in called lactobacillus and coliforms. When milk is allowed to warm up (out of the refrigerator) the coliform bacteria begin to do chemical reactions which make lactic acid, which is sour. We call this soured milk. STANDARDS GLE 0507.9.1 Observe and measure the simple chemical properties of common substances. GLE 0507.Inq.1 Explore different scientific phenomena by asking questions, making logical predictions, planning investigations, and recording data. GLE 0507.Inq.3 Organize data into appropriate tables, graphs, drawings, or diagrams. GLE 0507.Inq.4 Identify and interpret simple patterns of evidence to communicate the findings of multiple investigations. GLE 0507.T/E.4 Recognize the connection between scientific advances, new knowledge, and the availability of new tools and technologies.
EQUIPMENT NEEDED: A clean glass baby food jar (or small 40 ml beaker), milk, ph test strips. STUDENT JOBS: Chemists (everyone takes turns measuring ph); gofer (goes to get equipment and samples); group manager (responsible for insuring that all data is gathered and recorded before the next sample is taken...a chance for that bossy kid to try to develop positive group skills); equipment manager (director of cleaning the equipment...this is good for the hyperactive, wiggly kid!) OPTIONAL EQUIPMENT: 1-13 ph test paper, digital ph tester, ph Scale displayed on Promethean Board. TIME: 10 minutes per day for ONE WEEK PREVIOUS KNOWLEDGE: Not all chemicals have a chemical reaction (change) when they touch other chemicals. Sometimes nothing happens, such as when you drop a glass marble in water. There is no new or different molecule formed-it s still glass and water. Other times when molecules touch, changes occur. These changes are called chemical reactions or chemical change. For example, when a candle is lit, the wax molecule C25H52 is heated and combines with the air to form C25H52 + 38 O2 25 CO2 + 26 H2O. In other words when a wax molecule joins with exactly 38 O2 molecules during burning, things happen! The molecules form different molecules and voila, the wax is reorganized into 25 carbon dioxide molecules and 26 water molecules. None of the atoms were destroyed only changed into new molecules. (This is called the Law of Conservation of Matter and students will learn more about this in coming years.) So, everyone has seen a candle seem to disappear, but it s really turning into everyday carbon dioxide and water. Simple, right? Whenever atoms and molecules reorganize into new groups this is called a chemical reaction (or chemical change). Some common examples are a burning match, a burning candle, a sliced apple turning brown, a rusting nail, baking soda plus vinegar, and a burning piece of paper.
So how do scientists know if a chemical reaction has occurred? Remember, not everything is a chemical reaction (change); sometimes it s just a physical change, such as breaking a pencil, tearing paper, or melting ice. Chemists look for the following things to indicate (point out) that there has been a chemical change. THREE WAYS TO TELL THAT A CHEMICAL CHANGE HAS HAPPENED 1. A change of STATE 2. A change of COLOR 3. A change of TEMPERATURE Let s look at each of these briefly. 1. A change of state is the example of the candle. It changes from a solid wax candle into two gases. Baking soda (solid) and vinegar (liquid) turn into an invisible gas. 2. When iodine touches starch it turns black. When oxygen touches copper it turns green. This is why the Statue of Liberty is green! When oxygen touches a nail it turns orange (rust). 3. When some chemicals are mixed they either get hotter or colder on their own. Ever seen a cold pack. Hit it and mix the chemicals together and voila, it gets cold. A hand warmer generates heat when the molecules are mixed. When the chemicals in a match are ground together it gets VERY hot all by itself. These are some general guidelines. Students should learn them in identifying chemical reactions. Ask students to say the 3 ways to tell there s been a chemical reaction when you are talking about chemical reactions (change). This lesson focuses on a chemical change all right, but not a change of color, temperature or state. This milk, and the bacteria in them, will become more acidic over the next five days. There are special chemicals which will help to find certain chemicals and these are called INDICATORS. Chemists use indicators all the time such as testing a swimming pool for chlorine, aquarium water to check for excess acidity or alkalinity (or base). When indicators touch the molecule they react with, a color change occurs. This is much safer that tasting all the chemicals! There is a family of chemicals called the ACIDS and BASES. They are in the same chemical family even though they are opposites of each other. Scientists measure how strong the acids and bases are with something called the ph Scale.
The Acids go from 0 to 6. Neutral is 7 on the ph scale. From 8 to 14 are the Bases (also called Alkaline). The strongest ACID is a 0. The strongest BASE is a 14. The increment from number to number is a factor of ten. The weird thing is that a 5 is ten times stronger than a 6. A 4 is ten times stronger than a 5. That would make a 4 on the ph scale one hundred times stronger than a 6! This is a neat math scale. More on this later. There are many indicators for acids and bases. You can even buy a digital ph probe, but for this lesson we will use ph paper 1-13 over a five day period. In this lesson we will test MILK over time to see if it is ACID, NEUTRAL, OR BASE. In each case the indicator will turn a color (which is a sign of a chemical reaction occurring). Will the milk which little Whitney left out become basic, acidic, or not change at all? (See PRODUCTIVE TALK MOVES and STRUCTURING SCIENCE TALK packet. This gives you excellent ways to teach your students to move forward and get past yes/no answers and helps them develop strategies for thinking about the problems.)
OBJECTIVES: 1. Students will test for acids, neutrals and bases using milk left out at room temperature for a week with 100% accuracy. 2. Students will record the data with 100% accuracy. 3. Students will determine the relative strength of the acids or bases relative to neutral with 100% accuracy. This objective can be done the next day if the teacher has a short class. THE LESSON PREPARATION AND/OR ADVANCED PREPARATION 1. Buy some ph paper strips from Nasco.com. Get some which measure 1-13 ph. 2. Get a cup of milk (or a new, unopened student carton of milk from the cafeteria) THE EXPERIMENT 1. Pour a new, unopened milk carton of fresh milk into two clean glasses (or test tubes). Put an equal amount in each container. 2. Ask students to hypothesize what will happen to the ph of the milk if we left it out? Remind them that ph measures acid, neutral and base. 3. Discuss the following question: Is this question measurable and testable? In a lab? TO DO AN EXPERIMENT IT HAS TO BE TESTABLE? 4. Ask students to form a HYPOTHESIS for what happens to the milk when it sits out. Is it an acid, neutral, or a base? Record the HYPOTHESIS above the DATA Table. 5. Remind (guide their thinking) them that to be a fair test, perhaps there should be two samples of milk, one which remains in the fridge, and one which stays on a counter. Shouldn t they both be tested? (yes. The one which remains in the fridge is what scientists call a CONTROL GROUP) 6. Discuss with students this question: WHAT IS THE INDEPENDENT VARIABLE IS IN THIS EXPERIMENT? (the independent variable is the thing which you are going to do different to one group) What are we doing different to the one milk sample? (leaving it out of the refrigerator to get warm)
Normally we don t leave milk out in the room, we keep it in the refrigerator. In this case though, we are leaving the milk out ON PURPOSE. This is called the INDEPENDENT variable. 7. Now, discuss the DEPENDENT VARIABLE is in the experiment. (The dependent variable is the thing which will then be measured.) WHAT IS GOING TO BE MEASURED IN THIS EXPERIMENT? (the ph of the milk each day) 8. Students should tear off a one inch piece of ph paper strip for each glass. Dip the ph paper quickly (just a touch...don t linger!) in the milk and hold it up next to the color key which comes with the ph paper. HINT: it is easier to hold the ph paper strip by a pair of tweezers. Just rinse them off with water and dry them before reusing. Matching the color it turned to the key will tell you the ph number (perhaps 6.7 for milk). Test both milk samples and write the results in the DATA chart. (see below) 9. Repeat step 8 (above) each day for 5 days (MONDAY - FRIDAY. Each day review each of these things: CONTROL GROUP, EXPERIMENTAL GROUP, INDEPENDENT VARIABLE, DEPENDENT VARIABLE. Make little signs which you can hold next to the appropriate group, variable, etc. Write the ph for the warm milk and the refrigerated milk on the data chart each day. (See PRODUCTIVE TALK MOVES and STRUCTURING SCIENCE TALK packet. This gives you excellent ways to teach your students to move forward and get past yes/no answers and helps them develop strategies for thinking about the problems.)
HYPOTHESIS: DATA TABLE THE PH OF MILK SOURING DATA TABLE 1 DATE ph of milk in refrigerator ph of milk left out in room 2 3 4 5 CONCLUSION:
10. On day five graph the results. I suggest createagraph.com. Below is how the graph will most likely look for the warm milk which was at room temperature. YOU WOULD NEED ANOTHER GRAPH FOR THE CONTROL GROUP (milk in refrigerator). 11.This is a strange scale. It s NOT exactly like a normal number line, is it? 12. Think how powerful battery acid is (see chart at bottom) compared to water (ph 7 neutral)! It s super strong. That means that battery acid (ph 0) is 1,000,000 times stronger than milk (ph 6). Milk has lactic acid in it, making it barely an acid. Unless you let it ferment into yogurt (ph 4.5 to 5). Or it sours (4.7) 13. Have students write a CONCLUSION. Was the hypothesis correct and supported BY DATA?
In this lab students learned that a CHEMICAL INDICATOR will turn a color if it comes in contact with a certain type of molecule. Chemists know about dozens of different chemical indicators. These help scientists test for poisons, harmful chemicals, helpful chemicals, and even test for chemicals in the human body. Chemical indicators will show if a person has been drinking alcohol, using marijuana or other illegal drugs, is pregnant, or has diabetes, just to mention a few common indicator tests. So you see, chemical indicators are very useful. The ph papers were indicators. It was a much nicer way to find the ph of the two milk groups than TASTING! Yuk! VARIATIONS AND EXTENSIONS OF THE LESSON Are there other everyday milk product chemicals which students would like to test? Students may want to perform experiments on other milk products, such as yogurt to find out if they contain acid, neutral, or base molecules. QUESTIONS and CONCLUSIONS: 1. What patterns did you see in your results and data? ( the ph number decreased quickly after DAY 3) 2. How might technology help in testing for acid and base molecules? (could use a digital ph probe hooked into your ipad) 3. How does the data chart assist you in making logical predictions and plan addition investigations? (e.g., what might have happened on day 6) 4. Were these physical or chemical changes? Can you justify why these experiments were chemical or physical changes? 5. What TEN substances would you MOST like to test if you had the opportunity? (e.g. stomach acid, tears, the cafeteria food...) 6. Is this an important experiment? Why? Justify your answer with data! (See PRODUCTIVE TALK MOVES and STRUCTURING SCIENCE TALK packet. This gives you excellent ways to teach your students to move forward and get past yes/no answers and helps them develop strategies for thinking about the problems.) 2013 Ronald Boston
ph papers which test from 1-11 can be purchased from NASCO science for $11.25. One container will last for years. http://www.enasco.com/product/c08835n ph plastic strips from 1-13 can also be purchased from NASCO for $13.50. http://www.enasco.com/product/sb48831m