Chemical and Physical Changes Lab
Lesson 5 of 5
Objective: Students will be able to use evidence to determine if a reaction is a chemical or physical change.
This lesson is based on California's Middle School Integrated Model of NGSS.
PE: MS-PS1-2 – Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
Science & Engineering Practice 4 - Analyzing and Interpreting Data
Science & Engineering Practice 6 - Constructing Explanations and Designing Solutions
DCI: PS1.A: Structure and Properties of Matter – Each pure substance has characteristic: physical and chemical properties that can be used to identify it.
CCC: Cause and Effect – Cause and Effect relationships may be used to predict phenomena in natural or designed systems.
Lab Set-Up (Pre-lab)
In this lab student will use their knowledge of chemical and physical properties to perform five tests of common household materials to determine whether they have undergone a chemical or physical change. They will collect observations about five reactions and analyze their observations (S&E Prac 4: Analyzing and Interpreting Data) to figure out if the reaction supported evidence for a chemical reaction or was merely a physical reaction (MS-PS1-2). During the course of this lab they will also attempt to explain, with evidence, why a reaction should be classified as chemical or physical (S&E Prac 6: Constructing explainations and designing solutions).
This lab was inspired by McDougal Littell - Focus of Physical Science, CA Edition: Chapter Investigation p.156
I typically front-load the students with information about the Evidence of a Chemical Reaction and the Physical Properties of matter (PS1.A). Look out for a common misconception that no apparent reaction means they did something wrong - they may have only changed the volume within a container when they mixed two liquids, which is a physical change. While your students are working on this activity it is important to point out that these five reactions when mixed (cause) have a predictable outcome (effect) (CCC: Cause and Effect).
Chemical Materials: sugar cubes, granulated sugar, ice, tap water, vinegar, baking soda (sodium bicarbonate).
Equipment Materials: 2 empty baby food jars, metal spoon, 2 test-tubes, candle, test-tube rack, test-tube clamp, graduated cylinder, eye protections, triple-beam balance scale, large beaker (1000mL), timer.
TIP: These materials are for one group of students. IF you do not have access to graduated cylinders, approximations can be substituted. A large cup can be used in place of the large beaker. I buy cheap decorative candles with small glass holders. I have a large aluminum tray for the ice – I don’t want the kids playing with the ice or eating it so I tell them that many hands will be touching the ice over the course of the day and ask them how many of those students wash their hands after using the restroom. They NEVER want to play with it again. The question always comes up about what would happen if they eat any of the chemicals. My response is that it will cause painful diarrhea. Kids are not afraid of death by chemical ingestion, but they are terrified of any form of diarrhea – go figure!
I demonstrate how to perform the lab and use the equipment the day before, but I am very careful to not reveal any of the outcomes. I explain what to do and tell them that SOMETHING will happen.
Each student should receive a copy of Chemical and Physical Changes Lab. It includes a list of material for each group, a set of students directions, an observation chart, and a set of questions.
The observation page is the most important. Each student should record what they observed (practicing recording their observations in a scientific manor) and determine if each change is chemical or physical. (See Extension Section for ideas about reviewing chemical and physical changes.)
Student Directions (also found in student handout in resources)
1) Place an ice cube (H2O) in an empty baby food jar and let it sit for 10 minutes. Record your observations.
2) Use the metal spoon to grind a sugar cube (C12H22O11) into fine powder. Record how the sugar has changed. Dispose of sugar in trash.
3) Pour a half spoonful (about 2 g) of the granular sugar (C12H22O11) into a test tube. Have your teacher light the candle. Hold the test tube with the clamp so the bottom of the test tube is just above the flame for several minutes. Be sure the top of the test tube is pointed away from you. Place hot test tubes in the test tube rack when done. Record your observations.
4) Using the graduated cylinder, measure 50 mL of water (H2O). Pour contents into an empty baby food jar. Wipe out the graduated cylinder with a paper towel. Measure 50 mL of vinegar (CH3COOH). Add contents to the same baby food jar. Pour the contents down the drain. Record your observations.
5) Using the graduated cylinder measure 50 mL of vinegar (CH3COOH). Pour contents into the large beaker. Use the triple beam balance to measure 10 grams of sodium bicarbonate (NaHCO3). Add the sodium bicarbonate (NaHCO3) to the vinegar (CH3COOH). Pour the carbon dioxide gas (CO2) over the candle flame to extinguish the flame. Pour the remaining contents down the drain. Record your observations.
TIP: I teach my student that used dry chemicals should be placed in the trash, and certain liquid chemicals can be poured down the drain. Check with your Site Principal for specific direction in your state. I’m not concerned about pouring vinegar down the drain.
The last step in direction 5 is not related to this specific lab. I use this to introduce properties of carbon dioxide. Later in the year when we discuss carbon dioxide I often bring up this incidence as an example.
Student Lab Sheet
Teacher Cheat Sheet of Activities:
1) Melting ice: Physical reaction - the ice is changing from the solid state to the liquid state. Changes of state are physical.
2) Crushing sugar cube: Physical reaction - changing the size or shape of a material (i.e. cutting something in half) is physical.
3) Burning sugar in test-tube: Chemical reaction - burning sugar produces a sweet smell (odor), bubbles (gas), and turns the white sugar black (change in color/light).
4) Mixing water and vinegar: Physical Reaction – no evidence of a chemical reaction is observed (change in color/light, change in temperature, create a solid, produce an odor, form a gas).
5) Mixing baking soda and vinegar: Chemical – new bubbles will form (form gas).
My students record their reflections in their Interactive Journal. They build a decision wheel for chemical and physical reactions.
Pass out one Decision Wheel to each student. Have them cut on the dotted lines, insert a brad, and glue into their Science Interactive Notebook (if you use one).
Include a title for that paper. The top of each "pizza slice" should contain the following terms: change in color, change in temperature, form a solid, produce an odor, observe a gas, change texture, increase or decrease volume, change size, more or less mass, change in density. These terms can be placed in any order.
Inside each pizza flap each student must produce an example, definition, drawing, or hint that would allow a reader to determine if each change is a physical or chemical reaction.
TIP: Color can be either chemical or physical and is a part of both lists (crayons produce a change in color, but is physical, burning wood produces a change in color and is chemical). They have to describe in detail what a reader would need to observe to determine if a reaction is chemical or physical. This causes the students to reflect and understand the difference. This is the definition I most like to read for understanding.
They must use a least three color (color pencils/ high liters) to make their definitions. The act of making these definitions colorful provokes meaning in the students.
For determining a chemical or physical reaction I teach two mnemonic sentences;
1) Evidence for a Chemical Reaction
Cats Tell Stories of Ghosts.
Color/light, temperature change, formation of a solid (precipitate), produce an odor, observe a gas
2) Physical Properties of Matter
The very special cows meet daily.
Texture, volume, size, color, mass, density
TIP: I do warn my students that color can be either a chemical or physical change. If they only observe a change in color, I tell them to discuss the topic carefully and that they may wish to seek additional evidence.