Name: Block: Date: Student Notes

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1 Name: Block: Date: LCPS Core Experience Watersheds Student Notes OBJECTIVES Students will: investigate the difference between physical and chemical weathering identify the destination of the loss of mass of a substance after weathering has occurred 1. Describe what happens when you rub sandpaper on a piece of wood? 2. Have you ever seen a crumbled sidewalk? If so, how do you think it got crumbled? Materials Per Lab Group 1 water soaked marble chip 1 dry marble chip 1 water soaked piece of chalk 1 piece of dry chalk 2 50 ml beakers LINK 60 ml vinegar in 100 ml beaker forceps 1 triple beam balance 1 strip of 28cm x 11.5cm sandpaper stopwatch Background Weathering is the breakdown or decomposition of earth material, mainly rocks. Physical weathering (mechanical weathering) is the breakdown of rocks and soils through direct contact with atmospheric conditions such as heat, water, ice and pressure. The rock simply experiences physical changes. Causes of physical weathering include the breakage of rocks due to freezing and thawing cycles, heating and cooling cycles, the action of tree roots that causes rocks to split, gravity that causes rocks to fall and break. Rocks can also be broken up by friction and impact by other rock fragments and flowing water. Chemical weathering occurs when chemicals react with the minerals in a rock to convert the minerals to new compounds. A chemical change occurs. After millions of years of chemical weathering the chemical reactions will slowly break down the rock into smaller and smaller pieces and release gases. Page A

2 LCPS Core Experience Background (continued) Marble consists mostly of the mineral calcium carbonate along with other minerals. If marble is exposed to chemicals, such as acids, the calcium carbonate will react and chemical weathering will occur. Only the calcium carbonate and not the quartz will react. One naturally occurring acid that can chemically weather marble is carbonic acid, H2CO3, which is formed when carbon dioxide, CO2, in the atmosphere dissolves in water, H2O. Chemical weathering of the calcium carbonate in marble is shown in the following equation: CaCO 3 + 2H 2 CO 3 Ca HCO CO 2 + H 2 O Calcium carbonate Carbonic acid Calcium cation Bicarbonate anion Carbon dioxide Water Chemical weathering by carbonic acid occurs very slowly over millions of years because it is a weak acid. Stronger acids like hydrochloric acid, sulfuric acid and acetic acid will react more quickly with marble than carbonic acid. These acids are not naturally occurring so they do not usually weather rocks with the exception of sulfuric acid which is produced when coalburning power plants release chemicals into the air. EXPERIMENT Lesson One: Model Chemical Weathering Make a Prediction: What will happen when chalk and marble, which both contain calcium carbonate, are soaked in vinegar (acetic acid)? (Recall Properties of Matter Investigation TWO.) 1. Obtain a sample of pre-soaked chalk. Find its mass and record it in Table A. 2. Obtain a pre-soaked marble chip. Find its mass and record it in Table A. 3. Fill the beakers to the 30 ml mark with vinegar. 4. Drop the chalk and the marble chip into the separate beakers of vinegar at the same time. Start the stopwatch. 5. Begin Lesson Two and after 10 minutes, remove the chalk and marble chip from the vinegar with the forceps. Dry with a paper towel and find the mass of each and record in Table A. 6. Calculate the change in mass due to chemical weathering. Subtract columns 1 and 2. Record the change in mass on Table A in column 3. Page 2

3 LCPS Core Experience Table A: The Effect of Chemical Weathering on the Mass of Substances Chalk Marble Chip 1 Original Mass 2 Mass After Soaking in Vinegar 3 Change in Mass 4 EXTENSION Mass After Soaking Until the Next Class 5 Change in Mass 7. Question: Describe what has happened with the change in mass. 8. Extension: Follow directions from your teacher to let the marble chip soak in vinegar until the next class. 9. What was the change in mass after soaking the marble chip in vinegar since the last class? Subtract columns 1 and 4. Record the change in mass on Table A in column 5. Lesson Two: Model Physical Weathering Make a Prediction: What will happen when chalk and marble are rubbed against sandpaper? 1. Obtain a piece of dry chalk and a dry marble chip. Find the mass for each and record in Table B. 2. Drag the chalk sample across the sandpaper 15 times. 3. Find the mass of what is left of the chalk sample and record it in Table B. 4. Repeat steps 2 and 3 with the marble chip. 5. Calculate the change in mass due to physical weathering. Subtract columns 1 and 2. Record the change in mass on Table B column 3. Page B

4 LCPS Core Experience Table B: The Effect of Physical Weathering on Substance Mass 1 2 Original Mass After Dragging Mass on Sandpaper Chalk 3 Change in Mass Marble Chip 6. Question: Where did the change in mass go? LEARNING REVIEW 1. What is weathering? 2. What is physical weathering? 3. What is chemical weathering? EVALUATION 1. What is the difference between physical weathering and chemical weathering? 2. During physical weathering in our actual environment, where do the rock fragments go? Page 4

5 LCPS Core Experience 3. During chemical weathering in our actual environment, where do the rock fragments and other products go? 4. Observe the photographs below and describe what caused the face to fall off the mountain? The Old Man in the Mountain in 2001 (AP File Photo) The face of the Old Man in the Mountain recently fell from its mountainside in (AP Photo). Page C

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