Reflection: Checks for Understanding Decomposing Water Lab - Section 3: Student Activity (Lab)


I often have students that believe that the bubbles are coming from an air cavity inside the batteries, much like a leaking bike inner tube submerged under water or the battery is heating up the water and causing the water to boil.

Towards the end of this activity, at the point the students are trying to determine if a chemical reaction is occurring, I have students asking me about the bubbles. I ask reflection questions about where the bubbles are coming from. Almost all the kids identify the source of the bubbles as from inside the battery. When I remind them that the battery has no internal air space, then they start to understand that bubbles are coming from water. A misconception often develops that the bubbles observed are the same as boiling bubbles produced from hot water. I have to remind the students that the water is not hot. Having them touch the water to determine the temperature is a concrete test for them to understand the nature of the observed bubbles.

After spoon-feeding my students the possible answers they are able to arrive at the idea that bubbles are from the water (hydrogen and chlorine) and they are in the form of a gas when they are stripped from the water. Make sure your students understand that the bubbles are NOT from the internal structure of the battery and they are NOT produced from heating the water.  

To check for understanding, I briefly spot-check the student's drawings to see where they think the bubbles are coming from. Checking their drawings is much faster than reading student's written explanations. I can typically spot-check drawings rather rapidly at the end of the period. By placing the students in groups of four I typically only have to check one drawing within a group to determine if that group has developed a misconception. The whole process can be completed in a few minutes.   

  Students seeking evidence
  Checks for Understanding: Students seeking evidence
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Decomposing Water Lab

Unit 5: Chemical Reactions
Lesson 6 of 11

Objective: Students will use a 9v battery to decompose water into hydrogen and oxygen.

Big Idea: A safe lab that allows students to see the decomposition of water into its component elements.

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3 teachers like this lesson
  90 minutes
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