Most students today finished within 1.5hrs, and my last student left after 2.5 hrs. It is helpful to divide students into three groups and have each group start with a different part of the lab so that they aren't all waiting in line for the same chemicals. Also watch for students bringing chemical bottles back to their benches - they usually forget to put them back.
PRE-LAB - I first went over the grading for the lab reports from the past two weeks and discussed common mistakes - incorrect sig figs, missing units, missing observations, missing or illegible calculations and again what I expect to see in a good purpose, procedure and conclusion. There is plenty of time in lab today, so I wanted to make sure that my students understand my expectations - they pay more attention now than they did on check-in day since they already have experience with writing reports and have received grades back.
I gave examples of acid-base and precipitation reactions, including the molecular, complete ionic and net ionic equations for these types of reactions. They should have already covered this in class, but several still needed a review. We also discussed possible observations and what they mean (bubbles, color change, texture change, heat, turns cloudy). I also discussed the difference between 'clear' and 'colorless' since most students use these interchangably.
PART C - Almost every student thought that Na2CO3 should be a solid in the reaction and then wouldn't break up into ions. Some students thought this was a ppt rxn because they used too much Na2CO3 and assumed the solid in the bottom was a product (NaCl (s)) rather than leftover reactant. Make sure they stir/shake the reaction mixture.
Q4 - You can make up your own question, or omit - I had my students answer this question before doing Part C. I had them design an experiment to verify whether Na2CO3 is solid or aqueous. I expect them to be able to put a small amount of Na2CO3 into a test tube and add DI water to see whether it dissolves. They could verify the presence of ions using conductivity.
Waste - 2 beakers - the one with Ag waste goes in jug. Acid/base beaker gets neutralized and poured down the drain. Send them to look at the colors on the back of the universal indicator bottle to see the yellow/green a neutral solution should be. You might want to have them check with you when they get to this step - many add too much acid or base and then swing back and forth ending up with 500mL of waste at the end.