What could be two sources of error in my lab about the law of conservation of mass and popcorn kernels? 

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When popcorn is heated until it pops using hot-air popping, the mass of the resulting popcorn should be less than the mass of the kernels before they are popped. This is due to the reason the popcorn pops: the small amounts of water contained within the kernels is heated until it turns to steam, "popping" the corn into fluffy popcorn. The mass of the water that turned to steam is lost, so you should have seen a reduction in mass. Did you have a chance to repeat the experiment? Repetition is one of the ways to check results.

Sources of error in scientific experiments include limitations of the equipment used. In your case, if the balance you used to find the mass of the kernels and popped popcorn was not able to detect the small amount of mass lost, you would not be able to measure accurately. If either the kernels or the popped corn was contaminated with something, this could also throw off your results (a pebble in with the popped corn, or if you popped in oil instead--the oil could have added mass). There is also human error, but we don't usually put that in lab reports. You could have misread the scale, for example. We usually don't count such things as experimental error, though.

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