What are possible sources of error in a lab concerning the law of conservation of mass?

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The law of conservation of mass states that matter cannot be created nor destroyed on Earth. Theoretically, when performing a lab, the mass of the reactants should be the same as the mass of the products. However, lab error may occur so that the mass of the reactants and products are not equal. Lab error is defined as any error that is made by the scientist, errors caused by equipment used within the laboratory, errors in the recording of data, calculation errors, and/or errors in the analysis of the data used to derive a conclusion. For example, if the heating of a liquid was involved in your lab, then some mass of the reactant may have been lost due to evaporation. This would cause the mass of your final product to be lower in mass than you would expect. Keeping good notes while performing your lab and calibrating equipment can help reduce lab error within an experiment.

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

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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