Pinky's science fair project in junior high focused on an observation that the crickets outside her window chirped less frequently when the temperature was colder.

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Explanation:

The complete question: Pinky's science fair project in junior high focused on an observation that the crickets outside her window chirped less frequently when the temperature was colder. Consider if data was collected consisting of the average number of cricket chirps  at different temperatures in her backyard. What's the independent variable?

The independent variable would be the temperature.

Independent variables are variables that produce effects on another variable while remaining unaffected themselves. They can be manipulated in the course of a research in order to measure the kind of effects they exert on other variables - the dependent variables to be specific. The dependent variables are the actual variables needed to test the hypothesis of a research.

In this case, a variation in temperature was observed to have an effect on the chirping frequency of crickets. Hence, the independent variable would be the temperature.  

before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.

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