Astudent made the following observations about astronomical bodies. observation 1: mars reverses its direction of motion in the sky. observation 2: the stars appear brighter at different times of the year. which observation(s) can be explained using the heliocentric model? a. only observation 1 b. only observation 2 c. both observation 1 and observation 2 d. neither observation 1 nor observation 2

Use the table to identify the phase and phase changes of the elements under the given conditions. write the name of the substance, phase, or phase change

Agroup of students is studying convection current. they fill two identical balloons with the same amount of helium. one balloon is placed in a freezer and the other is in an area with warm air. after 10 minutes, the balloon are released from a height of 1 meter. which of the following to the students most likely observe? a) the warm balloon expands and rises. the cold balloon shrinks and sinks b) the balloon both rise. the cold balloon is larger than the warm balloon c) the cold balloon expands and rises. the warm balloon shrinks and sinks d) the balloon rise at the same rate. both balloons are the same size

Astudent reacts 13 moles of iron with 21 moles of oxygen according to the following equation: