19. Which statement best describes the reaction taking place in the core of a star during its main sequence stage? a. Decay of carbon-14 to carbon-12
b. Fusion of hydrogen into helium
c. Fission of helium into hydrogen
d. Fusion of hydrogen into water vapor

Astudent made a graph plotting the progress of a reaction over time. the student forgot to label the y-axis of the graph. a graph is shown with two graph lines. one graph line starts at a higher position on the y axis and slopes downwards towards the right. the other graph line starts at a lower position on the y axis and slopes upwards towards the right. the two graph lines stop short of intersecting each other and continue as separate lines which gradually become straight and parallel to the x axis. a vertical line is shown at a point where the two graph lines finally became parallel to the x axis. this vertical line is labeled equilibrium. the title on the x axis is time and an arrow pointing towards the right is shown above time. the title on the y axis is left blank. what best explains the label that the student should use on the y-axis? amount, because as the amount of product decreases, the amount of reactant increases over time. reaction rate, because forward and backward reaction become equal at equilibrium. amount, because the amounts of reactants and products become constant after equilibrium is reached. reaction rate, as the rate of forward reaction increases and rate of backward reaction decreases over time.

Which of the following is an example of physical change not a chemical change? a) a log gives off heat and light as it burns. b) a tree stores energy from the sun in its fruit. c) a penny lost in the grass slowly changes color. d) a water pipe freezes and cracks on a cold night.

An uncovered pot of water lies out in the sun. which statements correctly describe what happens at the surface of the liquid water? 1. the vapor pressure remains constant regardless of the water temperature. 2. the vapor pressure is produced by water molecules that have evaporated. 3. the vapor pressure increases as the sun heats the water in the pot. 4. evaporation stops once the vapor pressure reaches a certain point. 5. evaporation and condensation both occur on the liquid’s surface.