Which Changes Result in a Chemical Reaction?
Which of these changes produces a chemical change?
A. A stone fizzes and forms a gas when acid is dropped on it.
B. Limestone slowly dissolves in water and forms a cave.
C. Lava cools and changes from a thick fluid to solid rock.
D. A large rock is crushed into gravel.
Which of the changes listed above results in a chemical change?Answer:
Explanation:
Limestone (CaCO3) dissolves in water - with difficulty - but remains, and can be recovered as CaCO3
Solidifying lava and rock crushed to gravel are physical changes.
If acid dropped on a rock causes fizzing and gives off gas, that is a chemical reaction, e.g,
CaCO3 + 2HCl —> CaCl2 + H2O + CO2
Dissolution of limestone in water does result in dissociation into ions that may be hydrated, but CaCO3 is precipitated out as solutions become more concentrated.
CaCO3 + 2H2O —> Ca++ + CO3= + OH- + H3O+