Why are chemical reactions often carried out using solutions? the heterogenous nature of solutions prevents the reactants from freely mixing. the amount of each reactant can be controlled by adjusting the volume of each solution that is used. even very small amounts of reactants can be combined by using dilute solutions. a color change will always occur when a reaction is conducted in solution. the molecules or ions in a solution can freely intermingle. the solvent must always be water, so it is impossible for a fire to start.

The molecules or ions in a solution can freely intermingle.  

Explanation:

The reactant molecules and ions can move freely in solution, so thy are more likely to collide with each other and form products.

A is wrong. Solutions are homogeneous.

B and C are wrong. The statements are correct, but they are not the reason why reactions are carried out in solution.

D is wrong. Chemical reactions do not always produce a colour change.

F is wrong. Some chemicals react with water, so other (often flammable) solvents must be used.

they are made of gases.

they have a spherical shape.

they make energy through fusion.