In a double-replacement reaction,
the reactants are usually a metal and a nonmetal
one of the reactants is often water
energy in the form of heat or light is often produced
the reactants are generally two ionic compounds in aqueous solution

The correct answer is D: the reactants are generally two ionic compounds in aqueous solution.

Explanation:

A type of reaction in which two reactants participate and exchange ions with each other as the reaction proceeds. Such type of reaction is known as a double displacement reaction. In double displacement reaction, the product formed is precipitate.

Below is the pathway of the double displacement reaction:

AB + CD → AD + CB

In double displacement reaction usually, ionic compounds are involved. The bond formation between the reacting species may be ionic or covalent in nature.

1. 1 mole of anything contains 6.022x10^23 pieces  

d  

2. a  

3. c, number  

5. 1.24x10^24atoms / 6.022x10^23atoms/mole = 2moles h2 atoms  

6. fe2br3 mass = 2moles x mass fe + 3 moles x mass br  

molar mass = grams / number of moles = g/mole  

to find moles = g / molar mass  

to find mass = g = molar mass / moles

in cartography, a contour line (often just called a "contour") joins points of equal elevation (height) above a given level, such as mean sea level. a contour map is a map illustrated with contour lines, for example a topographic map, which thus shows valleys and hills, and the steepness or gentleness of slopes.

explanation: