2. Which of the following is good evidence that an unknown sample is a pure
substance.
A
After leaving a liquid to settle, it separates.
B
After leaving a liquid to settle, it does not separate.
С
After evaporating a liquid, there is nothing left behind.
D
After evaporating a liquid, a white powder is left behind.
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С. After evaporating a liquid, there is nothing left behind

Explanation:

In chemistry, a pure substance is that substance that has a definite chemical composition. It cannot be separated using physical means as the chemical bonds that forms it have to be broken. Pure substances include elements and compounds.

According to this question, a good evidence that an unknown sample is a pure substance would be that: After evaporating a liquid, there is nothing left behind. This means that after using physical separation technique, THE PURE SUBSTANCE COULD NOT BE SEPARATED, hence, nothing is left behind.

adding or remove a gaseous reactant or product changes the concentrations. if the concentration of reactant or product is increased, the system will shift away from the side in which concentration was increased (i.e. if the concentration of reactants is increased, the system will shift toward the products. if more products are added, the system will shift to form more reactants). conversely, if the concentration of reactant or product is decreased, the system will shift toward the side in which concentration was decreased (i.e. if reactants are removed, the system will shift to form more reactants. if the concentration of products is decreased, the equilibrium will shift toward the products).

explanation:

because u need two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei to form isotopes.

so since they all have the same shape? i mean maybe because m& m and reeses are both chocolates