Question

The atoms, molecules, or compounds present at the start of a chemical reaction that participate in the reaction.
Which part of a chemical reaction does this define?
A. products
B. reactants
C. subscripts
D. coefficients

Which is a chemical reaction?
A. solid chocolate melting into liquid chocolate
B. a peanut shell cracking open into two halves
C. water vapor condensing into liquid water on a window
D. hydrogen gas and oxygen gas synthesizing into liquid water

Answer 1

for the first one the answer is B. and the second one is D.

Answer 2

Answer:

For 1: The chemical species present at the start of the chemical reaction are reactants.

For 2: Hydrogen gas and oxygen gas synthesizing into liquid water is a chemical reaction.

Explanation:

• For 1:

In a chemical equation, the chemical species are termed as reactants or products.

Reactants are defined as the species which react in the reaction and are written on the left side of the reaction arrow.  They are present at the start of the reaction.

Products are defined as the species which are produced in the reaction and are written on the right side of the reaction arrow. They are present at the end of the reaction.

$\text{Reactants}\rightarrow \text{Products}$

Hence, the chemical species present at the start of the chemical reaction are reactants.

• For 2:

Physical change is defined as the change in which change in shape and size takes place. The chemical composition of a substance remains the same. No new substance is formed during this.

Chemical change is defined as the change in which change in chemical composition takes place. A new substance is formed in this.

For the given options:

Option A: A solid chocolate is melting into liquid chocolate by providing heat. The shape of the chocolate is changing. So, its a physical change.

Option B: When a peanut shell is cracked up, the size of the peanut shell is changed. So, it is a physical change.

Option C: When water vapor is cooled down to form liquid water, no new substance is getting formed. So, it is a physical change.

Option D: When hydrogen gas reacts with oxygen gas, it leads to the formation of water. As, new substance is getting formed. So, it is a chemical change.

$2H_2+O_2\rightarrow 2H_2O$

Hence, hydrogen gas and oxygen gas synthesizing into liquid water is a chemical reaction.