Suppose 20 atoms of hydrogen (H) react with chlorine (Cl) to produce 20 molecules of hydrochloric acid (HCl). According to the law of conservation of matter, how many atoms of chlorine reacted with the hydrogen?

According to the reaction between hydrogen and chlorine to produce hydrochloric acid (HCl), the balanced chemical equation is:

\[
H_2 + Cl_2 \rightarrow 2HCl
\]

This means that 1 molecule of \(H_2\) reacts with 1 molecule of \(Cl_2\) to produce 2 molecules of \(HCl\).

In your scenario, we have 20 atoms of hydrogen. Since hydrogen exists as diatomic molecules (\(H_2\)), to find out how many molecules of \(H_2\) correspond to 20 atoms, you can calculate:

\[
\text{Number of } H_2 \text{ molecules} = \frac{20 \text{ atoms of } H}{2 \text{ atoms per molecule}} = 10 \text{ molecules of } H_2
\]

According to the balanced equation, 10 molecules of \(H_2\) will react with 10 molecules of \(Cl_2\) to produce 20 molecules of \(HCl\).

Next, we convert the molecules of chlorine \( (Cl_2) \) to atoms. Each molecule of \(Cl_2\) consists of 2 chlorine atoms, so:

\[
\text{Number of } Cl \text{ atoms} = 10 \text{ molecules of } Cl_2 \times 2 \text{ atoms per molecule} = 20 \text{ atoms of } Cl
\]

Therefore, **20 atoms of chlorine reacted with the hydrogen**.