Suppose 10 atoms of carbon react with 20 atoms of oxygen to form 10 molecules of carbon dioxide. According to the law of conservation of matter, how many atoms are present in this reaction?

To determine the total number of atoms present in the reaction where 10 atoms of carbon react with 20 atoms of oxygen to form 10 molecules of carbon dioxide, we can count the atoms involved before and after the reaction.

1. **Reactants:**
- 10 atoms of carbon
- 20 atoms of oxygen

Total atoms in the reactants = 10 (C) + 20 (O) = **30 atoms**

2. **Products:**
- Each molecule of carbon dioxide (CO₂) contains 1 atom of carbon and 2 atoms of oxygen.
- For 10 molecules of CO₂, the composition is as follows:
- Carbon atoms = 10 (1 atom of C per molecule × 10 molecules)
- Oxygen atoms = 20 (2 atoms of O per molecule × 10 molecules)

Total atoms in the products = 10 (C) + 20 (O) = **30 atoms**

According to the law of conservation of matter, the total number of atoms remains constant before and after the reaction. Therefore, in this reaction, the total number of atoms present is:

**30 atoms.**

The answer is: **4. 30**