According to law of conservation of mass:

Options

A. reactants mass = products mass
B. gases are lighter than solids
C. kinetic energy changes with heat
D. reactants + oxygen = products

Explanation

**Step-by-Step Text Solution:**

1. **Understanding the Law of Conservation of Mass**:
The law of conservation of mass states that in a closed system, mass is neither created nor destroyed during a chemical reaction. This means that the total mass of the reactants (the substances that undergo the reaction) must equal the total mass of the products (the substances formed by the reaction).

2. **Defining Reactants and Products**:
In a chemical reaction, the substances that react are called reactants, and the substances that are produced are called products. For example, in the reaction of carbon (C) with oxygen (O2) to form carbon dioxide (CO2), carbon and oxygen are the reactants, and carbon dioxide is the product.

3. **Example Calculation**:
Let's consider a specific example:
- If we have 5 grams of carbon (C) and 10 grams of oxygen (O2), the total mass of the reactants is:
\[ \text{Total mass of reactants} = 5 \text{ grams (C)} + 10 \text{ grams (O2)} = 15 \text{ grams} \]
- When these reactants react, they form carbon dioxide (CO2). According to the law of conservation of mass, the mass of the products must also equal 15 grams. Therefore, the mass of CO2 produced will be:
\[ \text{Mass of products (CO2)} = 15 \text{ grams} \]

4. **Conclusion**:
From the above explanation and example, we can conclude that the mass of the reactants is equal to the mass of the products. Therefore, the correct statement according to the law of conservation of mass is that the reactant mass is equal to the product mass.

5. **Identifying the Correct Option**:
Among the given options, the first option states that "reactant mass is equal to product mass," which aligns with the law of conservation of mass. Hence, this option is the correct choice.