Which of these statements is incorrect regarding the laws of chemical combinations?

Options

A. Combination of elements in ratio by atoms is same as their combination ratio by mass
B. Elements may not combine with each other also. For instance, sodium and potassium don't combine to form compounds.
C. Elements can have multiple valencies
D. Fixed amount of an element should combine with different amounts of another element for law of multiple proportions.

Answer

To determine which statement is incorrect regarding the laws of chemical combinations, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Understanding the Laws of Chemical Combinations**:
- The laws of chemical combinations include the Law of Conservation of Mass, Law of Definite Proportions, and Law of Multiple Proportions.
- These laws explain how elements combine to form compounds and the ratios in which they do so.

2. **Analyzing the First Statement**:
- **Statement**: "Combination of elements in ratio by atom is the same as their combination ratio by mass."
- **Analysis**:
- For example, in carbon monoxide (CO), the atomic weight of carbon (C) is 12 and that of oxygen (O) is 16.
- The mass ratio of C to O is 12:16, which simplifies to 3:4.
- However, in a molecule of CO, the ratio by atoms is 1:1 (1 carbon atom and 1 oxygen atom).
- Since the two ratios (3:4 by mass and 1:1 by atoms) are not the same, this statement is incorrect.

3. **Analyzing the Second Statement**:
- **Statement**: "Elements may not combine with each other."
- **Analysis**:
- An example is given: Sodium (Na) and Potassium (K) do not combine to form a compound.
- This statement is correct as certain elements, especially those in the same group of the periodic table, do not react with each other.

4. **Analyzing the Third Statement**:
- **Statement**: "Elements can have multiple valencies."
- **Analysis**:
- Certain elements, such as copper and iron, exhibit variable valencies.
- This statement is correct as it is a known fact in chemistry.

5. **Analyzing the Fourth Statement**:
- **Statement**: "Fixed amount of an element should combine with different amounts of another element for the law of multiple proportions."
- **Analysis**:
- This statement correctly describes the law of multiple proportions, which states that when two elements combine to form more than one compound, the masses of one element that combine with a fixed mass of the other are in a ratio of small whole numbers.
- This statement is also correct.

### Conclusion:
The incorrect statement regarding the laws of chemical combinations is the **first statement**: "Combination of elements in ratio by atom is the same as their combination ratio by mass."