The rate of a reaction, which may also be called its velocity or speed, can be defined with relation to the concentration of any of the reacting substances, or to that of any product of the reaction. If the species chosen is a reactant which has a concentration c at time t the rate is - dc/dt, while the rate with reference to a product having a concentration x at time t is dx/dt. Any concentration units may be used for expressing the rate, thus, if moles per liter are employed for concentration and seconds for the time, the units for the rate are moles `"liter"^(-1) sec^(-1)`. For gas reactions pressure units are sometimes used in place of concentrations, so that legitimate units for the rate would be (mm. Hg) `sec^(-1)` and atm. `sec^(-1)`
The order of a reaction concerns the dependence of the rate upon the concentrations of reacting substances, thus, if the rate is found experimentally to be proportional to the `alpha^(th)` power of the concentration of one of the reactants A, to the `beta^(th)` power of the concentration of a second reactant B, and so forth, via.,
rate `kC_(A)^(alpha) C_(B)^(beta)" " (1)`
the over-all order of the reaction is simply
`n = alpha +beta + ` ---------(2)
Such a reaction is said to be of the `alpha^(th)` order with respect to the substance A, the `beta^(th)` order with respect to B and so on...
(Laidler, K. J., & Glasstone, S. (1948). Rate, order and molecularity in chemical kinetics. Journal of Chemical Education, 25(7), 383.)
In the following questions, a statement of assertion followed by a statement of reason is given. Choose the correct answer out of the following choices on the basis of the above passage.
Assertion: Reactions can occur at different speeds.
Reason: Rate of reaction is also called speed of reaction.

Options

A. Assertion and reason both are correct statements and reason is correct explanation for assertion.
B. Assertion and reason both are correct statements but reason is not correct explanation for assertion.
C. Assertion is correct statement but reason is wrong statement.
D. Assertion is wrong statement but reason is correct statement.

Answer

To solve the question regarding the assertion and reason statements about the speed of reactions and the rate of reaction, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Reactions can occur at different speeds." This is true because different reactions have different rates depending on various factors such as the nature of the reactants, temperature, and presence of catalysts.

**Hint**: Consider examples of fast reactions (like combustion) versus slow reactions (like rusting) to understand the variability in reaction speeds.

2. **Understanding the Reason**:
- The reason states that "Rate of reaction is also called speed of reaction." This is also true. The rate of a reaction quantifies how quickly reactants are converted into products, which can indeed be referred to as the speed of the reaction.

**Hint**: Recall the definition of rate in terms of concentration change over time, which aligns with the concept of speed.

3. **Evaluating the Relationship Between Assertion and Reason**:
- Both the assertion and the reason are true statements. However, the reason does not directly explain the assertion. The assertion talks about the variability of reaction speeds, while the reason defines what rate is.

**Hint**: Think about how one statement can be true without necessarily providing a direct explanation for the other.

4. **Conclusion**:
- Since both statements are true but the reason does not serve as a correct explanation for the assertion, we conclude that the correct answer is that both the assertion and reason are true, but the reason is not the correct explanation for the assertion.

**Final Answer**: Both the assertion and reason are true, but the reason is not the correct explanation for the assertion.