In this reaction 50% of the compound decompose in 10.5 min thus, it is half life of the reaction and denoted by symbol
.
(a) For first order reaction, rate constant and half life time are related to each other as follows:

Thus, rate constant of the reaction is
.
(b) Rate equation for first order reaction is as follows:
![k=\frac{2.303}{t_{1/2}}log\frac{[A_{0}]}{[A_{t}]}](https://tex.z-dn.net/?f=k%3D%5Cfrac%7B2.303%7D%7Bt_%7B1%2F2%7D%7Dlog%5Cfrac%7B%5BA_%7B0%7D%5D%7D%7B%5BA_%7Bt%7D%5D%7D)
now, 75% of the compound is decomposed, if initial concentration
is 100 then concentration at time t
will be 100-75=25.
Putting the values,

On rearranging,

Thus, time required for 75% decomposition is 21 min.
The answer is 5.4! I found it on a an online calculator!
When a substance is entering a phase change, the gain or loss of heat is a result of energy gained or lost in forming or breaking intermolecular interaction.
The constant temperatures occur when a substance is undergoing a phase transition. If heat is removed from a substance , such as in freezing and condensation , then the process is exothermic . In this instance , heat is decreasing the speed of the molecules causing then move slower.
Example : liquid to solid and gas to liquid .
These changes release heat to the surrounding.
To learn more about phase change,
brainly.com/question/12390797
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Explanation:
A chemical reaction is defined as the reaction in which bonds between the reactants either break or form which leads to the formation of a new substance.
For example, 
So, when we drop a sodium metal into water then it produces a frizzing sound which shows the metal is reacting with water.
We know that when two aqueous solutions chemically react with each other then it may lead to the formation of an insoluble substance which is known as precipitate.
This means that formation of a precipitate is also a chemical reaction.
Thus, we can conclude that following are the statements which show evidence for a chemical reaction.
- Dropping sodium metal into water produces fizzing.
- Mixing two aqueous solutions produces a precipitate.