We have to answer based on basic concept on electron.
The correct answer is: Since electrons repel each other, electrons pairs will be as far apart as possible.
We know, electrons are negatively charged particles and negatively charged particles repel to each other.
When electrons which are of similar charge repel each other, they go far apart from each other..
As the concrete has cement, gravel, crushed rocks, sand, and water of different physical state and can be separated by physical methods hence it should be called as a mixture.
As the components have significant physical boundaries which clearly identify their presence, the mixture is heterogeneous and not homogeneous.
Thus this is a concrete is a heterogeneous mixture of cement, gravel, crushed rocks, sand, and water
Answer : The equilibrium concentration of 
in the solution is, 
Explanation :
The dissociation of acid reaction is:
Initial conc. c 0 0
At eqm. c-x x x
Given:
c = 
The expression of dissociation constant of acid is:
![K_a=\frac{[H_3O^+][C_6H_5COO^-]}{[C_6H_5COOH]}](https://tex.z-dn.net/?f=K_a%3D%5Cfrac%7B%5BH_3O%5E%2B%5D%5BC_6H_5COO%5E-%5D%7D%7B%5BC_6H_5COOH%5D%7D)
Now put all the given values in this expression, we get:
![6.3\times 10^{-5}=\frac{(x)\times (x)}{[(7.0\times 10^{-2})-x]}](https://tex.z-dn.net/?f=6.3%5Ctimes%2010%5E%7B-5%7D%3D%5Cfrac%7B%28x%29%5Ctimes%20%28x%29%7D%7B%5B%287.0%5Ctimes%2010%5E%7B-2%7D%29-x%5D%7D)
Thus, the equilibrium concentration of in the solution is,
