Answer:When a substance undergoes a chemical change some of the chemical changes can be reversed with a chemical change.
Explanation:
A chemical change changes all of the element but some chemical changes but very few can be changed back to its original substance only with a chemical change.
Answer:
The frequency is 
Explanation:
From the question we are told that
The energy required to ionize boron is 
Generally the ionization energy of boron pre atom is mathematically represented as

Here
is the Avogadro's constant with value 
So

=> 
Generally the energy required to liberate one electron from an atom is equivalent to the ionization energy per atom and this mathematically represented as

=> 
Here h is the Planks constant with value 
So

=> 
Answer:
hope this answer will help you.
Increased density decreases the speed of sound in a medium. While increased density can mean increased rigidity, or stiffness, it is not always the case. Greater density can be due to each molecule or atom having more momentum, and being slower to respond to the vibration of its neighbor.