Explanation:
A change that does not cause any change in chemical composition of a substance is known as a physical change.
For example, mass, density, volume etc are all physical changes.
On the other hand, a change that brings change in chemical composition of a substance is known as chemical change.
For example, combustion, toxicity, reactivity etc are all chemical changes.
Hence, following statements are identified as chemical or physical changes as follows.
(a) Oxygen gas supports combustion : Chemical change
(b) Fertilizers help to increase agricultural production : Chemical change
(c) Water boils below 
on top of a mountain : Physical change
(d) Lead is denser than aluminum : Physical change
(e) Uranium is a radioactive element : Chemical change
<span>Platinum is solid at room temperature. SInce the catalyst is solid while the medium involves gases, this catalyst is characterized as heterogenous. Because they are different states of matter, they can be easily separated through physical means. So, the answer is B.</span>
Fluorine has C seven valence electrons.
Valence electrons are the electrons that are found on the outer shell of an atom that are capable of participating in chemical reactions. The easiest way to figure out how many valance electrons Fluorine has would be to look it up in a periodic table and notice that it a group 7 element and therefore has 7 valence electrons.
The other way to tell that Fluorine has 7 valence electrons is to notice that an element with an atomic number of 9 has 9 electrons. The electronic configuration of elements has the first 2 electrons go on the first shell or energy level 
, the leaving the next 7 to go to the second shell 
which can take up to 8 electrons.
This means that Fluorine has 7 seven valence electrons.
A. Their cells lack a nucleus
