Pure substances can or can not be chemically combined.
Pure substances can be either elements or compounds, but not mixture. Mixture are different substances mixing together without being chemically combined, such as air, which is a mixture of oxygen, carbon dioxide, water etc. Mixture can be separated by physical methods, like filtration or decantation.
Meanwhile, elements are the substances that cannot be further separated by any means. No matter physical or chemical methods. Examples of elements are oxygen, hydrogen, neon and all the other ones from the periodic table. Compounds are basically elements joining together, but they’re chemically combined which means their electrons (kind of subatomic particle) are either shared or given away. These elements can only be separated by chemical methods like electrolysis or heating.
Therefore, as long as the substance cannot be separated by physical methods, it can be considered as a pure substance. We can now conclude that pure substance can be (element) or can not be (compound) chemically combined.
Answer: 


Explanation:
Entropy is the measure of randomness or disorder of a system.
A system has positive value of entropy if the disorder increases and a system has negative value of entropy if the disorder decreases.
1. 
As 4 moles of gaseous reactants are changing to 2 moles of gaseous products, the randomness is decreasing and the entropy is negative
2. 
As 9 moles of gaseous reactants are changing to 10 moles of gaseous products, the randomness is increasing and the entropy is positive.
3. 
As 1 mole of solid reactants is changing to 2 moles of gaseous products, the randomness is increasing and the entropy is positive.
4. 
As 4 moles of gaseous reactants is changing to 5 moles of gaseous products, the randomness is increasing and the entropy is positive
5. 
As 4 moles of gaseous reactants is changing to 1 moles of gaseous products, the randomness is decreasing and the entropy is negative.
Answer is 0.289nm.
Explanation: The wt % of Fe and wt % of V is given for a Fe-V alloy.
wt % of Fe in Fe-V alloy = 85%
wt % of V in Fe-V alloy = 15%
We need to calculate edge length of the unit cell having bcc structure.
Using density formula,

For calculating edge length,

For calculating
, we use the formula

Similarly for calculating
, we use the formula

From the periodic table, masses of the two elements can be written


Specific density of both the elements are

Putting
and
formula's in edge length formula, we get
![a=\left [\frac{Z\left (\frac{100}{\frac{(wt\%)_{Fe}}{M_{Fe}}+\frac{(wt\%)_{Fe}}{M_{Fe}}} \right )}{N_A\left (\frac{100}{\frac{(wt\%)_V}{\rho_V}+\frac{(wt\%)_V}{\rho_V}} \right )} \right ]^{1/3}](https://tex.z-dn.net/?f=a%3D%5Cleft%20%5B%5Cfrac%7BZ%5Cleft%20%28%5Cfrac%7B100%7D%7B%5Cfrac%7B%28wt%5C%25%29_%7BFe%7D%7D%7BM_%7BFe%7D%7D%2B%5Cfrac%7B%28wt%5C%25%29_%7BFe%7D%7D%7BM_%7BFe%7D%7D%7D%20%20%5Cright%20%29%7D%7BN_A%5Cleft%20%28%5Cfrac%7B100%7D%7B%5Cfrac%7B%28wt%5C%25%29_V%7D%7B%5Crho_V%7D%2B%5Cfrac%7B%28wt%5C%25%29_V%7D%7B%5Crho_V%7D%7D%20%20%5Cright%20%29%7D%20%20%5Cright%20%5D%5E%7B1%2F3%7D)
![a=\left [\frac{2atoms/\text{unit cell}\left (\frac{100}{\frac{85\%}{55.85g/mol}+\frac{15\%}{50.941g/mol}} \right )}{(6.023\times10^{23}atoms/mol)\left (\frac{100}{\frac{85\%}{7.874g/cm^3}+\frac{15\%}{6.10g/cm^3}} \right )} \right ]^{1/3}](https://tex.z-dn.net/?f=a%3D%5Cleft%20%5B%5Cfrac%7B2atoms%2F%5Ctext%7Bunit%20cell%7D%5Cleft%20%28%5Cfrac%7B100%7D%7B%5Cfrac%7B85%5C%25%7D%7B55.85g%2Fmol%7D%2B%5Cfrac%7B15%5C%25%7D%7B50.941g%2Fmol%7D%7D%20%20%5Cright%20%29%7D%7B%286.023%5Ctimes10%5E%7B23%7Datoms%2Fmol%29%5Cleft%20%28%5Cfrac%7B100%7D%7B%5Cfrac%7B85%5C%25%7D%7B7.874g%2Fcm%5E3%7D%2B%5Cfrac%7B15%5C%25%7D%7B6.10g%2Fcm%5E3%7D%7D%20%20%5Cright%20%29%7D%20%20%5Cright%20%5D%5E%7B1%2F3%7D)
By calculating, we get

A metallic conductor moving at a constant speed in a magnetic field may develop a voltage across it. This is an example of Motional emf
Hope this helps!
The proton transfer reaction between Cyanide and water can be written as; X^- + H2O -----> HX + OH^-
<h3>What is a proton transfer reaction?</h3>
A proton transfer reaction is one in which a proton is moved from one chemical specie to another.It is in fact and acid - base reaction in the Brownstead - Lowry sense.
The proton transfer reaction between Cyanide and water can be written as(Let the cyanide ion be shown as X);
X^- + H2O -----> HX + OH^-
Learn more about proton transfer: brainly.com/question/861100?