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.
Extrusive Rocks. Igneous rocks which form by the crystallization of magma at the surface of the Earth are called extrusive rocks. They are characterized by fine-grained textures because their rapid cooling at or near the surface did not provide enough time for large crystals to grow.
Answer:
pH = 4.45
Explanation:
We need to find the pH of 
solution of HCl. We know that, pH of a solution is given by :
![pH=-log[H]^+](https://tex.z-dn.net/?f=pH%3D-log%5BH%5D%5E%2B)
Put all the values,
![pH=-log[3.5\times 10^{-5}]\\\\pH=4.45](https://tex.z-dn.net/?f=pH%3D-log%5B3.5%5Ctimes%2010%5E%7B-5%7D%5D%5C%5C%5C%5CpH%3D4.45)
So, the pH of the solution of HCl is 4.45.
Element Symbol Mass Percent
Sodium Na 27.367%
