Three signs that a chemical change has taken place is 1) color change 2) formation of gas bubbles 3) a change in temperature ( if you have ever bleached your hair the reason why it is warm to the touch is because of a chemical reaction)
Explanation:
Alkanes are the simple hydrocarbons that have only carbon and hydrogen atoms attached to each other through single bonds.
General chemical formula of an alkane is
, where n is any integer.
For example, if n = 5 then formula of the alkane will be as follows.

= 
= 
A suffix "-ane" is added in the name of an alkane. So, an alkane with five carbon atoms will be known as pentane.
Answer:
different acids include acetic acid,lactic acid,tartaric acids DNA(deoxyribonucliec acid) .
different bases include calcium oxide Potassium hydroxide ,Sodium hydroxide and Barium hydroxide .
Number of molecules in 1 dm³ Oxygen = 2.71 x 10²²
<h3>Further explanation</h3>
Conditions at T 0 ° C and P 1 atm are stated by STP (Standard Temperature and Pressure). At STP, Vm is 22.4 liters/mol.
The mole is the number of particles contained in a substance
1 mol = 6.02.10²³
1 dm³ of oxygen = 1 L Oxygen

n=mol=0.045
No = 6.02.10²³
