Chemical compounds have chemically bonded molecules so that they exhibit different properties (i.e. chemical) compared to the individual molecules comprising the compound. Mixtures are simply the combinations of different molecules and compounds that are not chemically bonded together, and can therefore be separated by physical means. Mixtures usually retain the properties of its components. The hydrogen and oxygen molecules in a mixture do not form strong bonds between each other. The molecules of both gases are only contained in the same space or volume and the individual molecules retain their chemical properties.
A compound containing hydrogen and oxygen molecules exhibit different chemical (and even physical) properties compared to the individual molecules themselves.
Water for example, is a compound with 2 hydrogen atoms and 1 oxygen atom, chemically-bonded together. Hydrogen gas is highly flammable, water is not. Oxygen gas is an essential reagent for combustion (or burning) reactions, water is not.
Thus, throwing a lighted match to a gaseous mixture of hydrogen

and oxygen

would create fire, or even an explosion (since hydrogen is flammable and oxygen feeds the reaction). Throwing a match to water vapor

would not create fire.
Answer:
1.5 moles
Explanation:
To find the number of moles of HCl in 500 mL of a 3 M solution of HCl, we consider moles in 1 liter/ 1000 mL.
3 moles HCl is contained in 1000 mL
x moles is HCl is contained in 500 mL

Hence the number of moles of HCl in 500 mL is 1.5 moles.
Alcohol consumption can damage the digestive system and can increase the risk of alcohol-caused cancer and liver disease.