? 發問於 科學及數學化學 · 9 年前

Lots of questions on Chemistry

1. Why pure gold doesn't react with dilute nitric acid?

Dilute nitric acid acts as an oxidizing agent and can react with metals with low reactivity like copper, but why it can't react with pure gold?

2. In pyrolysis, air shouldn't be present, otherwise, plastic wastes will be oxidized/burn/give CO2 and H2O.

Where do CO2 AND H2O come from? Are they products from combustion?

If not, can you explain why they are formed and can you show me the chemical equation of the process?

3. In the liquefaction of air, if there is the presence of CO2, the pipes will be blocked after cooling. Is it because the temperature around the cooling coils is low enough to turn CO2 to dry ice?

4. How can we test nitrogen gas?

5. What's the difference of a concentrated solution and a saturated solution? (Even there's an explanation in my book, I can't thoroughly/clearly understand their difference)

6. Why a hot solvent can usually dissolve more solute than a cold one? (more energy to break up intermolecular force, bonds, etc.?)

7. In the evaporation of sea water, we can either heat it directly or heat it with a steam bath. What's actually the purpose of heating it with a steam bath? Is it because larger crystals will form or something?

8. Why are the crystals formed in crystallization pure?

9. Why the crystals formed in crystallisation are smaller when the solution is evaporated quickly?

10. While conducting a flame test, why should we first "dip a clean nichrome wire into concentrated hydrochloric acid?

11. In a silver nitrate test, why is it necessary to add the solid sample into excess dilute nitric acid?

Thank you so much for your patience :)

2 個解答

評分
  • Ka ho
    Lv 5
    9 年前
    最愛解答

    1. because nitric acid do not have enough oxidizing power to oxidize gold.gold can react with halogen(very high oxidizing power) ,Aqua Regia and Cyanide.

    2.plastic is a very large group,there are a lot of kinds of plastic.they are all organic chemical.Some kinds of plastic are ester which contains oxygen itself.Duringpyrolysis,C H and O in plastic will be separated into CO2,H2O and CH4.

    3.Yes,CO2 's freezing point is -78C which is higher than other gases' boiling point(-183C ~ -196C).

    4.http://hk.knowledge.yahoo.com/question/question?qi...

    5.Concentrate something means to increase the concentration of solute=decrease the concentration of solute.Such as 25%HCl(g)+75%H2O(l)=>100%HCl(aq),

    decrease the concentration of water in hydrochloric acid by boiling.The product is called concentrated hydrochloric acid.

    Saturated solution means that the solvent cannot dissolve solutes anymore.Such as

    Ca(OH)2(l)+CaO(s)=>Saturated calcium hydroxide solution (lime water).

    Saturated solution can known as the maximum concentrated solution.

    6.dissolution means that solvent molecules and solute molecules attract each other and then join together.The state of chemical depends on their intermolecular force such as Van de Waal's force and hydrogen bonds.High temperature break those intermolecular force in solvent and more space for solute molecules to dissolve inside.

    7.Steam is the very easy to optain and it contain a large of heat energy due to very high specific heat

    capacity and latent heat of vaporization.It is a effective heater which is much cheaper than heating with fuels.

    2012-04-10 00:58:08 補充:

    8.Some specific chemical doesn't present independently, they will formed giant molecular structure or giant ionic structure.All salt can formed giant ionic structure.Giant ionic structure is what we called crystal.Giant ionic structure is formed with specific chemical thus all crystal are pure.

    2012-04-10 01:02:17 補充:

    我冇位打啦...用左知識+寄信0.0

  • 匿名
    9 年前

    answer some of them...

    some of them I'm not sure

    I'll use a bracket to indicate that

    Give the best answer to those who can answer it all

    1 You will learn sth so called metal reactivity series

    gold is quite inert

    it seldom react

    [2. you are right

    CO2 and H2O is the product of combustion

    For example

    ethene (simplest form of polythene)+ 3O2 -> 2CO2 + 2H2O]

    3. Yes.

    therefore, CO2 and dust are removed before liquefaction process

    [4. Nitrogen is also quite inert

    we seldom test for it because the method is quite complicated

    So, we usually said "there is no simple test for N2"]

    5. Saturated solution is base on the solvent-solute relationship

    however, conc. soln is base on the amount of solute to volume of soln

    S.S. : solvent dissolve max. amount of solute at a specific temp.

    C.S. : conc. of solute is high (usually 2M , which is 2mole solute per 1L soln)

    there is an obvious diff. as S.S. refers to the state that solvent dissolve max. amount of solute and it is impossible to dissolve more in a specific temp.

    C.S. do not mean the same thing, it may still dissolve more e.g. 2M -> 3M is possible

    [6. I think you are right

    more energy -> easier to break bond

    however, high temp=> hold less gas (it cannot be explained...]

    [7. watch glass is not possible for direct heating ]

    *Heating cannot form large crystals

    [8. it may only be an assumption...

    or... most of the crystal are ionic

    +-+- packing is adopted

    those non-charged substances e.g. dust will not enter the structure]

    9. More time => larger (forms a regular shape)

    10. in order to clean it up

    it may remain some sample of the previous test

    dip in to the conc. HCl => cleaning effect

    11. HNO3 can prevent the formation of other insoluble Ag compound which may interfere the result by provide excess ions...

    hope can help

    資料來源: self: )
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