標題:
why litmus paper or ph paper can change it's color?
發問:
i wanna ask why litmus paper can change from blue to red when in contact with dilute acids?and why ph paper can change it's color with different ph value?and why cobhalt paper can change it's color when contact with water?what is the reason behind ?what does the paper consist so that it will... 顯示更多 i wanna ask why litmus paper can change from blue to red when in contact with dilute acids? and why ph paper can change it's color with different ph value? and why cobhalt paper can change it's color when contact with water? what is the reason behind ?what does the paper consist so that it will react with the specific solution?and also please tell me the equation if can, thank you, please help me
最佳解答:
For litmus paper, it's about the dye on the paper. In short, the dye is like an acid-base indicator(e.g. phenolphthalein, methyl orange...etc) you use in titrations, that have different colour under different pH. The reason behind is outside CE syllabus, but just for your reference. Acid-base indicators are generally weak acid/bases that would ionize to different extent, depending on the pH of the solution they're in. For instant, an indicator which is a weak acid involves the ionization: HIn + H2O In- + H3O+ you can treat it like ionization of weak acid you know, like ethanoic acid. The acid from HIn has colour A and the alkaline form In- has another colour B. The colour shown would depend on the relative concentration of HIn and In- in the solution.(as the ionization is incomplete and the relative concentrations will be depending on a number of factors, and pH is one of them), mathematically, pH= pKa + log [In-]/[HIn], pKa is a constant for an acid under constant temp. In general, if the HIn is ten times more than In-, it will show colour A, and vice versa, a mixed colour of A and B will be shown in between. For cobalt chloride paper, you remember the word anhydrous before it? Cobalt(II) chloride, in its anhydrous from CoCl2, is blue in colour.(the hexahydrate is rose in colour) When the paper is dipped into water, dissolution occurs and the Co2+ ion formed, will form a complex [Co(H2O)6]2+ with water ,(in fact most metal ion, esp those transition metal ions, does not exist as simply Mn+, but forming complex) which is pink in colour.
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