In alkaline solution ,we have chromate ions in acidic solution we have dichrimate ions back .explain the reason.
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This predominance diagram from Wikipedia shows that the situation is a bit more complicated than stated in the question:

According to this, at high pH, chromate is the dominant species, but at low pH, either hydrogen chromate or dichromate can be the dominant species, depending on the concentration of Cr(VI). Adding dichromate to acid doesn’t cause it to turn into chromate, but can cause chromate to turn into dichromate.
It’s obvious why chromate tends to become protonated at low pHs, so let’s focus on the other two equilibria. The equilibrium represented by the blue line is:
dichromate + H2O <- - - - -> 2 chromate + 2H+
Since H+ appears on the right-hand side, lowering the pH will tend to drive the equilibrium to the left.
The equilibrium represented by the red line is:
dichromate + H2O <- - - - -> 2 hydrogen chromate
The equilibrium constant for this reaction is
K = [hydrogen chromate]^2 / [dichromate]
As the concentration of Cr(VI) is increased, if hydrogen chromate and dichromate were to increase in proportion, then the reaction quotient would increase since hydrogen chromate is squared. To counteract this, more hydrogen chromate must be converted to dichromate.
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According to this, at high pH, chromate is the dominant species, but at low pH, either hydrogen chromate or dichromate can be the dominant species, depending on the concentration of Cr(VI). Adding dichromate to acid doesn’t cause it to turn into chromate, but can cause chromate to turn into dichromate.
It’s obvious why chromate tends to become protonated at low pHs, so let’s focus on the other two equilibria. The equilibrium represented by the blue line is:
dichromate + H2O <- - - - -> 2 chromate + 2H+
Since H+ appears on the right-hand side, lowering the pH will tend to drive the equilibrium to the left.
The equilibrium represented by the red line is:
dichromate + H2O <- - - - -> 2 hydrogen chromate
The equilibrium constant for this reaction is
K = [hydrogen chromate]^2 / [dichromate]
As the concentration of Cr(VI) is increased, if hydrogen chromate and dichromate were to increase in proportion, then the reaction quotient would increase since hydrogen chromate is squared. To counteract this, more hydrogen chromate must be converted to dichromate.
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