Question

Explain:
How does the colour of ${K_2}C{r_2}{O_7}$ solution depend on pH of solution?

Answer 1

Hint: Chromate ion shows orange colour in acidic medium while it shows yellow colour in basic medium. And the pH of acidic solutions are less than $7$ and the pH of basic solutions are greater than 7. On the basis of this criteria we can answer this question.

Complete step by step solution:
Let us first define the pH range of acids and bases.
First of all we will discuss acids, bases and amphoteric compounds.
Acids: Those compounds which on ionization produce \[{H^ + }\]ions, are known as acids. They are sour in taste. Acids change the color of litmus from blue to red. When they react with bases then they become less acidic.
Bases: Those compounds which on ionization produce $O{H^ - }$ions, are known as bases. They are slippery in nature. Bases change the color of litmus from red to blue. When they react with acids then they become less basic.
Amphoteric Compounds: Those compounds which can react with both acids and bases are known as amphoteric compounds.
pH values: If the pH value is in between $0 - 7$ then the solution or compound will be acidic in nature and If the pH value is in between $7 - 14$ then the solution or compound will be basic in nature. And if the pH value is exactly seven then compound or solution will be neutral i.e. neither acidic nor basic in nature.
Now the compound potassium dichromate (${K_2}C{r_2}{O_7}$) is having the ion chromate in the compound. And the chromate ion shows orange colour in acidic medium while it shows yellow colour in basic medium. And the pH of acidic solutions are less than $7$ and the pH of basic solutions are greater than 7.
Hence, the colour of ${K_2}C{r_2}{O_7}$ solution does depend on pH of solution i.e. if the pH of the solution is less than $7$ it will show orange colour and if pH is greater than $7$ then it will show yellow colour.

Note:
Acids and bases are also defined as:
Electron pair acceptor and donor: This concept was given by Lewis. According to Lewis: Lewis acids are those substances or solutions which can accept electron pairs and Lewis bases are those substances or solutions which can donate electron pairs.