Critical vs. Crucial

The two words mean of the greatest importance. They have similarities and can be interchanged nearly in all cases:

Similarities

1-They both mean of the greatest importance to the way things might happen:

- These talks are critical /crucial important.

-Your decision is critical/crucial to our future.

- Classroom practice is critical/crucial in childcare's learning.

2- The two adjectives can take certain prepositions such as: in,for,to and also that:

- It is critical/crucial that a problem is tackled immediately.

- His work has been crucial/critical to the project.

- The money is crucial/critical for the research.

- Experience has been a crucial/critical factor in deciding who would be the best person for the job.

3- They collocate with a wide range of words:A crucial/ critical stage,factor,time,situation, issue, element, decision and etc.

- The conservation of tropical forest is of crucial/critical importance.

- It is absolutely crucial/ critical that we arrive there early.

And also:

Verbs:be, prove | become | remain | consider sth, deem sth, regard sth as, see sth as

Adverbs: really , fairly, quite , hardly ,clearly, obviously,

4- These words can appear as adverbs:

- Sleep in early life may play crucial/crucial role in brain development.

- He played a crucial/critical role/part in the politic.

- They play a crucial/critical role in smuggling the drugs through the country.

- crucially/critically, though,it failed it failed to secure the backing of the bank.

- Women play a crucial/critical role in promoting human rights.

- This is crucially/critically important.

Differences

Critical indicates that a time or a situation is extremely important and possibly dangerous because your future will depend on what happens or is decided at that time.

A- These accusations came at a critical phase in the negotiations.

Something that is crucial is the most important part of a process or situation, which everything else depend on it and has a major effect on something especially the success or failure of them.

B- Wining the contract is crucial to the success of the company.

In spite of the difference in the definitions between the two words, they are interchangeable in the sentences A&B and many other sentences like these and maybe sometimes with a subtle difference.

The only difference between critical and crucial I can think of is the following meaning of critical in which I think the former is not interchangeable with the latter.

Critical also mean indispensable; essential:

- A critical element of the plan.

- A second income that is critical to the family's well-being.

- Courage, honesty, and firmness are critical qualities that no politician should lack.

Here is an excerpt from jaced.com about the difference between the two words:

While the two words are practically interchangeable in everyday conversation, I suppose crucial is most appropriate when talking about something that pivots on a circumstance (visualize the point of intersection on a cross), with the outcome of the circumstance going either way (usually positive or negative). i.e. Speeding up is crucial to whether or not I’ll beat the red light. Critical is probably the better word when directly referring to the observably negative outcome of the crucial situation. i.e. It is critical that I beat that red light, otherwise I won’t get to the airport on time and I’ll miss my flight.

Sources:

Longman Activator.

ColinsCobuild- Lexicon

MacMillan English Dictionary -American

Cambridge Advanced Learner's Dictionary

Longman Advanced American Dictionary