Confusion about making binary search tree EMPTY in C

I want to define a function which makes binary search tree to empty with return-type void.

These are my codes as below :

_Node structure

typedef struct _Node {
    int data;
    struct _Node* l_child;
    struct _Node* r_child;
} Node;

BST_To_Empty

void BST_To_Empty(Node* root)
{
    if(root)
    {
        BST_To_Empty(root->l_child);
        BST_To_Empty(root->r_child);
        free(root);
    }
    printf("[BST_To_Empty] Now BST is NULL");
}

CheckEmpty

void isEmpty(Node* root)
{
    if (root == NULL)
    {
        printf("NULL");
    }
    else
    {
        printf("Not NULL");
    }
}

With these codes I made main function as below :

int main()
{
    Node* root = NULL;
    // Some Initialization
    BST_To_Empty(root);
    CheckEmpty(root);
}

So I think I can get a result that
"[BST_To_Empty] Now BST is NULL" and
"NULL"

But I got
"[BST_To_Empty] Now BST is NULL" and
"Not NULL"

I have a little confusion, why the result of "CheckEmpty" is "Not NULL" although
I made root free?

What should I modify to get result of "CheckEmpty" is "NULL"?

Thanks for your help.

The function BST_To_Empty declared like

void BST_To_Empty(Node* root)

deals with a copy of the value of the pointer root declared in main

Node* root = NULL;
BST_To_Empty(root);

Changing the copy of the value of the original pointer within the function leaves the original pointer unchanged. And moreover the function free also accepts a pointer by value and does not set the original pointer to NULL.

You need to pass the original pointer root to the function by reference.

In C passing an object by reference means passing it indirectly through a pointer to it.

That is the function will look like

void BST_To_Empty(Node **root )
{
    if( *root )
    {
        BST_To_Empty( &( *root )->l_child );
        BST_To_Empty( &( *root )->r_child );
        free( *root );
        *root = NULL;
    }
}

and be called like

BST_To_Empty( &root );

In turn the function isEmpty should be declared and defined like

int isEmpty( const Node *root )
{
    return root == NULL:
}

And the functions should not display any message. It is the caller of the functions that will decide whether to output a message as for example

if ( isEmpty( root ) )
{
    puts("NULL");
}
else
{
    puts("Not NULL");
}