Sometimes "食” could been use as verb.One word Chinese always say is "食言”，it means break one's promise. If your friend didn't keep his promise,you could say "你食言了！nǐ shí yán le !”

In current day Mandarin 吃 is used as a verb and 食 is usually as a noun In Cantonese and Classical Chinese, however, only 食 is used as both a noun and a verb.

There is no difference between "食” and “吃” It really depends which one is more idiomatic in your sentence . We have some fixed phrases you have to memorise, no shortcut. Eg. 食品 noun / 食物 吃饭 verb. 食饭（Cantonese） 抽烟 verb. 食烟 （Cantonese） 「食」is actually more old-fashioned in ancient Chinese written but also used in some dialects like Cantonese.