What is the difference between "a" and "á" in Portuguese?

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What is the difference between "a" and "á" in Portuguese? Basically just the sound of them. "a" is the definite article whereas "á" is never alone, it is always in a word, like " água, hábito". The sound of "á" is more open than the sound of "a". Watch the video and practice the difference of the sounds with me! :)