Flour is made of finely ground cereal grain and is comprised of both carbohydrates and proteins. Depending on how the flour is made and what grain is used, the level of protein will vary. All purpose flour has an average amount of protein, while cake and bleached flour have lower amounts of protein that result in softer baked goods. Bread flour has a high amount of protein and is considered a "hard" flour.
Hydrochloric acid is a mixture of hydrogen chloride in water and can be made or found naturally in the stomach. Aside from aiding digestion, hydrochloric acid is used to remove rust from steel, manufacture fertilizer and dyes, and to neutralize acidic solutions. The compound is corrosive and can damage human tissue on contact, which is why the stomach secretes a layer of mucus to protect itself from the acid.
Mixing a small amount of flour in with hydrochloric acid will result in the flour dissolving, as the hydrochloric acid will neutralize the flour and begin to break down the protein and starch components. With a small amount of hydrochloric acid in water, it will make the flour and water mixture less viscous, especially when heated past 100 degrees Fahrenheit, which can be useful for manufacturing purposes.
When attempting to make a bread without yeast, hydrochloric acid and flour can play a role as well. The lightness of the bread is due to the carbonic acid. Baking soda is mixed with flour for the bread, and either sour milk or diluted hydrochloric acid is added to disengage the carbonic acid from the baking soda, which is also called sodium bicarbonate. The end result is a different kind of bread than one produced with yeast, but still allows it to rise somewhat.