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blue
it turns acidic or very basic
Lemon juice contains citric acid so blue litmus paper dipped in lemon juice will turn red indicating that it is acidic.
Potatoes, like the majority of vegetables, are a low-acid food with a pH value of 6.1. Each brand of pH test strips may have slightly different color schemes. Litmus paper comes in two colors. Red litmus paper turns blue when exposed to a base. Blue litmus paper turns red in the presence of an acid. Litmus paper is of limited use since it only tells you whether you are dealing with an acid or base and gives no indication about strength. Universal indicator paper would show a very light yellow. Hydrion paper would show a 'dirty' yellow.
blue
Red
Lemon juice is an acid and blue litmus paper turns red when dipped in an acidic liquid.
It would remain red, red litmus paper only reacts to blue in the presence of a base. Lemon juice is acidic and thus wouldn't turn the paper a color. If blue litmus paper was used in the lemon juice it would turn red. Consequently dipping blue litmus paper in a base won't change the color because it does not react with bases, only acids.
Litmus is a dye extracted from lichens, a plant. Upon addition of acid, litmus turns red whilst on addition of base, litmus turns blue. Being acidic, containing citric acid, lemon juice is expected to turn violet litmus paper red.
A green-y yellow.
Apple juice is slightly acidic and therefore red litmus will not change color.
Litmus turns pink in acid solutions like citric acid...blue in basic solutions.