Distilled water is neutral in character, i.e., it is neither acidic nor basic. Neutral nature of distilled water can be verified by the use of blue and red litmus paper.

An acid reacts with a metal and gives a salt with evolution of hydrogen gas.
$\text{Acid}+\text{metal}\to \text{salt}+{\text{H}}_2$
Many reactive metals like sodium, potassium, etc. displace hydrogen from acids.
$2HCl+2Na\to 2NaCl+H_2$

The leftover food in the mouth is eaten by bacteria which releases acid into the mouth. This causes tooth decay.

A chemical indicator is any substance that gives a visible sign, usually by a colour change, of the presence or absence of a threshold concentration of a chemical species, such as an acid or an alkali in a solution.

Any one source
Tamarind is a natural source of tartaric acid.
Various other sources of tartaric acid are grapes, unripe mangoes, etc.

Name of the substance: 1 Mark
Mechanism: 1 Mark
Examples: 1 Mark
 For testing whether a substance is an acid or base, we use indicators.
 Indicators are special substances that show a change of colour or smell when added to a solution containing an acidic or a basic substance.
 Examples of indicators are: Turmeric, litmus, China rose petals, vanilla, etc.

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Colour: 1 Mark
Litmus is extracted from lichens. 
When added to an acidic solution, it turns red and when added to a basic solution, it turns blue.
It has mauve (purple) colour in distilled water which is neutral.

Litmus paper: 1 Mark
Explanation: 1 Mark
Litmus paper is an indicator. It is used to check whether a solution is acidic or basic.
When treated with a solution containing NaOH, the color of red litmus paper will change to blue as NaOH is a base and bases turn red litmus blue.