Acids are,
(i) sour in taste.
(ii) change the colour of blue litmus to red.

Bases are,
(i) bitter in taste.
(ii) change the colour of red litmus to blue.

Indicators are chemical substances which give different colours in acidic or basic solutions.

(i) Methyl orange gives pink colour with acid solution and yellow colour with base solution.

(ii) Phenolphthalein is colourless in acid solution while it turns into pink colour in base solution.

(iii) Litmus solution turns red in acid solution and blue in base solution.

(iv) Bromothymol blue is blue in base solution and is yellow in acid solution.

Antacids are mild alkalies and contain sodium hydrogen carbonate. These are used for getting relief from acidity and indigestion and sometimes, even headache. When taken orally, it reacts with hydrochloric acid present in the stomach and reduces its strength by consuming some of it. For example, Milk of Magnesia.

Olfactory indicators are those indicators that help to identify whether the given solution is acidic or basic by changing their smell instead of colour as other indicators do. For example onion, vanilla, clove, etc
Onion: Paste or juice of onion loses its smell when added with base. It does not change its smell with acid.
Vanilla: The smell of vanilla vanishes with base, but it’s smell does not vanishes with an acid.