Litmus is a water-soluble mixture of different dyes extracted from lichens. It is often absorbed onto filter paper to produce one of the oldest forms of pH indicator, used to test materials for acidity. Wet litmus paper can also be used to test for water-soluble gases that affect acidity or alkalinity; the gas dissolves in the water and the resulting solution colors the litmus paper. For instance, ammonia gas, which is alkaline, colors the red litmus paper blue.

Indicators may be used to measure pH, by making use of the fact that their color changes with pH. Visual comparison of the color of a test solution with a standard color chart provides a means to measure pH accurate to the nearest whole number. Litmus paper is most widely used to check the pH of a solution. pH meter gives the strength of acidity or basicity of a solution. Phenolphthalein can be used to distinguish between acids and bases.

The addition of dilute $HNO _3$ to unknown metal (2) (silver metal) produced an orange gas nitrogen dioxide $\displaystyle (NO _2)$.
Note: Various gases have following colours.
$Cl _2-$ green
$H _2-$ colorless
$O _2-$ colorless
$CO _2-$ colorless
$NO _2-$ orange/brown

$\displaystyle { H } _{ 2 }{ S }$ would separate $\displaystyle { Cu }^{ 2+ },{ Cr }^{ 3+ }$ at pH<7 .
$Cu^{2+} +  H _2S \uparrow \rightarrow  \underset {black  ppt} {CuS} + 2H^+$

pH is the negative log of hydrogen ion concentration in a water-based solution. The term 'pH' was first described by Danish biochemist Soren Peter Lauritz Sorensen in 1909. pH is an abbreviation for 'power of hydrogen' where 'p' is short for the German word for power and 'H' is the element symbol for hydrogen.

A pH indicator is a halochromic chemical compound added in small amounts to a solution so the pH (acidity or basicity) of the solution can be determined visually. Hence, a pH indicator is a chemical detector for hydronium ions ($H _3O^+$) or hydrogen ions ($H^+$). Bromophenol blue is used as a pH indicator, a color marker, and a dye. As an acid-base indicator, its useful range lies between pH 3.0 and 4.6. It changes from yellow at pH 3.0 to blue at pH 4.6; this reaction is reversible. Phenolphthalein is often used as an indicator in acid-base titrations. For this application, it turns colorless in acidic solutions and pink in basic solutions.

$pH$ paper is the litmus paper which helps in identification of whether the solution is acidic or basic in nature. It does not help to determine the strength of acidity or basicity of a solution.

The pH scale measures how acidic or basic a substance is. The pH scale ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic. A pH greater than 7 is basic.

$\begin{matrix}&pH=X= 8  \ or \ 4 \ (less\  than \ Y) \Cr _2O _7^{2-}&\rightleftharpoons&CrO _4^{2-}\orange&pH=Y= 10\  or \ 6  \ (more\  than\  X) &green\end{matrix}$

Unknown metal (1) could be zinc as it is dull gray solid with white oxide coating.
Note: Mercury is a liquid, copper is brownish, iron has orange brown coating of oxide (rust) and silver has gray-black colour.

The process involves the equilibrium between chromate (VI), dichromate (VI) and hydrogen ions:

Unknown metal (2) could be silver. It is a relatively inert metal. 
Note: Carbon is not a metal. Copper is brownish, zinc is reactive metal with whitish oxide coat in air. Sodium is highly reactive and explodes when it comes in contact with water. It reacts with acids even violently.

When dilute $HCl$ is added to the unknown metal  (1) (zinc), the transparent gas produced is hydrogen. 
Note: Chlorine and $NO _2$ gases are green and orange coloured respectively. $CO _2$ is not possible as no $C$ atoms are present.
Also, metals react with acids to form hydrogen gas.

The pH scale measures how acidic or basic a substance is. The pH scale ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic. A pH greater than 7 is basic.

The pH meter is a device used for measuring ph, which is either the concentration or the activity of hydrogen ions in an aqueous solution. It usually has a glass electrode plus a calomel reference electrode or a combination of an electrode.

$pH$ is a numeric scale used to specify the acidity or basicity of an aqueous solution. It is roughly the negative of the logarithm to base 10 of the concentration, measured in units of moles per liter, of hydrogen ions or $H^+$.

The use of pH is in soil analysis to test whether the soli is acidic or basic, water analysis to test whether water is pure or not and medical field to calculate the pH of blood, urine samples.

The scale showing the relative strength of the acids and alkalis is known as pH scale.

pH of soil generally varies from $3$ to $9$.

pH of clear rain water varies from $5.5$ to $6$.

Weak organic compounds which changes its colour in accordance with pH are known as indicators.

pH of urine varies from $4.8$ to $7.5$.