Identify the salts P, Q, R from the observation
Salt P has light bluish green colour. On heating, it produces a black coloured residue. Salt P produces brisk effervescence with dil. HCl and the gas evolved turns lime water milky, but no action with acidified potassium dichromate solution.

Explanation

To identify the salts P, Q, and R based on the observations provided, we will analyze the characteristics of salt P step by step.

### Step 1: Analyze the Color of Salt P
The observation states that salt P has a light bluish-green color. This color is typically associated with transition metal compounds, particularly those containing copper.

**Hint:** Look for transition metals, especially copper, when you see bluish-green colors in salts.

### Step 2: Heating Salt P
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