Arrange the following in increasing order of their pH values:

I. Human Blood

II. Stomach acid

III. Pure water

IV. Milk

pH Values: Understanding Acidity and Basicity

The pH scale is a measure that indicates how acidic or basic (alkaline) a solution is. This scale typically ranges from 0 to 14. A pH value of 7 is considered neutral. Solutions with a pH value less than 7 are acidic, with lower numbers indicating stronger acidity. Solutions with a pH value greater than 7 are basic or alkaline, with higher numbers indicating stronger basicity. Understanding the typical pH values of common substances is essential for arranging them correctly.

Stomach Acid pH: Strong Acidity

Stomach acid, which is primarily hydrochloric acid ($\text{HCl}$), is known for its extreme acidity. Its primary role in the human body is to break down food and to eliminate harmful microorganisms. This strong acidic nature means it has a very low pH value.

- Typical pH value range for Stomach acid: $\text{pH}\approx 1.0-3.0$

Milk pH: Slightly Acidic Nature

Milk is generally considered to be slightly acidic. The presence of lactic acid contributes to its relatively lower pH value compared to neutral water. The exact pH value can vary slightly depending on factors such as its freshness and how it has been processed.

- Typical pH value range for Milk: $\text{pH}\approx 6.5-6.7$

Pure Water pH: Neutral Reference

Pure water is the standard reference point for neutrality on the pH scale. This means it is neither acidic nor basic. Its pH value is exactly 7.0 under standard conditions.

- Typical pH value for Pure water: $\text{pH}=7.0$

Human Blood pH: Crucial Alkalinity

Human blood maintains a very precise and narrow range of pH values, which is vital for all bodily functions. It is slightly basic or alkaline. Any significant deviation from this specific pH range can lead to serious health issues, as enzymatic reactions and other biological processes are highly sensitive to pH changes.

- Typical pH value range for Human blood: $\text{pH}\approx 7.35-7.45$

Arranging Substances by pH Value

To arrange the given substances in increasing order of their pH values, let's list their approximate pH first:Now, let's arrange these substances from the lowest pH value (most acidic) to the highest pH value (most basic):

1. Stomach acid (II): This has the lowest pH, indicating strong acidity ($\approx 1.0-3.0$).
2. Milk (IV): This is slightly acidic, with a pH higher than stomach acid but still below 7 ($\approx 6.5-6.7$).
3. Pure water (III): This is neutral, with a pH of exactly 7.0.
4. Human blood (I): This is slightly basic, with the highest pH among the given substances ($\approx 7.35-7.45$).

Therefore, the increasing order of their pH values is: II (Stomach acid), IV (Milk), III (Pure water), and I (Human Blood).