Which one of the following is the correct order of pH for the given substances?
Lemon juice < Coffee < Blood < Milk of magnesia
Understanding pH and Substance Acidity/Alkalinity
The question asks for the correct order of specific substances based on their \(\text{pH}\) values. The order should be from the substance with the lowest \(\text{pH}\) to the substance with the highest \(\text{pH}\). A lower \(\text{pH}\) indicates that a substance is more acidic, while a higher \(\text{pH}\) indicates that it is more alkaline or basic.
What is \(\text{pH}\)?
The \(\text{pH}\) scale is a numerical scale used to specify the acidity or basicity of an aqueous solution. It typically ranges from 0 to 14.
- Solutions with a \(\text{pH}\) less than 7 are considered acidic. The lower the number, the more acidic the solution.
- A solution with a \(\text{pH}\) equal to 7 is considered neutral (like pure water at \(25^\circ\text{C}\)).
- Solutions with a \(\text{pH}\) greater than 7 are considered alkaline or basic. The higher the number, the more alkaline the solution.
Approximate \(\text{pH}\) Values of the Given Substances
To determine the correct order, we need to know the approximate \(\text{pH}\) values for Lemon juice, Coffee, Blood, and Milk of magnesia. These are commonly known values:
| Substance | Approximate \(\text{pH}\) Range | Nature |
|---|---|---|
| Lemon juice | 2.0 - 3.0 | Acidic |
| Coffee | 4.5 - 5.0 | Acidic |
| Blood (human) | 7.35 - 7.45 | Slightly Alkaline |
| Milk of magnesia | 10.5 | Alkaline (Basic) |
Looking at these values, we can see the relative positions on the \(\text{pH}\) scale:
- Lemon juice has the lowest \(\text{pH}\), making it the most acidic among these substances.
- Coffee has a \(\text{pH}\) higher than lemon juice but still below 7, so it is acidic.
- Blood has a \(\text{pH}\) slightly above 7, indicating it is slightly alkaline. The \(\text{pH}\) of blood is maintained within a very narrow range for health.
- Milk of magnesia has a \(\text{pH}\) significantly above 7, making it alkaline (basic) and often used as an antacid.
Ordering by \(\text{pH}\) from Lowest to Highest
Based on the approximate \(\text{pH}\) values, we can arrange the substances from the lowest \(\text{pH}\) (most acidic) to the highest \(\text{pH}\) (most alkaline):
- Lemon juice (\(\text{pH}\) 2.0 - 3.0)
- Coffee (\(\text{pH}\) 4.5 - 5.0)
- Blood (\(\text{pH}\) 7.35 - 7.45)
- Milk of magnesia (\(\text{pH}\) 10.5)
Using the < symbol to represent "less than" in terms of \(\text{pH}\) value, the correct order is:
Lemon juice < Coffee < Blood < Milk of magnesia
Matching with the Options
Now, let's compare this correct order with the given options:
- Option 1: Coffee < Lemon juice < Milk of magnesia < Blood (Incorrect, Coffee has a higher \(\text{pH}\) than Lemon juice)
- Option 2: Milk of magnesia < Blood < Coffee < Lemon juice (Incorrect, Milk of magnesia has the highest \(\text{pH}\), Lemon juice has the lowest)
- Option 3: Lemon juice < Blood < Coffee < Milk of magnesia (Incorrect, Blood has a higher \(\text{pH}\) than Coffee)
- Option 4: Lemon juice < Coffee < Blood < Milk of magnesia (Correct, This order matches our derived order from lowest to highest \(\text{pH}\))
Revision Table: Understanding pH Order
| Concept | Description | Application to Question |
|---|---|---|
| \(\text{pH}\) Scale | Measures acidity/alkalinity (0-14) | Used to classify substances |
| Acidity | \(\text{pH}\) < 7 | Lower \(\text{pH}\) means more acidic |
| Alkalinity (Basicity) | \(\text{pH}\) > 7 | Higher \(\text{pH}\) means more alkaline |
| Order | Ascending \(\text{pH}\) (Acidic to Alkaline) | Required sorting method |
| Substances | Lemon juice, Coffee, Blood, Milk of magnesia | Items to be ordered by \(\text{pH}\) |
Additional Information: Factors Affecting pH
While we use approximate \(\text{pH}\) values, the exact \(\text{pH}\) of substances can vary slightly depending on factors such as concentration, temperature, and the presence of other dissolved substances.
- Concentration: For acids and bases, higher concentration generally leads to a more extreme \(\text{pH}\) (lower for acids, higher for bases).
- Temperature: Temperature can affect the ionization of water and solutes, slightly altering \(\text{pH}\). The neutral \(\text{pH}\) of pure water is 7 only at \(25^\circ\text{C}\).
- Composition: The specific type of acid or base and other chemicals present influence the exact \(\text{pH}\). For example, the exact \(\text{pH}\) of coffee can vary depending on the beans and brewing method. Blood \(\text{pH}\) is tightly regulated by buffer systems to stay within its narrow range.
For this question, understanding the general nature (strongly acidic, weakly acidic, neutral, slightly alkaline, strongly alkaline) and the typical \(\text{pH}\) ranges of these common substances is sufficient to determine the correct order.
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