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PH scale

PH scale determines the acidic, neutral, or basic nature of the substance, material, or solutions. pH stands for power of Hydrogen. It means that the pH is determined on basis of the hydrogen ions present in the solution. The range of pH scale lies between 0 and 14. The pH of 7 is considered neutral. The best example of a neutral substance is water.

PH scale

The pH scale displays the range of acidic, neutral, and alkaline solutions, as shown above.

Here, we will discuss the following topics:

What is the pH scale?

Significance of acidic, neutral, and basic

Substances with different pH values

Litmus paper

Classification of soil and its pH ranges

pH value in the living system

Applications of the pH scale

What is the pH scale?

In chemistry, pH refers to power and hydrogen. It is used to determine the nature of the aqueous solution (acidic or basic). pH scale displays different colors for the range from 0 to 14. It means it shows 15 different colors that signify the nature of a substance.

The pH scale is shown below:

PH scale

pH below 7 is acidic, while pH above 7 is basic. As the value from 7 decreases, the acidic nature of the substance increases. Similarly, as the value from 7 increases, the basic nature of the substance increases.

We can also say that pH 0 is highly acidic, and pH 14 is a strong alkali. Alkali is the basic salt of alkali metal or earth metal. It is also called a base dissolved in water.

pH is generally measured at room temperature (25 degrees Celsius). The value of the pH scale can also exist less than 0 or more than 14, but those substances are powerful acids and bases.

Significance of acidic, neutral, and basic

There are three substances that lie on the pH scale, namely, acidic, neutral, and base. Acidic and bases are the two ends on the pH scale. Most of the substances come under these two categories. Let' discuss the significance of these three substances.


Any substance or material below pH 7 is considered as acidic. Water is a neutral substance, while rainwater is acidic. It is due to the dissolve acids in the rainwater from the atmosphere, such as carbon dioxide.

Why acids have lower pH values?

The acids are classified based on the presence of hydrogen ions. It easily provides hydrogen ions in the water. The stronger the acid, the more it donates the hydrogen ions (H+). It means that the higher concentration of hydrogen ions increases the acidic nature of the solution. Hence, acids have lower pH values.


The properties of acids are as follows:

  • The acid reacts with active metals to form hydrogen gas.
  • It conducts electricity.
  • Acids have a sour taste.
  • Acid can change the color of some indicators.
  • The example includes hydrochloric acid, lemon, vinegar, etc.
  • Hydrochloric acid plays a major function in the human body. It helps in the digestion of food. It also prevents the body from infections.


The substance with pH 7 is considered neutral. The most common example of a neutral substance is water. It is neither acidic nor basic. Distilled water is also a neutral substance. But, pure distilled water can sometimes be acidic as it absorbs carbon dioxide from the atmosphere.


  • The properties of neutral substances are as follows:
  • It has an equal number of hydroxide and hydrogen ions.


Substances with the pH above 7 are considered as bases. The bases produce hydroxide ions (OH-) when dissolved in water.

Why bases have a pH range higher than 7?

Bases are the substances that provide hydroxide ions, which are responsible for the lower pH value. It readily donates these hydroxide ions. When the hydroxide ions combine with hydrogen ions, it forms water, as shown in the below equation:

PH scale

The formation of water increases the pH value. Thus, we can say that strong bases dissociate entirely in water. Hence, bases have higher pH ranges.


The properties of bases are as follows:

  • Bases can also change the color of indicators, such as litmus paper.
  • Bases do not react with other metals as acids do.
  • The base tends to lose its capability when mixed with acids.
  • The taste of basic substances is generally bitter.
  • Bases do not react with carbonates (salt of carbonic acids).

Substances with different pH values

Let's discuss some popular substances with their pH scale. We will discuss the reason of the substance holding the corresponding pH value.

pH Scale Name of substances or objects Reason
pH 0 Battery acid Battery acid is a type of sulfuric acid that converts chemical energy into electrical energy and vice - versa. Hence, it is a strong acid.
pH 1 Gastric fluid Gastric fluid is also known as digestive fluid or stomach acid. The low pH value is due to the presence of one of the strong acids called HCl (hydrochloric acid). It helps in the digestion of food.
pH 2 Lemon juice Lemon contains a high amount of citric acid. Hence, it has lower pH.
pH 3 Soda, orange juice Juices and drinks are added with phosphoric acid or citric acid. Hence, it tends to lower the pH value.
pH 4 Acid rain Acid rain contains dissolve acids and particles in the rainwater, which makes it acidic in nature.
pH 5 coffee The coffee beans comprises of various acids, such as quinic, phosphoric, acetic, etc.
pH 6 Sweat
Sweat is slightly acidic in nature due to the presence of salts in it.
The pH value of milk lies between 6 and 7. It is mildly acidic due to the presence of lactic acid in it.
pH 7 Pure water As discussed, water is a neutral substance.
pH 8 Sea water The seawater contains dissolved gases, such as carbon dioxide, oxygen, etc., making it slightly basic.
pH 9 Baking soda Baking soda is added to the water bodies to increase alkalinity and clarity. Baking soda comprises sodium carbonate, which increases its pH value. Sodium carbonate has high pH value.
pH 10 Great salt lake Great salt lakes comprises of various dissolved minerals, which increases the pH of water.
pH 11 Ammonia solution The molecular formula of ammonia is NH3. When dissolved in water, it readily accepts a proton and produces ammonia ions and hydroxide ions in water.
pH 12 Soapy water Soap is a combination of a weak acid and a strong base. When soap is dissolved with water, its pH value increases.
pH 13 Bleaching powder The bleaching power also comprises of a strong base as one of the ingredients. Hence, it has high pH.
pH 14 Sodium hydroxide The addition of sodium hydroxide (NaOH) to the water causes a decrease in the concentration of hydrogen ions. Thus, it increases the pH.

Litmus paper

It is a form of paper that is used as a pH indicator. It tests the acidic or basic nature of the aqueous solution. It is simply paper and can be safely used at home. It is also used for experimental purposes in the laboratories.

Types of litmus paper

PH scale

There are three types of litmus paper.

Yellow litmus paper

Red litmus paper

Blue litmus paper

Yellow litmus paper

Yellow litmus paper turns red when dipped in the acidic solution. Similarly, it turns blue when dipped in the basic solution. If the yellow litmus paper turns green or violet, it means that the corresponding solution is neutral. It is shown below:

PH scale

Red litmus paper

Red litmus paper remains red when dipped in the acidic solution and turns blue when dipped in the basic solution. It is shown below:

PH scale

It does not appear exact red in color. It is generally used to test the basic or alkaline solution.

Blue litmus paper

Blue litmus paper turns red when dipped in the acidic solution and remains blue when dipped in the basic solution. It is shown below:

PH scale

Blue litmus paper is generally used to test the presence of acidic solution. Otherwise, it remains blue.

Thus, we can conclude that yellow litmus paper can test the acidic, neutral, and basic solutions. While the red litmus paper is used to test the presence of basic solutions, blue litmus paper is used to test only acidic solutions.

How does it work?

When the litmus paper is dipped in the aqueous solution, it changes its color. It means,

The color of litmus paper, when dipped in acidic solution is: Red

The color of litmus paper, when dipped in basic solution is: Blue

The litmus paper can also indicate the presence of a neutral solution. It means it turns green or purple when dipped in the aqueous solution comprising of neutral substances.

Calculation of pH value

The pH is calculated as the logarithmic of the hydrogen ion concentration in the solution. It is given by:

PH scale

Let's consider an example.

Example: Find the pH of the solution that has the hydrogen ion activity of 5 x 10^-6?

Solution: The pH can be calculated as:

PH scale

pH = log10 (1 / 5 x 10^-6)

pH = log10 (2 x 10^5)

pH = 5.3

Classification of soil and its pH ranges

There are various types of soils with different pH ranges. The classification of soil based on the pH ranges helps in the proper cultivation of different types of crops. The ideal pH range for soil lies between 6 and 7.

The soils are also classified as acidic and basic. The pH value of acidic soil lies below 7, while pH value of alkaline soil lies above 7. According to the pH ranges, the soils are classified as:

pH range Category of soil
< 3.5 Ultra - acidic soils
3.5 - 4.4 Extremely acidic soils
4.5 - 5.0 Very strong acidic soils
5.1 - 5.5 Strong acidic soils
5.6 - 6.0 Moderate acidic soils
6.1 - 6.5 Slightly acidic soils
6.6 - 7.3 Neutral soils
7.4 - 7.8 Slightly alkaline soils
7.9 - 8.4 Moderate alkaline soils
8.5 - 9.0 Strong alkaline soils
>9.0 Very strong alkaline soils

The above table displays the types of soil, its pH values, and the example of the plants suitable to grow in that pH range. Most of the crops are ideal for growing in the pH range of 5 - 7.5. It includes apple, turnip, pumpkin, broccoli, cabbage, lettuce, potato, wheat, corn, etc.

The high acidic nature of the soil leads to poor crop cultivation.

Effect of addition of fertilizers to the soil

The addition of fertilizers to the soil results in a variation in pH. It also affects the number of nutrients present in the oil. It is generally added to increase the production of crops. Some fertilizers can even harden the soil and lead to various disadvantages, such as water pollution, decreasing soil fertility, etc.

For example,

Phosphoric acid: It increases the acidic nature of the soil. It also creates an adverse impact on the soil, such as poor plant growth. The phosphorus helps in the transfer of energy in the plant.

Ammonia-based fertilizers: It is also added to lower the pH value of the soil. It also leads create direct damage to the leaves. Ammonia-based fertilizers produce fewer greenhouse gases as compared to other fertilizers. The nitrogen in ammonia also helps in plant growth.

pH value in the living system

The pH is the most important factor for survival on the Earth. The most important task of the human body is food digestion.

The acid present in the stomach has a pH between 1.5 and 3. It helps in the breakdown of the food and enzymes. It also helps to kill harmful viruses and bacteria.

Let's the pH value associated with different parts of the body. It is shown in the below table.

Compartments in human body pH Function
Gastric acid 1.5 - 3.5 Gastric juice helps in the digestion of food.
Lysosomes 4.5 Lysosomes present in the human body helps to destroy the invading virus and bacteria.
Human skin 4.7 The pH of human skin is slightly acidic. Such acidic nature is maintained by sweat, etc. The slightly acidic nature keeps the moisture in and prevents the skin from unwanted bacteria.
Urine 6.0 6.0 is the average pH of the urine. Urine is the form of waste- a product created by the body's urinary system.
Cytosol 7.2 Cytosol has various functions. For example, transportation of metabolites. These are small molecules transported from the production site to other parts of the cell.
Blood 7.34 - 7.45 Blood is an important fluid in our body that helps in the proper functioning of the body. The increase or decrease in the blood pH can lead to various diseases.
Cerebrospinal fluid 7.5 CSF or Cerebrospinal fluid acts as a buffer in our body. It provides protection to the brain present inside the skull. It is slightly alkaline and most of its percentage is water.
Mitochondrial matrix 7.5 It is known as the powerhouse of the cell. It breaks down the glucose into energy, which is used to fuel other cellular processes.
Pancreas secretion 8.1 Pancreas secretes enzymes that help in the breakdown of proteins, fats, carbohydrates, and acids. The pH of the pancreas is alkaline because the juice and enzymes produced by it are alkaline in nature.

The pH of blood flowing in our body lies between the range 7.3 and 7.4. The pH at a defined scale is essential for the proper functioning of the body.

Improper balance in the pH value can lead to various diseases. Let's discuss the pH imbalance in the human body in detail.

pH Imbalance in the human body

The lungs and kidneys of the human body play an essential role in balancing pH. The acid-base balance is essential for the proper functioning of the body. Such imbalance requires medical attention cannot be quickly resolved with dietary changes.

  • The major problems due to pH imbalance in the blood are termed acidosis or alkalosis. Both the problem arises when the pH of the blood becomes greater than 7.4 and less than 7.3. It arises due to respiratory problems.
  • The pH imbalance arises when the pH decreases in the pancreases. The acidification introduced to the lower pH can adversely impact the activity of the enzymes.
  • The imbalance in urine pH can cause problems. For example, lower urine pH can indicate the presence of stone in the kidneys.
  • The pH of the human skin can be affected by makeup, pollution, alcohol, etc. The imbalance in pH can cause inflammatory skin and other problems, such as acne.

Applications of the pH scale

PH scale

There are various applications of pH scales ranging from school practical to industrial use. Let's discuss some most common applications.


The medicines are created to treat the acidity in stomach, which has a pH between 1.5 and 3.0. It can cause heart burn, indigestion, and imbalance in the body's functioning. The medicine works as an antacid to lower the amount of acidity.

Water treatment

A pH scale is used to measure the pH of the water. The water is basic if it contains more hydroxide ions. Similarly, the water is acidic if it contains more free hydrogen ions. It helps in determining the chemical nature of the water and removes further impurities.


In chemistry, pH helps also helps to determine the acidic and basic nature of the aqueous solutions. It is also used in laboratories to perform experiments.


Agronomy is defined as the study of soil management. The ph scale is used to check the acid or basic nature of the soil.

It helps in determining the optimal crop range for suitable cultivation. Hence, it is quite popular in agriculture.

Industrial processes

The pH scale is also used to test the acid or basic nature of the food, products, etc. It helps to determine the taste and quality of different food products. For example, citric acid is added to the liquid package product as a preservative.

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