Acids and Bases

The PH scale: Its importance in everyday life

The PH Scale is a measure of the strength of an acid or base. The PH of a solution is measured using a paper impregnated with a universal indicator.

In the previous articles, we have learned that acids produce hydrogen or hydronium ions in a solution. The strength of an acid is proportional to the concentration of the hydronium ions. Therefore, if we have a way to measure the concentration of hydronium ions, then we would be able to quantify the strength of an acid. Similarly, the strength of a base is directly proportional to the concentration of the hydroxide (OH) ions.

The PH Scale is the log of the hydronium ion concentration. An acid is any substance whose hydronium ion concentration is more that 10-7 moles/litre. Therefore, an acid will always have a PH of less than 7. As the strength of the acid increases, the PH decreases. For example, lemon juice has a PH for 2.3. The hydrochloric acid in our stomachs has a PH of 1.5-2, and the PH of concentrated hydrochloric acid is nearly 0.

The PH scale is also used to measure the strength of bases. In general, bases have a PH greater than 7. Stronger the base, more is its PH. The maximum value possible on the PH scale is 14. A concentrated solution of sodium hydroxide would have a PH value of 14.

So, what is the importance of the PH scale in the everyday file?

Most life processes can occur within a narrow range of PH. For example, the PH of blood is tightly regulated in the range of 7.2 to 7.4. Similarly, the PH value of the soil is of great importance in agriculture. Most food crops grow best at a PH of 7-7.8. The growth of plants suffers when the PH value is outside this range. The PH of soil is usually tightly regulated by the microflora present in the ground. However, excessive use of ammonia-based fertilisers can make the soil acidic.

Have you heard of acid rain? The atmosphere contains gasses like nitrous oxide and sulphur oxide. When these dissolve in rain, they produce nitric and sulphuric acid. If the concentration of these acids in high in rainwater, it can lower the PH value. Acid rain has a value of less than 5.6 on the PH scale. ?Acid rain is another cause of the increase in soil acidity.

The PH scale plays a critical role in the human body. Different parts of the human body have different PH. The mouth has a slightly alkaline PH while the stomach has a very acidic PH. The skin has a slightly acidic PH, and the PH in the Vagina is again very acidic. Why is this so?

The saliva in the oral cavity makes the PH of the mouth alkaline. The alkaline PH facilitates the action of salivary enzymes. It also prevents tooth decay. Why do we wash our mouth after eating anything? The sugars present in the food can ferment to produce acid. This acid can damage the enamel of the teeth, thus causing tooth decay.

The PH of the stomach is between 1.3 -2. The glands in the stomach secrete a hydrochloric acid that makes the PH of the stomach acidic. The acidic PH serves two functions- it kills any microorganisms, and it facilitates the action of enzymes like trypsin.

Similarly, the acidic PH of the skin and vagina prevent colonisation by bacteria, thus serving as a natural defence against infection.

Thus, the PH scale plays a crucial role in everyday life. Students are frequently asked to calculate the PH in the board exams. Therefore, the PH scale is an important topic that the students of class 10 science should master.

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Acids: Properties and behaviour


What are the adjectives one would associate with acids? The first word would be corrosive. What about sour? Are acids sour in taste? Only a fool would taste a concentrated solution of acid. But what about a dilute solution of acids, like vinegar? You will note that acids are sour in taste.

So how does one identify an acid? Let us assume that you are given two test tubes with a colourless solution. One of them is an acid and the other is not. How do you know which one is the acid?

One method is to use a litmus paper. The Litmus paper contains a dye produced by a class of plants called ferns. This dye changes colour when exposed to acids. There are two types of Litmus papers- red and blue. When you dip the blue litmus paper in an acid, it will change colour to red while the red litmus paper will remain red on exposure to acids. The Litmus paper can also be used to identify a base. If you dip a red litmus paper in a basic solution, it turns blue in colour. Thus, by using a combination of these two Litmus papers, we can identify acids and bases. Apart from Litmus paper, we could also use substances like Turmeric, methyl orange and phenolphthalein to identify acids and bases. These pigments are also known as indicators.

These were the physical properties of acids. Let is now discuss a little about the chemical properties of acids

In the previous article, we learned about the definition of an acid. We learned that acids were in general proton donors. Therefore, acids can react with any substance that accepts a proton. These materials include bases and metals. Acids can also be defined on the basis of the transfer of electrons. An acid is a substance that accepts a pair of electrons from another species.

The following equation summarises the reaction between acids and metals:-

Metal + Acid = Metal Salt + Hydrogen gas

Consider this example:-

Zn + 2HCl = ZnCl2 + H2

In the above equation, one atom of Zinc reacts with two molecules of hydrochloric acid to form one molecule of hydrogen and zinc chloride. Here, hydrochloric acid is behaving as a Lewis acid as it accepts a pair of electrons from the zinc atom.

So, how do acids and bases react with each other?

Consider this equation:-

2HCl +2NaOH = 2H2O +2 NaCl

We can devise a simple experiment using the above equations. Take a test tube with 5 ml of NaOH. Add a few drops of phenolphthalein to this solution. What do you observe? You will note that the colour of this solution turns pink. Now, start adding hydrochloric acid to this solution drop by drop. What do you observe? You will note that the pink colour gradually fades and at some point, the solution will become entirely colourless. When you add hydrochloric acid to a solution of NaOH, the acid neutralises the base to produce salt and water. The basic solution will become transparent at the point where all the base is neutralised. ?In fact, you will be using this neutralisation reaction in Class 12 to quantify the exact concentration of an acid or base in a solution.

To summarise, acids are corrosive substances with a sour taste that change the colour of blue Litmus to red. Acids are capable of reacting with a multitude of materials. We will learn more about acids in the next article.

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Acid: Definition

Can you think of a few uses of an acid in everyday life? We use vinegar to preserve and flavour our food. Acid was and is still used to clean toilets and bathrooms, and car batteries also use acids. Apart from these uses, acid is also used in many industrial processes. In fact, acids are ubiquitous in our life. So, what exactly is an acid?

There are three definitions of an acid; the Arrhenius, the Br?nsted-Lowry & the Lewis definition.

Arrhenius definition

A Swedish chemist, Arrhenius, defined an acid is any substance that increases the concentration of hydrogen, or more accurately, hydronium ions in a solution. Study the following equation:-

H2O + H2O = H3O+ + OH

Most of the molecules of water exist as H2O. But, some molecules exist as hydronium, and some exist as hydroxide ions. As the number of hydronium and hydroxide ions is the same, water in neutral. However, if any substance were to increase the concentration of hydronium ions, then the solution will turn acidic. Conversely, any substance that decreases the concentration of hydroxide ions will also be an acid, as a reduction in hydroxide ions will turn a solution acidic.

Have you ever wondered why a PH < 7 is considered acidic? An acid is any solution that contains hydronium ions in excess of 10-7 moles per litre. Since PH is the negative log of hydrogen or hydronium ions, a PH< 7 is considered acidic.

Br?nsted- Lowry definition

Although the Arrhenius definition explains many reactions involving acids, there are some limitations to its use. Consider this equation:-


In the above reaction, there is no hydronium ion, yet we know that acetic acid (CH3COOH) is an acid. This flaw was rectified by Br?nsted and Lowry, two chemists, who independently postulated that an acid is any species that donates a proton. Thus acetic acid, in the above example, donates a proton (H+) ion to ammonia. Thus, it behaves as a Br?nsted acid.

Lewis definition

In the same year, another chemist, Gilbert N. Lewis, defined an acid in terms of electron transfer. He defined an acid as any species that accepts a pair of electrons from another substance. All Br?nsted acids are Lewis acids. However, all Lewis acids need not be Br?nsted acids. Consider this equation:-

BF3 + F = BF4

Here, the fluoride ion donates a pair of electrons to boron trifluoride. This reaction cannot be explained by the Br?nsted theory. However, by the Lewis definition, boron trifluoride is an acid as it accepts a pair of an electron from fluoride ion.

The Arrhenius and the Br?nsted definitions are the most relevant definitions for an acid, and the Br?nsted definition is the most commonly used method to define an acid.

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