Acids, Bases and Salts

Acids and Bases in the Laboratory

Chapter 2: Acids, Bases and Salts

Page 1 of 5: Acids and Bases in the Laboratory

You've probably noticed that some foods, like lemon and tamarind, taste sour, while others, like baking soda solution, taste bitter. This difference in taste is due to the presence of chemical substances we call acids and bases.

• Acids are substances that taste sour. The word 'acid' comes from the Latin word acidus, which means sour. Lemons, oranges, vinegar, and curd all contain acids.
• Bases are substances that are bitter in taste and feel soapy to the touch. Baking soda, washing soda, and soaps are common examples of bases.

But how can we identify acids and bases in a laboratory? We certainly can't taste every chemical! It would be extremely dangerous. This is where indicators come to our rescue.

What are Indicators?

Imagine you have three unlabeled test tubes, one containing distilled water, one an acid, and one a base. How would you identify them? You would use a special substance that changes its colour when it comes into contact with an acid or a base.

{{KEY: type=definition | title=Acid-Base Indicators | text=Indicators are substances that show a distinct change in colour (or odour) when brought in contact with an acid or a base, thus helping to distinguish between them.}}

These indicators can be natural or synthetic. Let's explore the most common ones you'll use in the lab and find in your own home!

Natural Indicators

These are obtained from natural sources like plants.

• Litmus: This is the most common indicator. It is a natural dye extracted from a type of plant called lichen. In a neutral solution (like pure water), it is purple.
  • When added to an acidic solution, it turns red.
  • When added to a basic solution, it turns blue. Litmus is available as a solution or as strips of paper—blue litmus paper and red litmus paper. Can you guess what happens when you dip blue litmus paper into an acid? That's right, it turns red!

{{VISUAL: diagram: Two test tubes, one labeled 'Acid' and the other 'Base'. In the acid tube, a strip of blue litmus paper is turning red. In the base tube, a strip of red litmus paper is turning blue.}}

• Turmeric (Haldi): You've seen this yellow powder in your kitchen! Turmeric is a fantastic natural indicator. Have you ever noticed that a curry stain on a white shirt turns reddish-brown when soap (which is basic) is scrubbed on it? That's turmeric in action!
  • It remains yellow in acidic and neutral solutions.
  • It turns reddish-brown in basic solutions.

Synthetic Indicators

These are man-made chemicals used specifically for testing acids and bases.

• Phenolphthalein: This is a colourless liquid.

  • It remains colourless in acidic solutions.
  • It turns pink in basic solutions.

• Methyl Orange: As the name suggests, this indicator is orange in a neutral solution.

  • It turns red in acidic solutions.
  • It turns yellow in basic solutions.

{{KEY: type=points | title=Common Indicator Changes | text=- Litmus: Red in Acid, Blue in Base.

• Phenolphthalein: Colourless in Acid, Pink in Base.
• Methyl Orange: Red in Acid, Yellow in Base.
• Turmeric: Yellow in Acid, Reddish-brown in Base.}}

Olfactory Indicators

Some substances have a characteristic smell that changes in acidic or basic solutions. These are called olfactory indicators (related to the sense of smell).

• Onion: Finely chopped onions have a strong, characteristic smell. When a basic solution like sodium hydroxide is added to a cloth strip treated with onion juice, the onion smell disappears. However, the smell remains when an acidic solution is added.
• Clove Oil & Vanilla Essence: Both have a characteristic smell that vanishes in a basic solution but persists in an acidic solution.

{{VISUAL: chart: A simple table comparing Litmus, Phenolphthalein, Methyl Orange, and Onion. Columns are 'Indicator', 'Colour/Smell in Acid', and 'Colour/Smell in Base'.}}

Chemical Properties of Acids: What Happens When Acids React?

Now that we can identify acids, let's explore how they behave chemically. One of the most important reactions is how acids react with metals.

Reaction of Acids with Metals

When an acid reacts with a reactive metal (like zinc, iron, or magnesium), it typically produces a metal salt and hydrogen gas.

The general reaction can be written as:

Acid + Metal → Salt + Hydrogen (H₂) gas

For example, when dilute sulfuric acid (H₂SO₄) reacts with zinc granules (Zn), the following reaction occurs: H₂SO₄ (aq) + Zn (s) → ZnSO₄ (aq) + H₂ (g)

Here, zinc sulfate (ZnSO₄) is the salt, and hydrogen gas is evolved. But how do we know the gas is hydrogen?

The "Pop" Sound Test: Hydrogen gas is flammable and burns with a characteristic ‘pop’ sound. To test for its presence, we can bring a burning candle or a matchstick near the gas being evolved. If it extinguishes with a popping sound, we can confirm the gas is hydrogen.

{{VISUAL: diagram: Experimental setup showing zinc granules in a test tube with dilute sulfuric acid. A delivery tube passes the evolving gas through a trough of soap solution. A burning candle is brought near a soap bubble filled with gas, showing a 'pop' sound effect.}}

{{KEY: type=exam | title=The 'Pop' Sound Test for Hydrogen | text=In exams, questions often ask how to test for the gas evolved when an acid reacts with a metal. Always mention bringing a burning splinter near the gas and listening for the characteristic 'pop' sound to confirm the presence of hydrogen gas.}}

Caution: Always handle acids with care. Strong acids like sulfuric acid and hydrochloric acid are highly corrosive and can cause severe burns. These experiments must always be performed under the supervision of a teacher.

How do Acids and Bases React with Metals?

{{FORMULA: expr=Acid + Metal → Salt + Hydrogen gas | symbols=Acid:a substance that produces H⁺ ions, Metal:an element that is a good conductor, Salt:an ionic compound, Hydrogen gas:H₂}}

How do Acids and Bases React with Metals?

In our last lesson, we learned to identify acids and bases using indicators. But what happens when we mix them with other substances? Let's start by investigating one of the most fundamental reactions in chemistry: the reaction between acids, bases, and metals. This is not just a test tube reaction; it explains everything from why batteries work to why we don't store pickles in brass jars!

1. Reaction of Acids with Metals

Imagine dropping a few granules of shiny, grey zinc metal into a test tube containing a clear, colorless liquid like dilute hydrochloric acid. What would you expect to see?

You'll immediately notice bubbles forming on the surface of the zinc granules and rising up. This bubbling or fizzing is called effervescence, and it indicates that a gas is being produced.

This observation is a classic example of an acid reacting with a metal. The general rule for this reaction is:

Acid + Metal → Salt + Hydrogen Gas

Let's break down our example with zinc and hydrochloric acid:

• Acid: Hydrochloric Acid (HCl)
• Metal: Zinc (Zn)
• Salt: Zinc Chloride (ZnCl₂)
• Gas: Hydrogen (H₂)

The balanced chemical equation for this reaction is: Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g)↑

Here, (s) stands for solid, (aq) for aqueous (dissolved in water), and (g) for gas. The upward arrow ↑ also signifies that a gas is evolving. The salt formed, Zinc Chloride, remains dissolved in the solution.

{{KEY: type=concept | title=Acid-Metal Reaction | text=When an acid reacts with a metal, it produces a metal salt and hydrogen gas. The metal displaces the hydrogen from the acid, leading to the evolution of H₂ gas. This is a type of single displacement reaction.}}

How do we confirm the gas is Hydrogen?

The gas produced is colorless and odorless, so how can we be sure it's hydrogen? Chemists use a simple yet definitive test called the pop test.

1. Set up the experiment as shown in the diagram, allowing the gas produced to pass through a delivery tube into a trough of soap solution.
2. Gas-filled soap bubbles will begin to form and rise.
3. Bring a burning candle or a lit matchstick near one of these bubbles.
4. If the gas is hydrogen, you will hear a characteristic 'pop' sound as the gas burns rapidly, extinguishing the flame.

{{VISUAL: diagram: laboratory setup showing the reaction of zinc granules with dilute sulphuric acid. A test tube contains the reactants, and a delivery tube passes the evolved gas through a trough with soap solution. A burning candle is brought near a gas-filled soap bubble, which bursts with a 'pop' sound.}}

This test is the standard confirmation for the presence of hydrogen gas in a laboratory setting.

2. Reaction of Bases with Metals

Now, a logical question arises: Do bases also react with metals to produce hydrogen gas?

The answer is yes, but with an important condition. This reaction is not as common as the acid-metal reaction. Only certain metals, like Zinc (Zn) and Aluminium (Al), react with strong bases.

Let's take the example of zinc reacting with a strong base, Sodium Hydroxide (NaOH). When zinc granules are heated with a solution of sodium hydroxide, hydrogen gas is also produced.

The general word equation is: Base + Metal → Salt + Hydrogen Gas

The specific chemical reaction is: 2NaOH(aq) + Zn(s) → Na₂ZnO₂(aq) + H₂(g)↑

The salt produced here has a more complex name: Sodium Zincate (Na₂ZnO₂). Just like with acids, the presence of hydrogen gas can be confirmed using the pop test.

{{VISUAL: photo: a close-up of a test tube containing zinc granules and a clear sodium hydroxide solution. The test tube is being gently heated by a Bunsen burner flame, and small bubbles of hydrogen gas are visibly forming on the surface of the zinc.}}

{{ZOOM: title=What are Amphoteric Metals? | text=Metals like zinc (Zn) and aluminium (Al) are called amphoteric because they can react with both acids and strong bases to produce hydrogen gas. This dual nature makes them special compared to other metals like iron or magnesium, which do not typically react with bases.}}

It is very important to remember that not all metals react with bases. For your syllabus, the reactions of zinc and aluminium with strong bases like NaOH are the primary examples.

{{KEY: type=exam | title=Important Distinction | text=Remember that while most reactive metals react with dilute acids to produce hydrogen gas, only a few metals like zinc (Zn) and aluminium (Al) react with strong bases (like NaOH) to produce hydrogen gas.}}

A Real-World Connection: Pickles and Metal Pots

Have you ever wondered why your grandmother insists on storing sour food items like curd, pickles, or lemon juice in ceramic or glass jars, and never in metal containers made of copper or brass?

This is chemistry in action in your kitchen!

• The Food: Pickles and curd contain acids (acetic acid, lactic acid, etc.).
• The Container: Brass is an alloy of copper and zinc. Both are metals.
• The Reaction: The acids in the food react with the metals of the container, just like in our experiments. Acid + Metal → Salt + Hydrogen.
• The Danger: The salts produced by this reaction can be toxic and harmful to health, spoiling the food and making it unsafe to eat.

This simple kitchen rule is a direct application of the acid-metal reaction, preventing food poisoning by avoiding the formation of harmful metal salts.

How do Acids and Bases React with each other?

{{FORMULA: expr=Acid + Base → Salt + Water | symbols=→:yields or produces}}

The Chemical Showdown: What Happens When Acids and Bases Meet?

We've explored acids and bases as two distinct families of chemical compounds with opposite properties. But what happens when you mix an acid with a base? Do they coexist peacefully, or is there a dramatic reaction? This is one of the most fundamental and important reactions in chemistry.

Imagine an acid, like hydrochloric acid (HCl), which donates H⁺ ions in water. Now, imagine a base, like sodium hydroxide (NaOH), which provides OH⁻ ions. When they are mixed in the same solution, the proton (H⁺) from the acid and the hydroxide ion (OH⁻) from the base find each other irresistible. They immediately combine to form a stable, neutral molecule of water (H₂O).

The remaining ions, the Na⁺ from the base and the Cl⁻ from the acid, are left in the solution and form an ionic compound called a salt. In this case, they form sodium chloride (NaCl), which you know as common table salt!

The Neutralisation Reaction

This reaction between an acid and a base to give a salt and water is known as a neutralisation reaction. The term "neutralisation" is used because the acid and base effectively cancel out or neutralise each other's properties. The resulting solution is often neutral (with a pH of 7), especially when a strong acid reacts with a strong base in the right proportions.

The general equation for this reaction is beautifully simple:

Acid + Base → Salt + Water

Let's look at our example again, this time as a balanced chemical equation:

HCl (aq) + NaOH (aq) → NaCl (aq) + H₂O (l)

Here, (aq) means aqueous (dissolved in water) and (l) means liquid.

{{VISUAL: diagram: schematic showing the ionic exchange in a neutralisation reaction. H⁺ and Cl⁻ ions from HCl are shown separately, and Na⁺ and OH⁻ ions from NaOH are shown separately. Arrows indicate that the H⁺ combines with OH⁻ to form a water molecule (H₂O), while the Na⁺ and Cl⁻ ions remain to form the salt NaCl.}}

{{KEY: type=definition | title=Neutralisation Reaction | text=A chemical reaction in which an acid and a base react quantitatively with each other to form a salt and water. In this reaction, the H⁺ ions from the acid combine with the OH⁻ ions from the base.}}

The neutralisation reaction is an exothermic reaction, meaning it releases heat. If you were to carefully mix an acid and a base in a test tube, you would feel the test tube become warm.

Expanding the Idea: Metallic and Non-Metallic Oxides

The concept of neutralisation isn't limited to just traditional acids and bases. We can extend this idea to understand the chemical nature of metal and non-metal oxides.

1. Reaction of Metallic Oxides with Acids

Have you ever noticed rust on an iron nail? Rust is iron oxide (Fe₂O₃), a metallic oxide. Most metallic oxides are basic in nature. This means they react with acids in a way that is very similar to a neutralisation reaction, producing a salt and water.

General Equation: Metal Oxide (Basic) + Acid → Salt + Water

A common laboratory example is the reaction of copper(II) oxide, a black solid, with dilute hydrochloric acid.

CuO (s) + 2HCl (aq) → CuCl₂ (aq) + H₂O (l) (Copper Oxide + Hydrochloric Acid → Copper(II) Chloride + Water)

You will observe that the black solid CuO dissolves, and the solution turns a beautiful blue-green colour due to the formation of the salt, copper(II) chloride.

{{KEY: type=concept | title=Basic Nature of Metal Oxides | text=Oxides of metals are generally basic in nature. They react with acids to form salt and water, just like bases do. This is why substances like magnesium oxide (MgO) can be used in antacids to neutralise stomach acid.}}

2. Reaction of Non-Metallic Oxides with Bases

Now, let's consider the oxides of non-metals, like carbon dioxide (CO₂), which you exhale. Non-metallic oxides are acidic in nature. They react with bases to form a salt and water, completing our understanding of these neutralisation-like reactions.

General Equation: Non-metal Oxide (Acidic) + Base → Salt + Water

A classic and very important example is the reaction between carbon dioxide and calcium hydroxide (limewater).

CO₂ (g) + Ca(OH)₂ (aq) → CaCO₃ (s) + H₂O (l) (Carbon Dioxide + Calcium Hydroxide → Calcium Carbonate + Water)

In this reaction, calcium carbonate (CaCO₃) is an insoluble white solid (precipitate), which causes the clear limewater to turn milky.

{{VISUAL: photo: A side-by-side comparison of two test tubes. The first test tube contains clear limewater (calcium hydroxide solution). The second test tube shows the same solution turned milky white after passing carbon dioxide gas through it, indicating the formation of calcium carbonate precipitate.}}

{{KEY: type=exam | title=Test for Carbon Dioxide Gas | text=The reaction of CO₂ with limewater (Ca(OH)₂) to form a milky white precipitate of CaCO₃ is the standard chemical test used to confirm the presence of carbon dioxide gas. This is a frequently asked concept in practical-based questions.}}

This beautifully demonstrates the acidic nature of CO₂. Let's summarize the nature of these oxides.

The dance of chemistry often involves opposites attracting. An acid neutralises a base, a metallic oxide neutralises an acid, and a non-metallic oxide neutralises a base—all variations on the same fundamental theme of forming salt and water.

What Do All Acids and All Bases Have in Common?

What Do All Acids and All Bases Have in Common?

In our previous discussions, we observed that acids share many chemical properties. They react with metals to produce hydrogen gas, and they react with metal carbonates to produce carbon dioxide. Bases also show similarities among themselves. This begs a fundamental question: What is the underlying chemical reason for this similar behaviour? Let's investigate.

The Common Thread in Acids: The Hydrogen Ion (H⁺)

To find out what's common to all acids, let's consider an experiment. Imagine setting up a simple circuit with a battery, a bulb, a switch, and two graphite rods (electrodes) dipped into a beaker.

What happens if we fill the beaker with a solution of hydrochloric acid (HCl) and complete the circuit? The bulb starts glowing! This indicates that the HCl solution is conducting electricity. We can repeat this with other acids like sulphuric acid (H₂SO₄) and acetic acid (CH₃COOH), and we'll see the same result.

Electricity is essentially the flow of charged particles. For the bulb to glow, there must be mobile ions in the solution to carry the charge. This phenomenon is called dissociation or ionization.

{{VISUAL: diagram: A beaker containing dilute HCl solution with two graphite rods connected to a 6V battery, a switch, and a bulb. The bulb is shown glowing, indicating the flow of current through the acidic solution.}}

When an acid is dissolved in water, it splits into ions. The common player in every acid is the hydrogen ion (H⁺).

• Hydrochloric acid: HCl(aq) → H⁺(aq) + Cl⁻(aq)
• Sulphuric acid: H₂SO₄(aq) → 2H⁺(aq) + SO₄²⁻(aq)
• Nitric acid: HNO₃(aq) → H⁺(aq) + NO₃⁻(aq)
• Acetic acid: CH₃COOH(aq) ⇌ H⁺(aq) + CH₃COO⁻(aq)

It is the presence of this H⁺(aq) ion that is responsible for the acidic properties of a substance. If we perform the same experiment with solutions of glucose (C₆H₁₂O₆) or alcohol (C₂H₅OH), the bulb does not glow. Although these compounds contain hydrogen, they do not dissociate in water to produce H⁺ ions, and hence, do not show acidic character.

{{KEY: concept | title=Acids and Electrical Conductivity | text=Acids conduct electricity in their aqueous solutions because they dissociate to produce mobile ions. The cation produced by all acids is the hydrogen ion (H⁺), which is responsible for their characteristic acidic properties. Compounds like glucose and alcohol contain hydrogen but do not ionize, so they do not conduct electricity or behave as acids.}}

A Closer Look: The Hydronium Ion (H₃O⁺)

The hydrogen ion, H⁺, is essentially just a proton. It's extremely small and highly reactive, meaning it cannot exist independently in a water solution.

As soon as an H⁺ ion is formed, it immediately latches onto a surrounding water molecule (H₂O). This combination forms the hydronium ion (H₃O⁺).

H⁺ + H₂O → H₃O⁺

So, a more accurate way to represent the dissociation of HCl is: HCl + H₂O → H₃O⁺(aq) + Cl⁻(aq)

{{VISUAL: diagram: A two-part illustration. Part A shows an HCl molecule in water splitting into H⁺ and Cl⁻ ions. Part B shows the H⁺ ion immediately combining with a polar H₂O molecule to form a hydronium ion, H₃O⁺.}}

For all practical purposes in Class 10, the hydrogen ion H⁺(aq) and the hydronium ion H₃O⁺(aq) are used interchangeably to represent the acidic particle in water.

The Common Thread in Bases: The Hydroxide Ion (OH⁻)

Now, let's apply the same logic to bases. What do common bases like sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)₂) have in common?

When dissolved in water, they also dissociate to produce ions. The common ion produced by all these bases is the hydroxide ion (OH⁻).

• Sodium hydroxide: NaOH(s) --(water)--> Na⁺(aq) + OH⁻(aq)
• Potassium hydroxide: KOH(s) --(water)--> K⁺(aq) + OH⁻(aq)
• Magnesium hydroxide: Mg(OH)₂(s) --(water)--> Mg²⁺(aq) + 2OH⁻(aq)

It is the presence of this OH⁻(aq) ion that gives a substance its basic properties.

{{KEY: definition | title=Alkali | text=A base that is soluble in water is called an alkali. For example, sodium hydroxide (NaOH) and potassium hydroxide (KOH) are alkalis. All alkalis are bases, but not all bases are alkalis.}}

The Indispensable Role of Water

This leads to a very important conclusion: the acidic or basic character of a substance is only exhibited in the presence of water.

Consider this classic experiment:

1. Take some solid sodium chloride (NaCl) in a dry test tube.
2. Carefully add some concentrated sulphuric acid (H₂SO₄). You will observe a gas coming out. This is hydrogen chloride (HCl) gas.
3. Bring a dry strip of blue litmus paper near the mouth of the test tube. You will see no change in colour!
4. Now, bring a moist (damp) strip of blue litmus paper. It instantly turns red!

{{KEY: exam | title=Dry HCl Gas Question | text=A very common question asks why dry HCl gas does not change the colour of dry litmus paper. The answer is that in the absence of water, HCl does not dissociate to form H⁺ ions. Only in the presence of moisture does it form H₃O⁺ ions, which are responsible for its acidic nature.}}

This experiment proves that water is essential for an acid to show its properties. The HCl molecule itself isn't acidic; it's the H⁺ (or H₃O⁺) ions it produces in water that are. The same principle applies to bases.

Dilution: Handling Acids and Bases Safely

Mixing a concentrated acid or base with water is called dilution. This process reduces the concentration of H₃O⁺ or OH⁻ ions per unit volume.

The process of dissolving an acid or a base in water is a highly exothermic reaction—it releases a large amount of heat. This has critical safety implications.

Safety Rule: Always add acid (or base) slowly to water with constant stirring. NEVER add water to a concentrated acid.

Why? If you add water to a concentrated acid, the heat generated is so intense and rapid that it can turn the water into steam almost instantly. This can cause the acid solution to splash out of the container, leading to severe burns. The glass container may also crack due to the sudden, localized heating.

When you add acid to a large volume of water, the heat is absorbed and distributed more gradually and safely.

{{VISUAL: diagram: Two beakers side-by-side. The first, labeled "CORRECT," shows acid being poured from a test tube into a large beaker of water, with a green checkmark. The second, labeled "INCORRECT," shows water being poured into a test tube of acid, with a red cross and a splash/explosion symbol.}}

{{KEY: points | title=Acid Dilution Safety | text=

• The process is highly exothermic.
• Always add acid to water, never water to acid.
• Add the acid slowly and with constant stirring.
• Wear safety goggles and gloves.}}

What Happens to an Acid or a Base in a Water Solution?

What Happens to an Acid or a Base in a Water Solution?

We've learned that acids are sour and turn blue litmus red, while bases are bitter and turn red litmus blue. We also know this is because of the presence of H⁺ ions in acids and OH⁻ ions in bases. But do substances like HCl (hydrochloric acid) or NaOH (sodium hydroxide) always have these free ions? Or is there a special condition required for them to show their acidic or basic character?

Let's investigate the crucial role that water plays in the chemistry of acids and bases.

The Role of Water in Acidic Behaviour

The properties of an acid are due to the hydrogen ions, H⁺, it produces. However, this production of ions, or dissociation, doesn't just happen on its own. It happens specifically in a water solution.

Consider a simple experiment:

1. Take about 1g of solid NaCl (common salt) in a clean, dry test tube.
2. Carefully add some concentrated sulphuric acid (H₂SO₄) to the test tube. You will observe a gas coming out.
3. This gas is hydrogen chloride (HCl). Test the gas with a dry blue litmus paper. You will notice no change in colour.
4. Now, test the gas with a moist (wet) blue litmus paper. The litmus paper instantly turns red!

{{VISUAL: diagram: A two-part diagram showing an experiment. Part A shows a stream of dry HCl gas from a test tube directed at a dry blue litmus paper with the label "No change". Part B shows the same gas stream directed at a moist blue litmus paper, which is turning red, with the label "Turns red".}}

What does this tell us? Dry HCl gas does not behave as an acid. It's only in the presence of water (from the moist litmus paper) that it shows its acidic properties.

This is because the separation of H⁺ ions from the HCl molecule can only occur in the presence of water.

HCl + H₂O → H₃O⁺ + Cl⁻

The hydrogen ions cannot exist alone. They are highly reactive and immediately combine with the surrounding water molecules to form the hydronium ion (H₃O⁺).

H⁺ + H₂O → H₃O⁺

So, when we talk about H⁺(aq) (aqueous hydrogen ions), we are actually referring to hydronium ions. The presence of these hydronium ions is the true indicator of acidic behaviour in a solution.

{{KEY: type=definition | title=Hydronium Ion (H₃O⁺) | text=The ion formed when a hydrogen ion (H⁺), produced by an acid, combines with a water molecule (H₂O). It is the species responsible for the acidic properties of a solution.}}

{{VISUAL: diagram: A molecular-level diagram showing one HCl molecule reacting with a water (H₂O) molecule. An arrow shows the H atom from HCl transferring to the H₂O molecule, resulting in a positively charged H₃O⁺ ion and a negatively charged Cl⁻ ion.}}

What About Bases in Water?

A similar process happens with bases. They produce hydroxide ions (OH⁻) when dissolved in water. For example, sodium hydroxide (NaOH), a solid, dissolves in water to produce sodium ions and hydroxide ions.

NaOH(s) ---[Water]---> Na⁺(aq) + OH⁻(aq)

Potassium hydroxide (KOH) behaves similarly:

KOH(s) ---[Water]---> K⁺(aq) + OH⁻(aq)

Bases that are soluble in water are called alkalis. This is an important distinction because not all bases dissolve in water. For example, copper(II) hydroxide, Cu(OH)₂, is a base but it is insoluble in water, so it is not an alkali.

{{KEY: type=definition | title=Alkali | text=A base that is soluble in water. All alkalis are bases, but not all bases are alkalis.}}

The presence of water is the essential medium that allows both acids and bases to ionize and exhibit their characteristic properties. Without water, they are just molecular compounds.

Mixing Acid or Base with Water: The Dilution Process

When you mix a concentrated acid or base with water, you are performing dilution. This process decreases the concentration of ions (H₃O⁺ or OH⁻) per unit volume of the solution.

This process, especially when dealing with concentrated acids like sulphuric acid (H₂SO₄) or nitric acid (HNO₃), is a highly exothermic process—it releases a large amount of heat.

A Critical Safety Warning: If you add water to a concentrated acid, the heat generated is so intense that it can cause the water to flash boil, splashing the concentrated acid out of the container. This can cause severe burns.

Therefore, the correct procedure is always:

1. Take the required volume of water in a beaker.
2. Slowly add the concentrated acid to the water, drop by drop or in a thin stream.
3. Continuously stir the solution to distribute the heat evenly.

This way, the large volume of water can absorb the heat generated gradually and safely. The same precaution should be taken when diluting a strong base.

{{VISUAL: diagram: A safety poster with two panels. The left panel, labeled "CORRECT", shows a hand slowly pouring acid from a test tube into a large beaker of water with a green checkmark. The right panel, labeled "INCORRECT", shows a hand pouring water into a small beaker of acid, with a red cross and a symbol for splashing/explosion.}}

{{KEY: type=concept | title=Dilution | text=The process of reducing the concentration of a solute in a solution, usually by mixing with more solvent (like water). In the case of acids and bases, dilution decreases the concentration of H₃O⁺ or OH⁻ ions per unit volume, resulting in a less acidic or less basic solution.}}

Mixing an acid or a base with water results in a decrease in the concentration of ions per unit volume. This is why a diluted acid is less corrosive and has a higher pH than its concentrated form.

{{KEY: type=exam | title=Safety Precaution Question | text=CBSE frequently asks a 2-mark question: "Why is it recommended to add acid to water and not water to acid while diluting a concentrated acid?" Your answer must mention that the process is highly exothermic and adding water to acid can cause splashing due to sudden boiling.}}