CHEMISTRY

Curdling of Milk – A Chemical Change

The curdling of milk is an example of a chemical change because the original substance undergoes a transformation and forms a new substance with different properties.During this process, lactic acid bacteria convert lactose (milk sugar) into lactic acid. The increase in lactic acid causes casein proteins present in milk to denature and coagulate, resulting in the formation of curd.The process is irreversible, which is a key characteristic of chemical changes. It is also accompanied by:

• Formation of a new substance (curd)
• Development of a characteristic smell
• Change in texture

Curdling is widely used in the production of yogurt and cheese.

Physical Changes (For Comparison)

Law of Conservation of Mass

The Law of Conservation of Mass states that mass can neither be created nor destroyed during a chemical reaction. Therefore, the total mass of reactants is always equal to the total mass of products.

Example

Reactants:

• Reactant A = 25 g
• Reactant B = 35 g

Total mass of reactants:

• 25 g + 35 g = 60 g

Products:

• Product C = 50 g
• Product D = ?

Applying the law:

• Total mass of products = Total mass of reactants
• 50 g + Product D = 60 g

Therefore:

• Product D = 10 g

About the Law

• Proposed by Antoine Lavoisier in the late eighteenth century.
• Forms the foundation of modern chemistry.
• Ensures chemical equations are balanced.

Applications

• Chemical Engineering
• Environmental Science
• Industrial Chemical Processes
• Stoichiometric Calculations

Heterogeneous Mixture

A heterogeneous mixture is a mixture in which the components are not uniformly distributed and the composition varies from one part to another.Such mixtures often contain physically distinct regions, each possessing different properties. The appearance is generally non-uniform, and individual components can usually be observed separately.Examples include:

• Salad
• Sand and water
• Oil and water

The components of heterogeneous mixtures can often be separated using physical methods such as filtration or sorting.Heterogeneous mixtures are important in fields such as:

• Geology
• Chemistry
• Material Science

Reactivity Series – Position of Magnesium

In the reactivity series, metals are arranged according to their tendency to lose electrons and form positive ions.Magnesium is placed above aluminium and zinc because of its higher reactivity. It loses electrons readily and participates actively in oxidation-reduction reactions.

Characteristics of Magnesium

• More reactive than aluminium.
• Highly reactive metal.
• Reacts with oxygen to form magnesium oxide (MgO).
• Reacts with acids to release hydrogen gas.

Applications

• Alloys
• Fireworks
• Medicines

Its position in the reactivity series makes it important in understanding displacement reactions and metal reactivity trends.

Prevention of Rancidity in Food

Antioxidants are substances that slow down or prevent oxidation, which is the primary cause of rancidity in packaged foods.Oxidation occurs when food reacts with oxygen present in the air, causing fats and oils to deteriorate and develop unpleasant odours and flavours. Antioxidants prevent the formation of peroxides, which are responsible for rancid smell and taste.

Common Antioxidants

• BHA (Butylated Hydroxyanisole)
• BHT (Butylated Hydroxytoluene)
• Ascorbic Acid (Vitamin C)

These are particularly effective in foods rich in unsaturated fats, including:

• Snacks
• Oils
• Baked products

Benefits

• Extends shelf life
• Maintains nutritional value
• Preserves flavour and quality

Antioxidants are also used in:

• Cosmetics
• Pharmaceuticals
• Other industrial products

Factors That Promote Spoilage

True Solution and Colloid

True Solution

A true solution is a homogeneous mixture of two or more substances. In a true solution, the solute particles are completely dissolved in the solvent, forming a single-phase system. Since the particles are extremely small and uniformly distributed, true solutions appear transparent.

Colloid

A colloid is a heterogeneous mixture in which the solute particles do not dissolve completely but remain dispersed throughout the solvent. A colloid consists of:

• Dispersed Phase – substance present in smaller amount.
• Dispersion Medium – substance present in larger amount.

Thus, colloids form a two-phase system. The particles are larger than those in true solutions but remain invisible to the naked eye. Colloids are generally opaque or translucent.

Difference Between True Solution and Colloid

Bee Sting and Methanoic Acid

Bee stings are painful because bees inject methanoic acid (formic acid) into the skin.

Effects of Methanoic Acid

• Causes a burning sensation.
• Produces redness, swelling, and inflammation.
• Triggers the body's immune response.
• Leads to release of histamines, resulting in pain and irritation.
• Although methanoic acid is a weak acid, its concentrated injection causes significant discomfort.
• Severity depends upon:
  • Individual allergies.
  • Number of stings received.

Acid Reaction with Metal Oxide

When an acid reacts with a metal oxide, a neutralization reaction takes place.

General Reaction

Acid + Metal Oxide → Salt + Water

Example

Hydrochloric acid reacts with magnesium oxide:HCl + MgO → MgCl₂ + H₂O

Key Points

• Metal oxides behave as basic substances.
• Acid neutralizes the basic metal oxide.
• Products formed are salt and water.

Covalent Compounds and Their Melting/Boiling Points

Covalent compounds are formed by sharing of electrons between atoms.

Characteristics

• Generally possess low melting points.
• Generally possess low boiling points.
• Individual molecules are held together by:
  • Van der Waals forces
  • Dipole-dipole interactions

These intermolecular forces are much weaker than ionic or metallic bonds; therefore less energy is required to separate molecules.

Examples

• Water (H₂O)
• Methane (CH₄)
• Carbon dioxide (CO₂)

Comparison

Mixture and Compound

Composition

Mixture

• Components can be present in any ratio.
• Composition is variable.

Compound

• Elements combine in a fixed ratio.
• Composition remains definite.

Formation

Mixture

• Formed by physical mixing.
• No chemical bond formation.

Compound

• Formed through chemical reactions.
• New chemical bonds are formed.

Properties

Mixture

• Retains properties of constituent substances.

Example:

Iron filings + Sulphur retain properties of both iron and sulphur.Compound

• Possesses properties different from constituent elements.

Example:

Water (H₂O) differs from hydrogen and oxygen.

Separation

UV-B Radiation

Wavelength Range

UV-B : 280–315 nm

Characteristics

• Wavelength is shorter than UVA and longer than UVC.
• UVC (100–280 nm) is completely blocked by the ozone layer.
• UV-B partially reaches Earth's surface.

Biological Effects

• Highly biologically active.
• Damages DNA by forming thymine dimers.
• Causes:
  • Sunburn
  • Mutations
  • Increased skin cancer risk

Skin Penetration

• Penetrates only superficial skin layers.
• Produces delayed tanning, burning, and aging.
• UVA penetrates deeper but causes less direct DNA damage

1. Mass Percent of NaCl in Solution

Question

A solution is prepared by dissolving 40 g of NaCl in 200 g of water. Calculate the mass percent of NaCl.

Formula

Mass Percent = (Mass of Solute / Mass of Solution) × 100

Given

• Mass of NaCl = 40 g
• Mass of Water = 200 g

Mass of Solution = 40 + 200 = 240 g

Calculation

Mass Percent = (40 / 240) × 100= 16.67%

Answer

Mass Percent of NaCl = 16.67%

2. Molecular Mass of Sulphur (S₈)

Question

Atomic mass of sulphur is 32 u and sulphur exists as S₈ molecules. Find the molecular mass.

Formula

Molecular Mass = Atomic Mass × Number of Atoms

Calculation

= 32 × 8= 256 u

Answer

Molecular Mass of Sulphur = 256 u