What is Chemistry?
Chemistry is the branch of science which deals with the properties, composition and the structure of substances. It also deals with the physical and chemical changes in matter and the laws or principles which govern these changes.
Key Concepts:
- Composition represents finding out percentages of elements and compounds in a sample of matter.
- Structure of matter means the arrangement of atoms in matter.
- Both physical and chemical changes may be brought about by absorption or evolution of energy.
Tips & Tricks
Remember the three main aspects of chemistry: Properties, Composition, and Structure. Use the acronym PCS to recall these fundamental concepts.
Branches of Chemistry
To understand the widely spread complex subject of chemistry and to concentrate on its specific aspects, chemistry is divided into many distinct branches.
- Physical Chemistry – Investigates how substances behave at atomic and molecular levels.
- Inorganic Chemistry – Study of elements and compounds that contain little or no carbon.
- Organic Chemistry – Deals with carbon compounds (hydrocarbons and their derivatives).
- Environmental Chemistry – Scientific study of chemical and biochemical phenomena in the environment.
- Analytical Chemistry – Deals with the analysis of different substances.
- Biochemistry – Understanding life through chemical processes.
- Nuclear Chemistry – Deals with reactions in the nucleus of an atom.
- Polymer Chemistry – Focuses on properties, structure and synthesis of polymers.
- Geochemistry – Study of chemical composition of Earth and its minerals.
- Medicinal Chemistry – Design and synthesis of medicines or drugs.
- Astrochemistry – Study of molecules and ions in space and interstellar space.
Tips & Tricks
To remember the main branches, use the mnemonic: PIO EAB NPG MA (Physical, Inorganic, Organic, Environmental, Analytical, Biochemistry, Nuclear, Polymer, Geochemistry, Medicinal, Astrochemistry).
Elements, Compounds and Mixtures
Matter in this world exists in the form of elements, compounds and mixtures.
| Elements | Compounds | Mixtures |
|---|---|---|
| Simplest form of matter | Pure substance formed by chemical combination | Impure substance with multiple components |
| Contains same kind of atoms | Formed by fixed ratio of elements | Components mixed in any ratio |
| Cannot be broken down by ordinary chemical reactions | Can be broken into constituent elements by chemical reactions | Components retain identity and can be separated by physical methods |
| Represented by symbols (Na, Ca) | Properties different from constituent elements | May be homogeneous or heterogeneous |
Tips & Tricks
Remember: Elements are pure and simple, Compounds are combined chemically, Mixtures are mixed physically.
Solutions, Colloidal Solutions and Suspensions
Solutions can be classified into three main types based on particle size and behavior:
| Property | True Solution | Colloidal Solution | Suspension |
|---|---|---|---|
| Particle Size | < 1 nm | 1 nm – 1000 nm | > 1000 nm |
| Visibility | Not visible | Not visible to naked eye | Visible to naked eye |
| Filtration | Pass through filter paper | Pass through filter paper | Do not pass through filter paper |
| Settling | Do not settle | Do not settle | Settle on standing |
| Examples | Sugar in water | Milk, starch solution | Sand in water |
Unsaturated vs Saturated Solutions:
- Unsaturated Solution – Can dissolve more solute at a particular temperature.
- Saturated Solution – Contains maximum amount of solute that can be dissolved at a particular temperature.
Tips & Tricks
Remember the order of particle size: True Solution < Colloid < Suspension. Use the acronym TCS (Tiny, Colloidal, Suspended) to remember the visibility and behavior differences.
Solved Exercises
1. Tick (✓) the correct answer.
(i) Matter is present in neon signs in the state of:
Explanation: Neon signs contain ionized gas, which is the plasma state of matter.
(ii) Hazardous effects of shopping bags are studied in:
Explanation: Environmental chemistry deals with the effects of chemicals on the environment.
(iii) The man-made polymer is:
Explanation: Starch, proteins, and cellulose are natural polymers, while polystyrene is synthetic.
(v) Which liquid among the following is a colloidal solution?
Explanation: Milk is a colloidal solution where fat globules are dispersed in water.
(vi) Which of the following is a heterogeneous mixture?
Explanation: Concrete is a heterogeneous mixture as its components are not uniformly distributed.
(viii) When the tiny visible particles of a substance are dispersed through a medium, the mixture is named as:
Explanation: In suspensions, particles are large enough to be visible and settle on standing.
2. Questions for Short Answers
i. Why is there a need to divide Chemistry into many branches? Give three reasons.
Answer: Chemistry is divided into branches because:
- The subject is too vast and complex to study as a whole
- It allows scientists to specialize and focus on specific areas
- It enables deeper understanding and breakthroughs in specialized fields
ii. Reactions may take place due to electrons present outside the nucleus or they may take place inside the nucleus. Which branches of Chemistry cover these two types of reactions?
Answer:
- Reactions due to electrons outside the nucleus: Physical Chemistry, Organic Chemistry, Inorganic Chemistry
- Reactions inside the nucleus: Nuclear Chemistry
v. Why are supercritical fluids important?
Answer: Supercritical fluids are important because:
- They have properties between liquids and gases
- They can dissolve materials like liquids
- They can effuse through solids like gases
- They are used in extraction processes (like decaffeination of coffee)
- They are environmentally friendly solvents
vi. In which state does matter exist in the Sun?
Answer: Matter exists in the plasma state in the Sun. Plasma is the fourth state of matter consisting of ionized gases.
3. Constructed Response Questions
i. How does a supercritical state look like?
Answer: A supercritical fluid appears as a homogeneous phase that has properties of both liquids and gases. It doesn’t have a distinct liquid-gas interface. It looks like a hazy or foggy substance that can diffuse through solids like a gas but dissolve materials like a liquid.
ii. In what way is plasma created in a fluorescent tube?
Answer: Plasma is created in a fluorescent tube through these steps:
- Electric current passes through the tube containing mercury vapor
- The current ionizes the mercury atoms, creating plasma
- The plasma emits ultraviolet light
- The UV light strikes the phosphor coating on the tube’s interior
- The phosphor fluoresces, emitting visible light
iii. Most of the molecules we study in biochemistry are organic in nature. Where does the difference exist in organic and biochemistry branches of Chemistry?
Answer: The main differences are:
- Organic Chemistry studies all carbon compounds, their synthesis, properties, and reactions
- Biochemistry specifically studies chemical processes in living organisms
- Biochemistry focuses on biomolecules (proteins, nucleic acids, etc.) and metabolic pathways
- Organic chemistry is broader, including both natural and synthetic carbon compounds
viii. Why graphite is slippery to touch? Which property of graphite enables it to be used as lubricant?
Answer: Graphite is slippery because of its layered structure where carbon atoms are arranged in hexagonal sheets. These sheets can easily slide over one another due to weak van der Waals forces between layers. This slippery nature and layered structure make graphite an excellent solid lubricant.
4. Descriptive Questions
i. Mention the name of the branch of Chemistry in which you will study each of the following topics.
Answer:
- (a) Rate of a reaction: Physical Chemistry
- (b) Digestion of food in human body: Biochemistry
- (c) Properties of plasma: Physical Chemistry
- (d) Ecosystem: Environmental Chemistry
- (e) Reactions taking place during fireworks: Inorganic Chemistry
- (f) Measurement of the absorption of wavelength with the help of ultraviolet spectrometer: Analytical Chemistry
iii. What are supercritical fluids. How are they different from ordinary liquids?
Answer: Supercritical fluids are substances at temperatures and pressures above their critical points, where distinct liquid and gas phases do not exist. They differ from ordinary liquids in these ways:
- They have densities similar to liquids but viscosities similar to gases
- They can diffuse through solids like gases
- They don’t have surface tension
- Their solvating power can be tuned by changing pressure and temperature
v. What types of movements are present in gaseous and liquid molecules?
Answer:
- Gaseous molecules exhibit:
- Random translational motion
- Vibrational motion
- Rotational motion
- High kinetic energy and free movement
- Liquid molecules exhibit:
- Vibrational motion
- Rotational motion
- Limited translational motion (sliding past each other)
- Moderate kinetic energy with some intermolecular forces
vi. Differentiate between the areas which are studied under inorganic and organic chemistry.
Answer:
| Inorganic Chemistry | Organic Chemistry |
|---|---|
| Studies elements and compounds with little or no carbon | Studies carbon compounds (hydrocarbons and derivatives) |
| Includes metals, nonmetals, salts, acids, bases | Includes compounds found in living organisms |
| Focuses on synthesis, properties, structure of inorganic compounds | Focuses on structure, formation, properties, reactions of organic compounds |
| Applications: fertilizers, medicines, catalysts, pigments | Applications: pharmaceuticals, plastics, fuels, dyes |
5. Investigative Questions
i. Preparation of solutions leads to an important process in chemistry which enables us to purify a compound through crystallization. Describe a process in which potassium nitrate is purified by crystallizing it in water.
Answer: The process of purifying potassium nitrate through crystallization:
- Prepare a saturated solution of impure potassium nitrate in hot water
- Filter the hot solution to remove insoluble impurities
- Allow the solution to cool slowly
- As the solution cools, the solubility of KNO₃ decreases, causing pure crystals to form
- Collect the crystals by filtration
- Wash the crystals with a small amount of cold water to remove surface impurities
- Dry the crystals to obtain pure potassium nitrate