1.1 What is Chemistry?
Chemistry is the branch of science that deals with:
- The properties, composition, and structure of substances
- Physical and chemical changes in matter
- The laws or principles governing these changes
Composition refers to finding percentages of elements and compounds in matter.
Structure means the arrangement of atoms in matter.
Changes may involve absorption or evolution of energy.
Branches of Chemistry
Physical Chemistry
Studies behavior of substances at atomic and molecular levels.
Inorganic Chemistry
Studies elements and compounds with little or no carbon.
Organic Chemistry
Studies carbon compounds (except simple salts).
Environmental Chemistry
Studies chemical phenomena in the environment.
Analytical Chemistry
Deals with analysis and composition of substances.
Biochemistry
Studies chemical processes in living organisms.
Nuclear Chemistry
Studies reactions in atomic nuclei.
Polymer Chemistry
Studies properties and synthesis of polymers.
Geochemistry
Studies chemical composition of Earth and minerals.
Medicinal Chemistry
Designs and synthesizes medicines and drugs.
Astrochemistry
Studies molecules and ions in space.
1.3 Element, Compound and Mixture
E Element
- Simplest form of matter
- Contains same kind of atoms
- Cannot be broken down by ordinary chemical reactions
- Exists as solid, liquid, or gas
- Examples: Sodium (Na), Oxygen (O₂), Gold (Au)
C Compound
- Pure substance made of two or more elements in fixed ratio
- Properties differ from constituent elements
- Can be broken down by chemical reactions
- Examples: Water (H₂O), Carbon dioxide (CO₂), Sodium chloride (NaCl)
M Mixture
- Impure substance with elements/compounds in any ratio
- Components retain their properties
- Can be separated by physical methods
- Can be homogeneous or heterogeneous
- Examples: Air, Soil, Milk, Tap water
1.5 Differences between Elements, Compounds and Mixtures
| Property | Element | Compound | Mixture |
|---|---|---|---|
| Composition | Same kind of atoms | Different elements in fixed ratio | Elements/compounds in any ratio |
| Separation | Cannot be broken down | Can be broken by chemical methods | Can be separated by physical methods |
| Properties | Definite properties | Different from constituent elements | Sum of properties of components |
| Examples | Oxygen, Gold | Water, Salt | Air, Soil |
1.6 Solution, Colloidal Solution and Suspension
S Solution (True Solution)
- Solute particles completely homogenized in solvent
- Particles cannot be seen with naked eye
- Pass through filter paper
- Example: Salt in water
C Colloidal Solution
- Particles larger than in true solution but smaller than in suspension
- Particles not visible to naked eye
- Do not settle down
- Pass through filter paper
- Example: Milk, Starch solution
S Suspension
- Solute particles do not dissolve
- Particles visible to naked eye
- Particles settle down when left undisturbed
- Do not pass through filter paper
- Example: Chalk in water
1.7 Formation of Unsaturated and Saturated Solutions
U Unsaturated Solution
- Can dissolve more solute at a particular temperature
S Saturated Solution
- Contains maximum amount of solute that can dissolve at a particular temperature
- Additional solute will not dissolve
Solubility: Different substances have different solubilities in the same solvent.
At 20°C:
- 36g NaCl dissolves in 100g water
- 203.9g sugar dissolves in 100g water
Exercise Questions
Multiple Choice Questions:
(ii) Hazardous effects of shopping bags are studied in:
Answer: (d) Environmental Chemistry
(iii) The man-made polymer is:
Answer: (b) Polystyrene
(v) Which liquid among the following is a colloidal solution?
Answer: (a) Milk
(vi) Which of the following is a heterogeneous mixture?
Answer: (d) Concrete mixture
(viii) When the tiny visible particles of a substance are dispersed through a medium, the mixture is named as:
Answer: (c) Suspension
Short Answer Questions:
i. Why is there a need to divide Chemistry into many branches? Give three reasons.
Answer:
- To understand the widely spread complex subject of chemistry
- To concentrate on specific aspects of chemistry
- To allow scientists to focus on specific areas and achieve breakthroughs
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 nucleus: Physical Chemistry, Organic Chemistry, Inorganic Chemistry
- Reactions inside nucleus: Nuclear Chemistry
vi. In which state does matter exist in the Sun?
Answer: Plasma state
viii. Which form of matter do most of the material things in this world belong to?
Answer: Mixtures
Constructed Response Questions:
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: Organic chemistry studies all carbon compounds (except simple salts), while biochemistry specifically studies chemical processes and substances in living organisms.
iv. Give the reason of brilliance shown by diamond. Can you improve it?
Answer: Diamond’s brilliance is due to its high refractive index and the way it reflects and refracts light. It can be improved through precise cutting and polishing to maximize light reflection.
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
vi. Differentiate between the areas which are studied under inorganic and organic chemistry.
Answer:
- Inorganic Chemistry: Studies elements and compounds with little or no carbon (metals, salts, acids, bases)
- Organic Chemistry: Studies carbon compounds (hydrocarbons and their derivatives) except simple salts like carbonates
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:
- Dissolve impure potassium nitrate in hot water to make a saturated solution
- Filter the hot solution to remove insoluble impurities
- Allow the solution to cool slowly
- As the solution cools, potassium nitrate crystals form because its solubility decreases with temperature
- Filter the crystals from the solution
- Wash the crystals with a small amount of cold water to remove surface impurities
- Dry the crystals to obtain pure potassium nitrate