CBSE Class 7 Science Notes: Your Ultimate Revision Guide

Key Points to Remember

• Photosynthesis: Plants prepare their food using carbon dioxide, water, sunlight, and chlorophyll in a process called photosynthesis, producing glucose and oxygen.
• Heat Transfer: Heat can be transferred by Conduction (solids), Convection (liquids and gases), and Radiation (no medium required).
• Acids, Bases, and Salts: Acids are sour and turn blue litmus red. Bases are bitter, soapy, and turn red litmus blue. Salts are formed by the neutralization reaction between an acid and a base.
• Electric Current Effects: An electric current can produce heating, magnetic, and chemical effects. The heating effect is utilized in appliances like heaters and geysers.
• Physical vs. Chemical Changes: Physical changes are reversible and don't form new substances (e.g., melting ice). Chemical changes are irreversible and form new substances (e.g., burning wood).
• Respiration: All living organisms respire to release energy from food. Aerobic respiration requires oxygen, while anaerobic respiration occurs without oxygen.
• Motion: Objects can undergo various types of motion: rectilinear (straight line), circular (circular path), periodic (repeats at regular intervals), and rotational (about an axis).
• Reflection of Light: Light travels in straight lines and bounces off surfaces. A plane mirror forms a virtual, erect, and laterally inverted image of the same size as the object.

Essential Science Terminology

Autotrophs

Organisms that prepare their own food, such as green plants.

Heterotrophs

Organisms that depend on other organisms for their food.

Conductor

Materials that allow heat or electricity to pass through them easily.

Insulator

Materials that do not allow heat or electricity to pass through them easily.

Neutralization

A chemical reaction between an acid and a base, producing salt and water, often with the release of heat.

Indicator

A substance used to test whether a solution is acidic or basic by changing its color.

Speed

The distance covered by an object in a unit of time (distance/time).

Magnetic Effect of Electric Current

The phenomenon where an electric current flowing through a wire produces a magnetic field around it.

Understanding Photosynthesis: The Basis of Life

Photosynthesis is the fundamental process by which green plants and some other organisms convert light energy into chemical energy, in the form of sugars. This process is crucial for life on Earth as it produces the food consumed by almost all living beings and releases oxygen into the atmosphere. The key components required for photosynthesis are carbon dioxide, which is absorbed from the atmosphere through tiny pores called stomata on the leaves; water, absorbed from the soil by the roots; and sunlight, captured by a green pigment called chlorophyll, present in specialized organelles called chloroplasts. The overall chemical reaction can be summarized as:

Carbon dioxide + Water + Sunlight (Chlorophyll) → Glucose (food) + Oxygen

The glucose produced is used by the plant for its growth and other metabolic activities, or stored as starch. The oxygen is released as a byproduct into the atmosphere. Without photosynthesis, the carbon dioxide levels would increase unsustainably, and oxygen, essential for respiration, would deplete, making life impossible for most organisms.

Physical vs. Chemical Changes

Mini-Examples for Quick Grasp

• Calculating Speed Q: A car travels a distance of 180 km in 3 hours. What is its speed? A: Speed = Distance / Time = 180 km / 3 h = 60 km/h.
• Neutralization Reaction Q: What happens when you mix dilute hydrochloric acid and sodium hydroxide solution? A: They undergo neutralization to form sodium chloride (salt) and water. HCl (acid) + NaOH (base) → NaCl (salt) + H₂O (water).
• Magnetic Effect Q: If you bring a compass near a wire carrying electric current, what do you observe? A: The compass needle deflects, indicating that the electric current produces a magnetic field around the wire.

Exam Ace Tip: Visualise & Define

For Class 7 Science, definitions are key! Examiners often award marks for precise definitions of terms like photosynthesis, conduction, acids, or speed. Don't just memorise; understand the concept behind each term. Practice drawing simple diagrams (e.g., electric circuit, plant cell, heat transfer) to solidify your understanding. Label diagrams clearly. When describing processes, use sequential bullet points or numbered steps. Pay attention to distinguishing characteristics in comparisons (like physical vs. chemical changes) as these are common question types. Always revise using past year's questions to identify frequently asked topics and common pitfalls.

Quick Check: Test Your Knowledge

• Q: Name the process by which green plants make their own food. A: Photosynthesis.
• Q: Give an example of a physical change and a chemical change. A: Physical Change: Melting of ice. Chemical Change: Burning of paper.
• Q: What is the main function of an indicator in chemistry? A: To determine if a solution is acidic or basic by changing its color.
• Q: How is heat mainly transferred in liquids and gases? A: Through Convection.

Frequently Asked Questions