General Science for UPSC Prelims — Physics, Chemistry, Biology Made Simple

Q: A student measures a liquid and finds its pH is 3. What does this tell us?

The pH scale runs from 0 to 14. A value of 7 is neutral, above 7 is basic, and below 7 is acidic. Since 3 is well below 7, the liquid is acidic, similar to lemon juice or vinegar.

Learn the core everyday-science ideas UPSC Prelims tests most: light, sound, heat, atoms, acids and bases, vitamins, and the human body.

By the PadhoDost Team · 📖 8 min read · Updated 4 August 2026

Part of UPSC Civil Services prep

UPSC does not ask you to solve hard physics numericals. It asks whether you understand how the everyday world works. Why does a straw look bent in water? Why do we get vitamin D from the sun? What gas makes soda fizzy? This chapter builds the small, solid base of Physics, Chemistry, and Biology that Prelims tests again and again.

🧠 Science is a toolbox, not a textbook

Think of General Science like the tools in a home. You do not need to know how a screwdriver is made. You just need to know which tool fixes which problem. UPSC works the same way: it tests not deep theory, but knowing which simple idea explains a real event. Learn the tool, not the whole factory.

Physics: light, sound, and heat

Physics in Prelims stays close to daily life. Light travels in straight lines and bends (refracts) when it moves from air into water, which is why a straw looks broken in a glass. Sound needs a medium such as air, water, or a solid to travel, so it cannot pass through the vacuum of space. Heat moves in three ways: conduction through solids, convection through liquids and gases, and radiation through empty space, like the Sun's heat reaching Earth.

Physics facts worth memorising

✓Light travels far faster than sound. That is why you see lightning before you hear thunder.
✓A convex lens converges light and is used in a magnifying glass; a concave lens spreads light out.
✓Red light scatters the least and passes through fog well, so it is used for danger and stop signals.
✓The sky looks blue because air scatters shorter blue wavelengths more than red (scattering of light).
✓Sound cannot travel in a vacuum, but light can.

Chemistry: atoms, acids, and bases

Everything around you is made of atoms, and atoms join to form molecules. Substances are grouped as acids, bases, or salts. An acid tastes sour (like lemon or vinegar) and a base tastes bitter and feels soapy. We measure how acidic or basic something is on the pH scale, which runs from 0 to 14. Below 7 is acidic, exactly 7 is neutral (like pure water), and above 7 is basic (also called alkaline).

Substance	Nature	Everyday example
Lemon juice, vinegar	Acidic (pH below 7)	Sour taste, used in food
Pure water	Neutral (pH 7)	Neither sour nor bitter
Baking soda, soap	Basic / alkaline (pH above 7)	Bitter, slippery feel
Common salt (NaCl)	Neutral salt	Made from acid + base reaction

💡 An easy memory hook: Acids are Angry and sour; Bases are Bitter and soapy. When an acid and a base react, they neutralise each other and form a salt plus water.

Biology: the body and nutrition

Biology questions often focus on the human body and food. Your body needs vitamins in small amounts to stay healthy, and a shortage of any one causes a specific disease. Blood carries oxygen using haemoglobin, which needs iron. A lack of iron causes anaemia, leaving a person tired and weak.

Vitamin	Main source	Deficiency disease
Vitamin A	Carrots, green vegetables	Night blindness
Vitamin C	Citrus fruits, amla	Scurvy (bleeding gums)
Vitamin D	Sunlight, milk	Rickets in children
Vitamin B1	Whole grains, pulses	Beriberi

How your body handles the air you breathe

1You breathe in air, which is about 78% nitrogen and 21% oxygen.
2In the lungs, oxygen passes into the blood and binds to haemoglobin.
3Blood carries oxygen to every cell, where it helps release energy from food.
4Cells produce carbon dioxide as waste, which the blood carries back to the lungs.
5You breathe out the carbon dioxide, and the cycle repeats.

📝 Applying the ideas to a real question

Question idea: Why does a person feel breathless at high mountains?

At high altitude, the air is thinner, so there is less oxygen in each breath.

The blood carries less oxygen to the cells, causing tiredness and breathlessness.

This is why climbers often carry oxygen cylinders on very high peaks.

Must-remember science bullets

✓The pH scale runs 0 to 14: below 7 acidic, 7 neutral, above 7 basic.
✓Iron deficiency causes anaemia; vitamin C deficiency causes scurvy.
✓Photosynthesis in plants takes in carbon dioxide and releases oxygen.
✓Water is made of hydrogen and oxygen (H2O); table salt is sodium chloride (NaCl).
✓Sound needs a medium to travel; light does not.

ℹ️ For Prelims, focus on cause-and-effect: which deficiency causes which disease, which gas does what, and why a common event happens. You rarely need exact numbers beyond a few durable ones like pH 7 for neutral water.

⚡ Quick check

A student measures a liquid and finds its pH is 3. What does this tell us?

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