CBSE Class 10 Science free practice questions with answers
Read question 1 and its full worked answer from paper 1 of our 20-paper CBSE Class 10 Science set, then download the 4-page PDF. These are original practice questions, not official past papers. Page updated 3 November 2026.
During photosynthesis, the oxygen that is released comes from:
A chlorophyll
B water
C carbon dioxide
D glucose
Correct answer: B — water
- Light energy splits water molecules (photolysis) in the chloroplast: 2H₂O → 4H + O₂.
- The hydrogen reduces CO₂ to carbohydrate; the oxygen is released.
- So the oxygen released comes from water.
- Why the other options are wrong:
- A (chlorophyll): Chlorophyll absorbs light; it is not split to release oxygen.
- C (carbon dioxide): Carbon dioxide supplies the carbon that ends up in glucose; the oxygen released comes from splitting water.
- D (glucose): Glucose is a product that stores the carbon, not the source of oxygen.
Photolysis of water in the light reaction releases oxygen; CO₂ is the source of carbon.
Your sample is downloading. Liked it? CBSE Class 10 Science Paper 1 is the full paper: 39 questions, 80 marks, 3 hours in Biology, Chemistry and Physics sections with internal choice, written to the CBSE 2026-27 Class X Science (086) syllabus and sample paper, with a worked solution for every question.
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Questions about this sample
What is in the CBSE Class 10 Science free sample?
The first 3 questions from paper 1 of the set, in the real exam layout, followed by the step-by-step worked answers. It is a 4-page PDF; this page shows question 1 with its answer.
How many full CBSE Class 10 Science papers are there?
20 original papers for Class 10 students in India preparing for the CBSE board examination in Science (086). Each is 39 questions, 80 marks, 3 hours in Biology, Chemistry and Physics sections with internal choice, written to the CBSE 2026-27 Class X Science (086) syllabus and sample paper, with a worked solution for every question. Papers start at A$11.99 and download instantly as a PDF.
Are these official past papers?
No. The sample is taken from our own original practice paper and is not an official past paper or affiliated with any exam authority.