Trace how a blockchain tracks transactions
Explain blockchain as a time-stamped digital ledger and trace how a transaction becomes part of its record.
Section 5 · The internet and its uses
This sequence follows Cambridge IGCSE Computer Science 0478, 2026–2028 syllabus Version 6. It covers subject content sections 5.1–5.3 only. The separately published 2029–2031 syllabus is not mapped here. Practice is original, self-checked and does not award an official Cambridge grade. Full timed papers and human subject-teacher review remain outstanding.
Before you begin
- No programming language is required. Be comfortable with place value and multiplication; each lesson introduces the computer-science notation it uses.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What does a blockchain record?
What you will learn
- Define blockchain as a digital ledger
- Describe time-stamped transaction records
- Explain why later changes are detectable and difficult to make
A blockchain is a linked transaction ledger
In its basic form, a blockchain is a digital ledger: a time-stamped series of transaction records that is designed not to be altered. A transaction is submitted, checked according to the system's rules and added to a record shared by the network.
Apply and check the rule
New records are linked to earlier records, commonly using a cryptographic hash. Changing an old entry would break the later links and conflict with copies or checks held by the network. This makes alteration detectable and difficult; it is safer to describe the syllabus model as immutable than to promise that every system is impossible to attack.
Pause and explain
Why are later changes to a linked ledger detectable?
Worked example
A transaction is added to a blockchain. Why can the ledger help track the transfer?
Show the worked solution
- The transaction is recorded with its time and details.
- The record is linked into the ledger's sequence.
- Later users can inspect the sequence to trace recorded transfers.
Answer The time-stamped transaction becomes part of the linked ledger, so recorded transfers can be followed in sequence.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Put the simplified stages in order: record is linked into ledger, transaction submitted, system checks transaction.
Give me a hint
A record is not added before the system processes the submission.
Compare my reasoning
- The user submits the transaction.
- The system checks it under its rules.
- A valid transaction is recorded and linked into the ledger.
Submit → check → record and link.
Look for these in your work
- Submission first
- Check second
- Record last
Explain how time-stamped linked records help someone trace a series of payments.
Give me a hint
Consider ordering and references between records.
Compare my reasoning
- Each record carries a place in the time sequence.
- Records link to earlier records.
- Following the sequence shows the recorded transfer history.
Timestamps order the records, while their links connect transactions into a sequence that can be followed.
Look for these in your work
- Time order
- Linked sequence
- Tracking outcome
A learner says a blockchain record can be silently changed without affecting anything else. Explain why that conflicts with the ledger design and describe the accurate limitation.
Give me a hint
Use the links between records, then avoid an absolute security claim.
Compare my reasoning
- Later records rely on links or checks connected to earlier records.
- Changing old data can break those links or disagree with other ledger copies.
- The design makes unnoticed changes difficult, but security still depends on the actual system.
An edit can disrupt subsequent links and conflict with network-held copies, making changes detectable and difficult. This design does not justify an absolute claim that every implementation is impossible to attack.
Look for these in your work
- Link dependency
- Detection reason
- Qualified conclusion
Common mistakes
- Calling blockchain a currency instead of a ledger process
- Claiming no blockchain could ever be changed under any circumstances
What can you explain now?
What is a blockchain in its basic form?
Compare with the explanation
A time-stamped digital ledger of records designed not to be altered
Linked records allow transactions to be tracked over time.
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Make it stick
What is a blockchain ledger, and how does it help track digital-currency transactions?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.