Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 3% of your exam marks.
increasing
Rare
Increasing3%
Blockchain and cryptocurrency are a newer addition; questions are growing as the topic becomes more established.
A blockchain is a digital ledger of every transaction in a currency, organised as a chain of blocks linked by cryptographic hashes, with copies stored across many computers in a decentralised network.
Three big ideas in that definition:
- A ledger: a record of who owns what and who sent what to whom.
- A chain of blocks: transactions are grouped into "blocks" and each block links to the previous one through a hash, so the whole history is connected in order.
- Distributed across many computers: no single computer holds "the" blockchain; many computers (nodes) keep copies and agree on the same version.
What is inside a block?
Each block typically contains four things:
| Field | What it holds |
|---|---|
| Block number | The position of this block in the chain (the first block is block 0, the genesis block) |
| Transactions | A list of new transactions being recorded in this block (who sent how much to whom, when) |
| Timestamp | The exact time the block was added |
| Hash of the previous block | The cryptographic fingerprint of the block before this one, linking the chain backwards in time |
| This block's own hash |
What a hash is
A hash is the fixed-length cryptographic fingerprint of some data, produced by a one-way function. Even a tiny change in the data produces a completely different hash.
Two important properties of a hash:
- Deterministic: the same input always produces the same hash.
- Sensitive to change: changing even one character in the input produces an utterly different output. This is what makes hashes useful for spotting tampering.
How blocks form a chain
Each block stores the of the previous block in its own header. This is the "link" in the chain:
Block 0 Block 1 Block 2 Block 3
[ hash: H0 ] [ hash: H1 ] [ hash: H2 ] [ hash: H3 ]
[ prev: H0 ] [ prev: H1 ] [ prev: H2 ]
[ data ] [ data ] [ data ] [ data ]
The first block (block 0) is special and is called the genesis block; it has no previous block, so its prev field is zero or empty.
When a new block is added, the network calculates the hash of the latest block and stores it as the new block's prev. The chain grows one block at a time.
Exam tip
Describing how blockchain records transactions
What comes up: "Describe the process of blockchain in digital currency" or fill-in-the-blank sentences about how a blockchain stores transactions (2 marks).
Write (two marks): (1) A blockchain acts as a digital ledger that records each transaction with a time stamp, linking each new record to the one before it. (2) Because records are only ever added (never altered), the full history of every transaction can be traced and verified.
Watch out: The mark scheme credits the idea that transactions "cannot be altered, only new transactions added." Avoid saying the blockchain "stores" or "deletes" old data — always frame it as an append-only chain of time-stamped records.