To read a process diagram, follow the arrows and ask four questions: what goes in, what happens, what comes out, and does anything loop back. The answers tell the story of the factory in four lines.
This lesson is part of industrial chemistry. It pairs with relating manufacturing choices to material properties, because every step in a diagram exists for a reason linked to a property.
What are the four questions?
- Inputs: which materials and energy enter the process?
- Steps: what happens in each box, in order?
- Outputs: what leaves, including the product and any waste or gas?
- Loops: does any arrow go backwards, and what does it return?
Use them on any diagram, including one you have never seen. The diagram is new, but the four questions are not.
Worked example: making glass bottles
An original diagram shows this flow.
Raw materials (sand, soda ash, limestone) → Furnace (heated until molten) → Moulding machine → Cooling oven → Inspection → Finished bottles
An arrow from Inspection returns rejected bottles to a crusher, and the crushed glass goes back to the Furnace.
Inputs: sand, soda ash and limestone, plus heat for the furnace.
Steps, in order: melting, moulding, cooling, inspection.
Outputs: finished bottles and, in this model, gas released while the raw materials melt.
Loop: rejected bottles are crushed and returned to the furnace. The loop saves raw materials because glass can be melted again.
A simple yield calculation
The same diagram gives these numbers for one batch. The furnace receives 800 kg of raw materials and 200 kg of crushed glass. It produces 950 kg of finished glass, and 50 kg leaves as gas.
Step 1: find the total input. 800 + 200 = 1 000 kg.
Step 2: check the outputs. 950 + 50 = 1 000 kg, so the masses balance.
Step 3: calculate yield. 950 ÷ 1 000 × 100 = 95%.
Step 4: state the assumption. The figure counts finished glass as the useful product, and it assumes that no glass is left behind in the furnace.
The mistake that costs marks
Two misreadings cost marks. The first is reading an arrow backwards, so the loop looks like a product leaving. The second is listing the recycled glass as a new raw material.
| Wrong | Right |
|---|---|
| “Crushed glass is a raw material” | “Crushed glass is a recycled input that loops back” |
| “Bottles go into the furnace” | “Rejected bottles go to the crusher, then to the furnace” |
| “Gas is not an output” | “Gas is a waste output and must be counted in the mass balance” |
The repair is to trace each arrow with a finger from its tail to its head and say the material aloud.
Check yourself
A plastic recycling plant follows this flow: Collect → Sort by type → Wash → Shred → Melt → Pellets. An arrow from Sort sends non-recyclable plastic to Landfill. The plant receives 500 kg, sends 60 kg to landfill and loses 40 kg of water-soaked dirt at the Wash step. Which step separates the different plastics, what is the final output, and what is the yield of pellets if 400 kg of pellets are made?
Answer
The Sort step separates the plastics by type. The output is pellets.
Check the balance: 60 + 40 + 400 = 500 kg, so every kilogram is accounted for.
Yield = 400 ÷ 500 × 100 = 80%. The assumption is that the pellets are the only useful product.
What to study next
Test the whole cluster with the industrial chemistry practice set. Keep a note of repeated misreadings in the mistake log and paper-error review.
For help with diagram questions, see online one-to-one Science tuition.