What Is Substitution in Science?

Substitution

Replacing a variable (letter) in an algebraic expression or scientific formula with a specific numerical value and then calculating the result.

In science, we constantly use formulas — mathematical rules that connect different quantities. Letters like $v$ , $m$ , $t$ , and $E$ represent physical quantities that can change. Substitution is the process of swapping those letters for actual measured values so we can calculate a result.

Think of a variable like a parking space — it holds a spot for a car (a number) that hasn't arrived yet. When the car pulls in, you can see exactly what's there.

Analogy

In a scientific investigation, you measure values for your independent and dependent variables. Substitution is how you plug those measured values into a formula to process your data and find a result — this is the heart of Criterion C: Processing and Evaluating.

Example

Evaluate $3 x + 5$ when $x = 4$

Write the expression: $3 x + 5$
Replace $x$ with $4$ : $3 (4) + 5$
Multiply first: $12 + 5$
Add: $= 17$

Exam Tip

When you substitute a number into an expression, it is good practice to put the number inside brackets. This helps avoid sign errors, especially with negative numbers.

Variables in Science vs. Algebra

Before we practise substitution, it helps to understand what variables mean in a scientific context.

In a science investigation, variables have specific roles:

Variable type	Meaning	Example
Independent	The one you change deliberately	Time elapsed, temperature set
Dependent	The one you measure as a result	Distance travelled, volume of gas
Controlled	Kept the same throughout	Same equipment, same starting conditions

When scientists write formulas, the letters stand for these measurable quantities — usually with SI units (the international standard).

Example

Common scientific formulas you will use this year:

Speed: $v = \frac{d}{t}$ — where $v$ = speed (m/s), $d$ = distance (m), $t$ = time (s)
Kinetic energy: $E_{k} = \frac{1}{2} m v^{2}$ — where $m$ = mass (kg), $v$ = speed (m/s), $E_{k}$ = energy (J)
Area of a rectangle: $A = l w$ — where $l$ = length (m), $w$ = width (m)
Density: $ρ = \frac{m}{V}$ — where $m$ = mass (g), $V$ = volume (cm³), $ρ$ = density (g/cm³)

Note

Always include units in your final answer when substituting into a scientific formula. A speed without units (m/s or km/h) is incomplete and would lose marks in an MYP Sciences assessment!

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