Concentration of solution → refers to the amount of solute dissolved in a unit volume of solution. Mole Concept and Stoichiometry Main Note
As concentration increases → the number of reacting particles per unit volume also increases. Collide more frequently → frequency of effective collisions increases.
Sample
Rate of Reaction increases with an increase in concentration:
- there are more reacting particles per unit volume, the particles are closer together
- resulting in more collisions between the reacting particles per unit time.
- thus frequency of effective collisions increases
- hence rate of reaction increases.
Sample
Rate of Reaction decreases with decrease in concentration
- there are fewer reacting particles per unit volume, the particles are further apart from each other
- resulting in fewer collisions between the reacting particles per unit time.
- thus frequency of effective collisions decreases
- hence speed of reaction decreases
Real Sample: Explain why higher concentration of acid affects rate of reaction between acid and magnesium ribbon
- at higher concentration, there are more reacting particles of acid per unit volume, particles are closer together
- this results in more collision between reacting particles (of acid and Mg or H+ ions and Mg atoms) per unit time
- thus frequency of effective collisions increases and the rate of reaction also increases

Summary
increase in concentration of reactants increases rate of reaction
i didn’t add it but try to add per unit time in these answers as it makes it more accurate and greater chance of getting it correct.