Rates of Reaction

Increasing the Concentration (in solution).

Increasing the concentration of a substance in solution
means that there will be more particles per dm³
of the substance.

The more particles that there are in the same volume,
the closer to each other the particles will be.
This means that the particles collide more frequently
with each other and the rate of the reaction increases.

In the reaction between sodium thiosulfate solution
and dilute hydrochloric acid

HCl + sodium thiosulfate sodium chloride + sulfur dioxide + sulfur + water.
HCl_(aq)   +  Na₂S₂O_3(aq)            NaCl_(aq)      +    SO_2(g)      +      S_(s) + H₂O_(l)

Solid sulfur (S_(s)) is formed in the flask.
Increasing the concentration of sodium thiosulfate means
that the solid sulfur will be produced more quickly and
there will be less time before the cross can no longer be seen.

When making different concentrations of sodium thiosulfate
 a fair test must use the same total volume
of sodium thiosulfate plus hydrochloric acid.

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