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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 that 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 thiosulfatesodium 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
take care to use the same total volume of sodium thiosulfate
plus hydrochloric acid for the comparison to be fair.

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