The Hydrochloric Acid, Sodium Thiosulphate Reaction

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The Hydrochloric Acid, Sodium Thiosulphate Reaction


"Hydrochloric acid is a strong colourless acid formed when hydrogen

chloride gas dissociates in water, used in industrial and laboratory

processes. Hydrochloric acid is also produced in the stomach, where it

initialises the digestion of proteins.

Sodium thiosulphate is a white crystalline salt used as a fixer in

photography and as a bleach."

Encarta World English Dictionary

When hydrochloric acid reacts with sodium thiosulphate, sulphur is

created. Sulphur is a pale yellow colour; after the reaction the

sulphur stays in the same container as the other products, meaning

that the solution will be a pale yellow colour.



H++ Cl-









Sodium thiosulphate


Hydrochloric acid

Sodium chloride



Sulphur dioxide


"The sulphur does not start to form immediately, but begins to appear

after a short time, turning the mixture cloudy. You cannot tell

precisely when the reaction has stopped." AQA GCSE Higher

Sulphur dioxide (SO2) is a colourless gas, belonging to the family of

gases called sulphur oxides (SO_). It reacts on the surface of a

variety of airborne solid molecules, is soluble in water and can be

oxidised within airborne water droplets.

Many factors can affect the rate of reaction; concentration,

temperature, ratio of the substances, amount, stirring, and catalysts

are these factors. Rate of reaction is the speed of reaction.

In essays written by young scientists, they have used concentration as

a variable in their experiments; some have used a light sensor,

connected to a data logger, to measure the amount of light passing

through the "cloudy" liquid.

1 moles = 6.02 ´1023

The particles in the hydrochloric acid and sodium thiosulphate need to

collide to react, this is called the collision theory; a minimum force

of collision is required for the reaction to take place; this is

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