Rational Explanation Two preliminary investigations on the rate of reactions were experimented, one on the effect of concentration between Sodium Thiosulfate and Hydrochloric Acid and the other between Hydrochloric Acid and Calcium Carbonate. After the group tested each of the experiments and delivered results, it was decided that the reaction of Sodium Thiosulfate and Hydrochloric Acid was going to be investigated. The concentration for both these investigations was the independent variable and the dependant variable was time. These investigations both had equal level of difficulty to control. The similarity between the two investigations is that the reactions contained Hydrogen Peroxide, which is a skin and eye irritant. The differences between the practicals is that one used Sodium Thiosulfate which is a medium hazard as the high concentrations may cause eye, skin and respiratory irritation. However Sodium Thiosulfate was not considered a high standard hazard as only 0.25molL-1 was used whereas the reaction that contained Calcium Carbonate was considered a minor safety hazard, as it is a colourless crystalline solid that occurs naturally in the minerals such as marble/stone so if it comes into contact with the skin it would not result in irritation. When investigating the rate of reaction with Hydrochloric Acid and Sodium Thiosulfate, the endpoint was hard to determine due to human error when identifying the disappearance of the black cross. This can be improved by using an electronic light censor to identify the endpoint accurately. The endpoint was determined by the mass of Hydrochloric Acid changing by 0.2g when adding Calcium Carbonate. However when using Calcium Carbonate chips it was difficult to measure exact amounts of the chips, the surface area of the chips may have contributed to a random error, as the scales may have been sensitive to movement with the minor weight changes, which made the endpoint difficult to determin