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Evidence of chemical reactions Transitional Reasoning Practice

Chemistry · Matter · Chemistry · Properties & Changes of Matter

1 Chemistry practice questions on Evidence of chemical reactions. Questions that build towards applied reasoning.

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  1. 1.

    A student adds a clear, colorless aqueous solution of lead(II) nitrate to a clear, colorless aqueous solution of potassium iodide in an insulated flask at room temperature. Within seconds, a bright yellow solid forms and settles to the bottom, while a digital thermometer records a temperature increase from 21.5 °C to 27.0 °C without external heating. Based on these observations, which statement best explains whether a chemical reaction has occurred?

    • A) A physical change occurred because mixing two liquid solutions frequently causes suspended solute particles to settle out purely by mechanical separation.
    • B) A chemical reaction occurred because both the spontaneous formation of an insoluble precipitate and the unexpected temperature increase indicate the synthesis of new chemical substances.
    • C) A physical change occurred because the observed temperature rise simply reflects heat being absorbed into the mixture from the surrounding room environment.
    • D) A chemical reaction occurred solely because liquids always undergo chemical decomposition whenever their combined physical volume increases.

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