Wrong concept 考考你區

Q3   When dry cobalt(II) chloride paper is dipped into a liquid, it turns blue. What can you deduced?

✘ans: The liquid is water

What’s WRONG?

Dy cobalt(II) chloride paper turns blue only if the liquid contain water. That mean NaCl solution, NaOH solution also can turn the paper blue. So we only can deduce the solution contain water.

Q2   How to show the metal is copper?

✘ans: We can carry out flame test which give greenish flame

What’s WRONG?

Flame is used to test metal ion but not metal. To test copper, HNO₃ can be used which blown gas is given out and blue solution is observed.

Cu + NO₃^2- + H⁺  →  Cu²⁺ + NO₂ + H₂O

H₂SO₄ is also used which give colourless bubble and blue solution.

Cu + SO₄^2- + H⁺  →   Cu²⁺ + SO₂ + H₂O

Q1   What is the relative atomic mass of Mg?

✘ans: relative atomic mass of Mg = 24.3 g

What’s WRONG?

Relative atomic mass should have no unit because it is relative

        relative atomic mass of Mg = 24.3 g

        molar mass of Mg = 24.3 g mol^-1