Chemistry – Experimental techniques and chemical analysis - Separation and purification | e-Consult

(a) The white powder is likely to be Sodium Chloride (NaCl).

Justification: The melting point range of 135-140 °C is relatively low compared to the melting points of the other substances listed. The melting point of NaCl is 801 °C, which is significantly lower than the melting points of CaCO₃, MgO, and Na₂SO₄. This suggests that the substance is a salt with a lower melting point. The provided melting point range is closer to the melting point of NaCl than to the melting points of the other substances.

(b) The melting point of a pure substance is considered a physical property because:

• It is a characteristic of the substance: The melting point is a specific value that is unique to a particular substance. It does not change the identity of the substance.
• It can be observed or measured without changing the chemical composition: Determining the melting point involves observing the change in state (solid to liquid) and measuring the temperature at which this occurs. This can be done without altering the chemical makeup of the substance.
• It is independent of the amount of the substance: The melting point of a pure substance is the same regardless of whether you have a small sample or a large sample.

Physical properties are those that can be observed or measured without changing the chemical composition of a substance. The melting point fits this definition perfectly.