Questions: Separating Mixtures: Filtering and Evaporating

Question 1

You have a mixture of sand and salt water. Which method would you use first to separate the sand from the salt water?

• A. Evaporate the water so both the sand and salt are left behind together
• B. Filter the mixture — the sand gets caught in the filter while the salt water passes through
• C. Freeze the mixture so the sand floats to the top
• D. Shake the mixture harder to separate it

Answer: (B)

Filtering is the best first step. The sand particles are large enough to get trapped in a filter, while the dissolved salt passes through with the water. Then you can evaporate the filtered salt water to recover the salt separately. This gives you clean sand AND clean salt.

Question 2

You can use a filter to separate salt from salt water.

• T. True
• F. False

Answer: False

False. Dissolved salt particles are far too tiny to be caught by a filter — they pass right through with the water. To separate salt from water, you need to evaporate the water. The water turns to gas and leaves, and the salt stays behind as solid crystals.

Question 3

How does evaporation help you recover salt from salt water?

Model answer: When salt water is heated or left in a warm place, the water evaporates (turns into gas and escapes into the air), but the salt cannot evaporate. The salt is left behind as solid crystals on the dish.

Evaporation separates substances based on which one turns to gas more easily. Water evaporates at normal temperatures, but salt does not — it would need extremely high temperatures to become a gas. So the water leaves and the salt stays.