Why water dissolves so many things
Grade 8 Science Worksheet
Circle the best answer, or write your answer on the line.
A teacher evaporated the water. Ari says salt melts in water. Does the table support that?
- No: the salt was destroyed in the water, so the recovered salt must be new salt
- No: salt sinks in water because it is heavier, and sinking solids cannot melt or dissolve
- Yes: the salt turned clear in the water, which is what happens when a solid melts
- No: the salt came back unchanged, so it dissolved; melting salt would take 801 °C
Read this first
Sugar and sand in water (example data)
mixture after stirring after 1 hour does the solid pass the filter paper? sugar in water clear clear passes through sand in water cloudy sand on the bottom stays on the paper What does the table show about dissolved sugar particles, Bo?
- They have been destroyed, since the sugar is gone when it dissolves, and that is why the paper catches no sugar
- They are too small to see, settle out or catch in a filter, unlike sand grains
- They have melted into a liquid in the water, so the filter paper cannot catch the sugar
- Sugar particles are large, but water dissolves any solid, so they pass through the paper too
A teacher warms the oil. Which statement does the table support, Esme?
- Oil dissolves salt and sugar more slowly than water does, so the liquids work the same way
- Water dissolves salt and sugar but not wax, so different liquids dissolve different solids
- Oil dissolves wax since oil floats on water, and floating liquids dissolve solids
- Water dissolves salt and sugar, so water dissolves any solid, and wax is just slow
Sugar dissolving (example data)
setup water temperature (°C) stirring seconds for 10 g to dissolve A 20 none 150 B 50 hard 20 Jun wants to test only the effect of temperature. Which change fixes the table?
- Time setup A and setup B once more each, keeping no stirring in A and hard stirring in B
- Remove setup A, since a test with no stirring cannot be compared with any other test
- Rerun setup A with hard stirring, so only the temperature differs between the setups
- Rerun setup B with gentle stirring at 80 °C, so both settings differ from the first run
Kira wants to know how much sugar dissolves at 20 °C and at 50 °C, not how fast. Which plan is best?
- Stir 10 g of sugar into 100 g of water at each temperature and record whether it dissolved or not
- Stir sugar into 100 g of water at each temperature until some stays undissolved, and record the grams added
- Time how long 10 g of sugar takes to dissolve at each temperature, since faster dissolving means more dissolves
- Stir sugar into 100 g of water at 20 °C and 200 g of water at 50 °C, and record the grams added
Potassium nitrate in 100 g of water (approximate reference data)
water temperature (°C) grams that dissolve 0 13 20 32 40 64 60 110 A teacher handles potassium nitrate.
A teacher already dissolved 60 g of potassium nitrate in 300 g of water at 20 °C. How many more grams can dissolve for Milo?
Noah's teacher already dissolved 40 g of potassium nitrate in 50 g of water at 60 °C. How many more grams can dissolve?
Solubility in 100 g of water (approximate reference data)
water temperature (°C) potassium nitrate (g) table salt (g) sugar (g) 0 13 36 179 20 32 36 204 40 64 36 238 60 110 37 287 A teacher handles potassium nitrate.
A teacher cools 100 g of water, saturated at 60 °C, to 0 °C. Which solid forms the most crystals, Ren?
- Sugar, since its solubility falls by 108 g and potassium nitrate's falls by 97 g
- Potassium nitrate, since its solubility changes the most compared with its starting amount
- Table salt, since a saturated salt solution at 60 °C holds the least and crystallizes first
- The three form the same amount, since each solution is saturated at the same 60 °C
Tala needs to dissolve 50 g of a solid in 100 g of water at 20 °C. Which solids can do it?
- Potassium nitrate can, since it dissolves more as the water warms, which makes up the difference
- Sugar can, since 204 g dissolves in 100 g at 20 °C; the other two hold 32 g and 36 g
- Sugar and table salt can, since both are kitchen solids that dissolve easily in water
- Table salt can, since it dissolves the same amount at each temperature, which is enough
Zee says warming water greatly increases how much of any solid dissolves. Which solid is the best counterexample?
- Table salt: about 1 g more dissolves between 0 °C and 60 °C, so warming barely helps
- Potassium nitrate: it rises from 13 g to 110 g, which shows the claim has exceptions
- No solid is a counterexample, since each column goes up at least a little with temperature
- Sugar: 287 g dissolves at 60 °C, far more than at 0 °C, which goes against the claim