Week 13 · Theme 2 Matter and Energy, Unit 2 Particles in Motion, Concept 2.3 Comparing Changes in Matter (C) · 3 lessons
1
Does mixing change the mass?
Discovery Education Science P5: Term 1, Unit 2, pp. 192–196
GoalPredict, measure and explain what happens to the total mass when substances are mixed.
New words
dissolve · to mix into a liquid so completely that the solid cannot be seentotal · all the parts added togetherpattern · something that happens again and again in the same way
Explanation
When we mix two things, does the total mass change? Scientists first predict, then measure. We weigh each substance on a balance or kitchen scale, add the masses to predict the total, mix inside a closed container, and weigh again. Safe kitchen materials work well: flour, sugar, salt, water, oil and lemon juice. For example, 30 g of flour and 20 g of sugar make 30 + 20 = 50 g of mixture. When salt dissolves in water it seems to disappear, but its particles are still there, mixed among the water particles, so the mass does not change. In a class, the results may differ by a gram or two because of small measuring mistakes. Look for the pattern in all the data: the mass after mixing is the same as the total before, as long as nothing escapes from the container.
Mixture
Masses before (g)
Total predicted (g)
Mass after (g)
flour + sugar
30 + 20
50
50
salt + water
15 + 100
115
115
oil + water
90 + 100
190
190
Worked examples
Nada has 25 g of flour and 10 g of salt. She predicts 25 + 10 = 35 g. After she shakes them in a closed jar she measures 35 g, so her prediction was right.
Group
Before (g)
After (g)
1
115
115
2
112
113
3
118
118
4
110
109
5
120
120
Five groups mix salt and water and weigh before and after. Their results differ by at most 1 g. The pattern is clear: the mass stays the same, and the small differences come from measuring.
salt 10 g + water 170 g→stir until it dissolves→salt water 180 g
Karim stirs 10 g of salt into 170 g of water. The salt seems to vanish, but the salt water still has a mass of 10 + 170 = 180 g. Nothing was lost, so the salt is still there.
Class activity
Mixing and weighing (a grown-up helps, a kitchen scale and a jar with a lid). Weigh 20 g of flour and 20 g of sugar in two cups. Write your prediction for the total. Pour both into the jar, close the lid and shake. Weigh the jar with the mixture and subtract the mass of the empty jar. Repeat with salt and water, stirring until the salt dissolves. Do not taste anything.
Practice
1
Mostafa mixes 40 g of flour with 25 g of sugar in a closed jar. What total mass do you predict?
40 + 25 = 65 g.
2
30 g of salt dissolves in 200 g of water. What is the mass of the salt water? Where is the salt?
30 + 200 = 230 g. The salt is still there, spread out among the water particles.
3
One group measures 85 g before mixing and 87 g after. What do you think happened?
Probably a small measuring mistake of 2 g. The pattern in the class data is that the mass stays the same.
