what happens if you boil the water closed flask ? Does the mass remain constant
Answers
Answer:
yes
Explanation:
because there is no outlet of anything but it increases pressure
Answer:
for many students the idea that matter is conserved is not a natural one. They observe that sugar disappears when mixed with water, a large log burns away to a small amount of ash, cars rust and big holes appear, water boils away, frost and condensation appear from apparently nowhere and trees grow apparently from nothing but the soil.
It may seem to students that matter disappears or appears during processes such as dissolving, burning, evaporation, boiling, rotting, respiration, rusting, condensation and growth of plants. Invisible gases are involved in many of these processes leading to many of these alternative student conceptions.
Research: Driver (1985), Russell, Harlen & Watt (1989)
Students also often believe that matter is exchanged or converted into energy (for example, they believe that wood turns into heat during combustion and food turns into energy when we metabolise it) or they confuse the energy of food (listed on packets as kilojoules) with the weights of the listed ingredients. Students also often believe that the sun’s energy is changed into plant matter during the process of photosynthesis.
The process of evaporation may also challenge students’ conceptions as many students believe that substances become lighter if they change into the gas state. If a liquid evaporates inside a sealed container then they believe that the combined weight of the container and the liquid will be reduced by the weight of the liquid because it has apparently disappeared.
Research: Stavy (1990)
Although at this level the majority of students will have an understanding of the particle nature of atoms, for many the numbers of atoms are not conserved during chemical reactions. For example, the number of atoms appears to grow in the bark of trees, and their numbers drop during processes such as combustion or decay and increase during photosynthesis.
Scientific view
The idea of indivisible atoms helps to explain the conservation of matter. If the number of atoms stays the same no matter how they are rearranged, then their total weight stays the same.
In all physical and chemical changes, the total number of atoms remains the same, hence when substances interact with one another, combine or break apart, the total weight of the system remains the same.
Growing plants obtain their new carbon (the great majority of their dried weight) from carbon dioxide i.e. from the air. When we lose weight by dieting or exercise, most of the loss is from breathing out the carbon atoms metabolised from our fat as carbon dioxide. When a liquid evaporates in a sealed container, the weight stays the same; the gas particles are affected by gravity in the same way as tennis balls and they consequently hit the bottom surface of the container with more force than they hit the top.
Albert Einstein’s prediction that mass can be converted into energy has been experimentally validated by numerous nuclear experiments. A consequence of this is that the statement, “the total weight of a system remains the same” is more correctly only a very good approximation for all non nuclear changes.
Critical teaching ideas
In physical and chemical changes:
particles just don't disappear or get created, rather their arrangements change
in any change involving matter, all of the matter must be accounted for. Matter does not turn into or appear from energy
particles are rearranged to create substances different from the original
there is no change in weight when substances move in and out of the gas state.
Explore the relationships between ideas about conservation of mass in the Concept Development Maps - (Conservation of Matter, States of Matter, Flow of Matter in Ecosystems)
In your teaching of conservation of matter and hence weight, students need to be encouraged to change their views from those based on their everyday experiences to more scientific views such as the idea that there are only a fixed number of particles in the world and these building blocks are continually being rearranged into new things.