You are provided with an experimental setup as shown in Figures (a) and (b). On keeping the test tube (Figure b) in a beaker containing hot water (~ 70°C), the water level in the glass tube rises. How does it affect the density?

Question

Bright Tutorials · Accepted Answer

The density of water in setup (b) will decrease.
Class 8 The Amazing World of Solutes, Solvents, and Solutions Question Answer (Activities)
Activity 9.1: Let us Investigate (Page 136)
Take a clean glass tumbler and fill it half with water.
Add one spoon of salt into it and stir well till it dissolves completely.
Gradually add a spoonful of salt into the glass tumbler and stir.
Observe how many spoons of salt you can add before it stops dissolving completely.
Record your observations in the Table.
Answer:
Quite a few spoons of salt could be dissolved before some of it remained undissolved.
Water has a high capacity to dissolve salts.
Activity 9.2: Let us Experiment (Demonstration Activity) (Page 137)
Take about 50 mL of water in a glass beaker and measure its temperature using a laboratory thermometer, say 20°C. Add a spoonful of baking soda (sodium hydrogen carbonate) to the water and stir until it dissolves. Continue adding small amounts of baking soda while stirring, till some solid baking soda is left undissolved at the bottom of the beaker. Now, heat the contents to 50°C while stirring (figure).
What happens to the undissolved baking soda?
You will observe that it has dissolved.
Continue adding more baking soda while stirring at this temperature until some solid baking soda remains undissolved.
Again, heat the contents further to 70°C while continuing to stir. What do you observe?
The undissolved baking soda dissolves.
What do you infer from this experiment?
Answer:
Water at 70°C dissolves more of the baking soda than at 50°C. It is inferred that for most of the substances, the solubility increases with temperature. It can also be concluded that a saturated solution becomes an unsaturated solution at a higher temperature.
Activity 9.3: Let us Measure (Page 142)
Switch ON the digital weighing balance.
Note the reading displayed on the balance, which gives the mass of the stone, say 16.400 g.
Answer:
The difference measured between the weight of the watch glass and the watch glass with the object gives the weight/mass of the object. In case of liquids, a watch glass is replaced with a measuring bottle.
Activity 9.4: Let us Observe and Calculate (Page 143)
What is the maximum volume it can measure?
Answer:
The maximum volume that can be measured in a cylinder depends on its size.
How much is the volume difference indicated between the two bigger marks (for example, between 10 mL and 20 mL)?
Answer:
The volume difference between two bigger marks (say 10 mL and 20 mL) is 10 mL.
How many smaller divisions are there between the two bigger marks?
Answer:
There are 10 smaller divisions between the two bigger marks.
How much volume does one small division indicate?
Answer:
One small division indicates 0.1 mL.
The smallest volume that the measuring cylinder can read is 1 mL.
Activity 9.5: Let us Measure 50 mL of Water (Page 144)
Place a clean, dry measuring cylinder on a flat surface.
Read the mark on the measuring cylinder that coincides with the bottom of the meniscus for water or other colourless liquids.
Answer:
While reading the measuring cylinder, the eye level should be on the lower meniscus for colourless liquid and on the top or upper meniscus for coloured liquid.

The Amazing World of Solutes Solvents Solutions — Question 12