Home School - Invstigate Dissolving- Part 2

A couple of weeks ago the boys and I undertook the experiment where they were discovering which materials would dissolve and which wouldn't.

I wanted to spend a couple of lessons looking at the concept of dissolving and then if temperature would effect the rate of change. So we repeated the experiment but with Hot water!

That some substances can dissolve in water.

That temperature affects the rate of dissolving.

That some substances dissolve and others are suspended.

 

To select and explore activities and resources independently

To use basic vocabulary as well as extend to learning new sounds, words and meanings

To explore, record and summarise the different changes

To ask questions about why things happen and how things work

Materials

1.       Hot Water

2.       Sand

4.       Salt

5.       Soil

6.     Flour

7.     Coffee

8.     Small bowls/See through glasses

9.       Spoons

We started off by recapping the previous lesson and discussing the difference between the substances provided. Both of the boys could distinguish the difference and could recall each substance carefully and what reaction happened. 

I started by boiling the kettle and pouring small amounts into see through glasses.


Once I had warned the boys to be careful they started the experiment by using  sand.

They both recalled that the sand didn't dissolve and even after stirring stayed as the same consistency. I asked them what they thought would happen when put in hot water.

William said he didn't think anything would happen and explored and explained that as it had no response previously then it wouldn't again. James was different; he thought that with hot water and stirring that it would change.

 

William had predicted correctly but what became apparent was his own doubt. Once the water and sand had settled and you could clearly see it sitting at the bottom of the glass he was starting to make other predictions. 

We then tried coffee.

James was quick on his predictions and instantly said that coffee would dissolve and it would happen quickly. When I asked William to recall the previous lesson he did remember that there was some residual granules. He did say that he thought it would happen again but there wouldn't be any left over because the water was hot; he had pre-existing knowledge that I have my coffee with hot water so it must work better!

The coffee as a whole started dissolving nearly instantly within the hot water. I think William expected an instant change and was surprised that it didn't go from one form to another as quickly as expected. He did stir the glass to add more motion as he declared that I do this when making a drink; the water became darker and it appeared that everything had fully dissolved once he did this.

The biggest comparison was made by James where he suggested that it was quicker. He pointed out that the wasn't any residual granules left and all the stirring did was to make it happen even quicker.

We next tested soil.

I wanted to use soil after the coffee to see the colour comparison and if the boys would relate to the coffee and make a prediction from that or if they used their knowledge of the sand.

Interestingly that both went for different options; William said there will be no change just like the sand and James said there would be because the coffee improved with hot water.

 

Putting the soil in the water just made the soil sink to the bottom. The colour of the water changed to a dirty brown colour but didn't go to the colour that the boys expected. When James stirred the water with the soil in we discovered that the soil was more likely to either clump together in bigger pieces or to be broken down more and change the water colour darker; something that William pointed out happened last time!

Just like the sand, the soil settled eventually at the bottom making a murky coloured water.

We then tested the salt and sugar at the same time.

Both of the boys discussed this for a while. William was trying to tell James that these would have the same result as the coffee.

He was using 2 points of reference to make this assumption.

1. Salt and Sugar are both items that we use every day and that would me it would work like the coffee.

2. In the last experiment he remember that they both disappeared after stirring which is what happened to the coffee.

Once he had explained this he declared that they would both dissolve and each would happen quicker because the water was hot. 

 

Just like the experiment before and the coffee previously the reacting was quick and quicker in the hot water.

Finally we tested the flour again.

In the last experiment there was some confusion caused because they knew that it was used in things we digest so they made the assumption that the reaction would be the same as coffee, sugar and salt but in fact the reaction is more like sand and soil.

William had remembered this and he changed his prediction.

He suggested that the whole water and flour would change from a liquid to a solid. The basis for this point was that the heat in an oven can transform from one state to another therefore the heat in the class would create the same chemical reaction.

I absolutely LOVED this logic!

He had recalled the previous experiment, assessed that heat had increased and changed an outcome in coffee, sugar and salt (even if that reaction was just dissolving quicker) and then used outside understanding to make a prediction.

With this set in place the boys waited with anticipation!

Obviously the reaction wasn't as severe as William had predicted and even with hot water and stirring the mixture just formed a milky white suspension.

I explained again that flour never really dissolves and that it just gets smaller and sits next to the water. I also explained that this is why we use it in cooking and baking and if it dissolved then it wouldn't be able to do that. (Like using self raising flour to create a rise in baking. The combination of salt, flour and baking powder remains within the mix)

(leave the glasses to settle for over an hour to really have the opportunity to confirm if substances dissolve)

I discussed with them that when a solid is mixed with a liquid that it becomes a solution or a suspension. A solution (Sugar and Salt) will go clear and never settle where a suspension (Flour and Soil) is cloudy and will settle eventually even if it takes longer with different substances. I said that coffee was the only thing that broke this rule; explaining that coffee is a solution as it is a solid that dissolves but doesn't produce a clear liquid.

I then told them that the addition of hot water in replace of the cold is an important factor when considering substances that turn into solutions.

They would find that the speed of dissolving will happen faster in hot water than it does in cold water because hot water has more energy than cold water. When water is heated, the molecules gain energy and, thus, move faster this creates the heat.

We demonstrated this by rubbing our hands together really quickly to demonstrate the heating process that can come from our hands.

A similar concept happens to substances. The molecules move faster, they come into contact with the substance, like sugar, more often, causing it to dissolve faster with the reaction being increased and therefore breaking up the bonds between them.

In theory this is a near on identical activity and experiment to last time and there will be a lot of repetitive actions and comments but this is exactly what is needed!

Having a similar test will hopefully grow an understanding, build upon outside and pre-existing knowledge to make logical assumptions; something that I feel both boys did. Yet, the premise of having hot water and why that makes a difference is a great lesson.

So what do you think? Will you try both parts to our experiment?