Definitions

soluble - Something that dissolves in a solvent

insoluble - Something that does not dissolve in a solvent

solute - the substance that is being dissolved in a solvent to form a solution.

solvent - the substance that does the dissolving to form a solution

solution - the mixture of a solute and a solvent.

filtrate - the substance that passes through a filter

residue - the substance left behind in the filter

Solubility.

Some solids will dissolve in water e.g. salt or sugar whilst others do not e.g. chalk. We can use this physical property to separate out mixtures.

To separate salt from sand we can add water to dissolve the salt. The sand is insoluble so does not dissolve.  we can then filter the mixture and the water (and salt) goes through the filter whilst the sand stays behind as the residue. We can recover the salt by evaporating the water over a radiator or more vigorously with a Bunsen burner. The solution should not be boiled dry but the last bit allowed to evaporate naturally as it may start spitting.

Factors that affect the rate of dissolving.

There are several factors affecting the rate of dissolving everyday substances in water, i.e. the temperature of the solvent, particle size of the solute and stirring.  The videos below help to explain this further.

Fair testing

Scientists use an experiment to search for cause and effect relationships in nature. In other words, they design an experiment so  that changes to one item cause something else to vary in a predictable  way.

When doing an investigation, it is important that fair testing is carried out so that each factor can be examined in isolation. 

These changing quantities are called variables. A  variable is any factor, trait, or condition that can exist in differing amounts or types. An experiment usually has three kinds of variables:  independent, dependent, and controlled.

Dependent, independent and controlled variables

Dependent variables

A dependent variable is the variable that you observe and measure. You have no control over the dependent variable; you want to observe what happens to the dependent variable when you change the independent  variable. For example, if the child is testing the effects of light on plant growth, how much the plant grows after some period of time would be the dependent variable. The child can measure how much the plant in  the closet grows relative to how much the plant near the window grows.

Independent variables

An independent variable is a variable that you can control. One way to explain it to a child is that it is the variable that the child can  change during the experiment. For example, in an experiment on the  effect of light on plant growth, the child can control how much light a plant receives. He can put one plant near a window and another plant in a dark closet.

Controlled variables

Besides independent and dependent variables, every good experiment needs to control certain variables so that they do not influence the outcome  of the experiment systematically. A controlled variable is one that you keep the same for all the conditions of your experiment. A good  scientist needs to think through all the variables that might need to be controlled so that they do not interfere with the experiment. For  example, to get reliable results for the plant experiment, both the plant near the window and the plant in the closet need to receive the  same amount of water so that testers would know that it was the  differences in light and not the differences in water that made one plant grow more than the other.

In a good experiment, the scientist must be able to measure the values for each variable. Weight or mass is an example of a variable that is very easy to measure. However, imagine trying to do an experiment where one of the variables is love. There is no such thing as a "love-meter." You might have a belief that  someone is in love, but you cannot really be sure, and you would  probably have friends that don't agree with you. So, love is not  measurable in a scientific sense; therefore, it would be a poor variable to use in an experiment.

Drawing graphs

Graph drawing is a very important skill. There are several different types of graph that you need to be able to draw. Graphs have two axes, the horizontal, X and vertical, Y.

We put what we are controlling on the X axis.  This is also called the independent variable . What we are looking at goes on the Y one. This is the dependent one as it depends on the independent variable.

for example, if you were making a bar graph of people's favorite colors, you'd put the colors (independent) on the x axis. then you'd survey everyone and put your newfound results (dependent) on the y axis.

Bar charts

Bar charts are useful where the independent variable is discrete.  that is to say it can only e a fixed value. E.g. favourite colour or car type.

line graphs

these are good where both the independent and dependent variables can be a range.  E.g. weight hanging from a spring vs. the length of the spring.

Here are some rules for drawing a good graph

1. Always give your graph a title

Let's say that you're doing a graph where you're studying how the temperature on the speed of a reaction of dissolving sugar. In this reaction, you're changing the temperature to known values, so the temperature is your independent variable. Because you don't know the speed of the dissolving and speed depends on the temperature, the speed of dissolving is your dependent variable.

2. The x-axis of a graph is always your independent variable and the y-axis is the dependent variable.

For the graph described above, temperature would be on the x-axis (the one on the bottom of the graph), and the time taken to dissolve the sugar would be on the y-axis (the one on the side of the graph)

3. Always label the x and y axes and give units.

Putting numbers on the x and y-axes is something that everybody always remembers to do (after all, how could you graph without showing the numbers?). However, people frequently forget to put a label on the axis that describes what those numbers are, and even more frequently forget to say what those units are. For example, if you're going to do a chart which uses temperature as the independent variable, you should write the word "temperature (degrees Celsius)" on that axis so people know what those numbers stand for. Otherwise, people won't know that you're talking about temperature, and even if they do, they might think you're talking about degrees Fahrenheit.

4. Never, EVER, connect the dots on your graph!

Hey, if you're working with your little sister on one of those place mats at Harvester, you can connect the dots. When you're working in science, you never, ever connect the dots on a graph.

Why? When you do an experiment, you always muck something up. . It's probably not a big mistake, and is frequently not something you have a lot of control over. However, when you do an experiment, many little things go wrong, and these little things add up. As a result, experimental data never makes a nice straight line. Instead, it makes a bunch of dots which kind of wiggle around a graph. This is normal, and will not affect your grade unless your teacher is a Nobel prize winner. However, you can't just pretend that your data is perfect, because it's not. Whenever you have the dots moving around a lot, we say that the data is noisy, because the thing you're looking for has a little bit of interference caused by normal experimental error.

To show that you're a clever young scientist, your best bet is to show that you KNOW your data is sometimes lousy. You do this by making a line (or curve) which seems to follow the data as well as possible, without actually connecting the dots. Doing this shows the trend that the data suggests, without depending too much on the noise. As long as your line (or curve) does a pretty good job of following the data, you should be fine. 

6. Make sure your data is graphed as large as possible in the space you've been given.

Let's face it, you don't like looking at little tiny graphs. Your teacher doesn't either. If you make large graphs, you'll find it's easier to see what you're doing, and your teacher will be lots happier.