CAMBRIDGE INTERNATIONAL AS AND A LEVEL CHEMISTRY COURSEBOOK PDF

Sign up for updates about changes to the syllabuses you teach We revise qualifications regularly to make sure that they continue to meet the needs of learners, schools and higher education institutions around the world and that they are up-to-date with current thinking. We have updated this syllabus and all assessment materials to improve the clarity and consistency between Biology, Chemistry and Physics. How has the syllabus changed? Where data are required these will be provided in the question. All papers will contain a data table of commonly used constants and a Periodic Table.

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Figure 1. Email Email encryption uses asymmetric encryption. This means that recipients of emails must have the private key that matches the public key used to encrypt the original email. Encrypting an email will also encrypt any attachments. How encryption protects data Encryption only scrambles the data so that if it is found, it cannot be understood. It does not stop the data from being intercepted, stolen or lost.

The purpose is to ensure the data is sensible and conforms to defined rules. A railway season ticket will have an expiry date. The season ticket is valid until it expires. Once it expires it is invalid. The rule here is that the date the season ticket is used must be before its expiry date. When data is validated, if it conforms to the rules then it will be accepted. If it does not conform to the rules, then it will be rejected and an error message will be presented. Validation does not ensure that data is correct.

TASK Create a flow chart to describe the process of validation. Range check A range check ensures that data is within a defined range. A limit check has a single boundary. This could be the highest possible value or the lowest possible value. A range check includes two boundaries, which would be the lower boundary and the upper boundary. These different checks are the different types of rules that are used. If data is entered, then it is accepted. If data is not entered, then the user will be presented with an error message asking them to enter data.

Please fill in the mandatory fields. The respondents have to be at least 18 years old. The lower boundary is There is no upper boundary, so this is a limit check. Only the letters A—E are valid grades. The grade must be less than F. The upper boundary is E. There is no lower boundary, so this is a limit check. The lower boundary is 5 and the upper boundary is 28, so this is a range check. This could be written as: Figure 1. Data that is outside the boundaries is invalid.

Data that is valid and within the boundaries is not necessarily correct. A grade of C could be entered when a grade A should have been entered. C is valid but incorrect.

Type check A type check ensures that data must be of a defined data type. It is sometimes known as a picture check and the data has to follow a pattern.

Data that is valid and of the correct data type is not necessarily correct. The date is valid because it is a date data type, but it is clearly incorrect. Length check A length check ensures data is of a defined length or within a range of lengths. Data that is valid and of the defined format is not necessarily correct. An email address of fdc jb meets the rules above but is clearly incorrect.

Lookup check A lookup check tests to see if data exists in a list. It is similar to referential integrity in Chapter 9, but uses a list defined within the validation rule. Data that is of the allowed length is not necessarily correct. For example, a valid date might require six digits. A date of 2ndFeb would be a valid length because it contains six characters, but it would not be correct because it does not follow the required format.

A lookup validation rule would check to see that the values are within this list. Students taking a qualification could be issued grades of pass, merit and distinction. Consistency check A consistency check compares data in one field with data in another field that already exists within a record, to see whether both are consistent with each other. When entering data about dispatching products, it would not be possible to mark an item as being dispatched until after it has been packaged.

Check digit A check digit is a number or letter that is added to the end of an identification number being input. It is a form of redundancy check because the check digit is redundant not needed for the identification number, but just used for validation. When the identification number is first created, an algorithm a series of calculations is performed on it to generate a check digit.

When the identification number is input, the same algorithm is performed on it. The result of the algorithm should match the check digit. If it matches, then the data is valid. If it does not match then the data is invalid. It is the last digit shown on a barcode. The algorithm for calculating the check digit is: 1 Add all the digits in even numbered positions together.

They match. Data is valid. They do not match. Data is invalid. There are a variety of calculations that can be performed to determine what the check digit should be. The important thing is that the same calculation used to create the check digit in the first place should be used to confirm the check digit when the identification number is input. The result of the algorithm is 1. Invalid example In this example, the ISBN has been entered incorrectly as two numbers have been transposed 7 and 3 accidentally: Visual checking A method of verification can be for the user to visually check that the data entered matches the original source.

This can be done by reading the data displayed on screen and comparing it with the original data. If the data matches, then it has passed the verification process. If it does not match, then it has failed the verification process and needs to be re-entered. Visual checking does not ensure that the data entered is correct.

If the original data is wrong, then the verification process may still pass. Double data entry Another method of verification is to input data into the computer system twice. The two items of data are compared by the computer system and if they match, then they are verified. If there are any differences, then one of the inputs must have been incorrect.

The result 3 is compared with the check digit of 1 that was entered. The ISBN entered is invalid. TASK Use the website www. It is only for 13 character barcodes that the even digits are multiplied by 3. Find out how to calculate a check digit for 10 digit barcodes. Verification Verification is the process of checking that the data entered into the computer system matches the original source.

This is because it is critical that the password is entered correctly in order that the user can gain access to the system in the future. If the new passwords match, then the password will be changed. It is still possible to pass double entry verification and for the data to be incorrect. If the data is entered incorrectly twice, then the two values may match.

For example, if the CAPS key is left on by mistake then both entries would match. The need for both validation and verification As you will have seen in the two sections above, it is possible to enter valid data that is still incorrect. It is also possible to verify incorrect data. By using both validation and verification, the chances of entering incorrect data are reduced. If data that is incorrect passes a validation check, then the verification check is likely to spot the error.

N is entered. This passes the validation check but is clearly incorrect. When verified using double entry, the user enters N first followed by M the second time.

The verification process has identified the error. However, it is still possible that the user could enter N twice and both the validation and verification processes would fail. Proof reading Proof reading is the process of checking information.

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